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Magnetic Resonance Materials in Physics, Biology and Medicine 
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ISSN (Print) 0968-5243 - ISSN (Online) 1352-8661
Published by Springer-Verlag
[2216 journals]
[3 followers] Follow Subscription journalISSN (Print) 0968-5243 - ISSN (Online) 1352-8661
Published by Springer-Verlag
[2216 journals]- High resolution morphologic imaging and T2 mapping of cartilage at 7 Tesla: comparison of cartilage repair patients and healthy controls
- Abstract: Object
Our objective was to use 7 T MRI to compare cartilage morphology (thickness) and collagen composition (T2 values) in cartilage repair patients and healthy controls.
Materials and methods
We scanned the knees of 11 cartilage repair patients and 11 controls on a 7 T MRI scanner using a high-resolution, gradient-echo sequence to measure cartilage thickness and a multi-echo spin-echo sequence to measure cartilage T2 values. We used two-tailed t tests to compare cartilage thickness and T2 values in: repair tissue (RT) versus adjacent cartilage (AC); RT versus healthy control cartilage (HC); AC versus HC.
Results
Mean thickness in RT, AC, HC were: 2.2 ± 1.4, 3.6 ± 1.1, 3.3 ± 0.7 mm. Differences in thickness between RT–AC (p = 0.01) and RT–HC (p = 0.02) were significant, but not AC–HC (p = 0.45). Mean T2 values in RT, AC, HC were: 51.6 ± 7.6, 40.0 ± 4.7, 45.9 ± 3.7 ms. Differences in T2 values between RT–AC (p = 0.0005), RT-HC (p = 0.04), and AC–HC (p = 0.004) were significant.
Conclusion
7 T MRI allows detection of differences in morphology and collagen architecture in: (1) cartilage repair tissue compared to adjacent cartilage and (2) cartilage repair tissue compared to cartilage from healthy controls. Although cartilage adjacent to repair tissue may be normal in thickness, it can demonstrate altered collagen composition.
PubDate: 2013-05-09
- Abstract: Object
Our objective was to use 7 T MRI to compare cartilage morphology (thickness) and collagen composition (T2 values) in cartilage repair patients and healthy controls.
Materials and methods
We scanned the knees of 11 cartilage repair patients and 11 controls on a 7 T MRI scanner using a high-resolution, gradient-echo sequence to measure cartilage thickness and a multi-echo spin-echo sequence to measure cartilage T2 values. We used two-tailed t tests to compare cartilage thickness and T2 values in: repair tissue (RT) versus adjacent cartilage (AC); RT versus healthy control cartilage (HC); AC versus HC.
Results
Mean thickness in RT, AC, HC were: 2.2 ± 1.4, 3.6 ± 1.1, 3.3 ± 0.7 mm. Differences in thickness between RT–AC (p = 0.01) and RT–HC (p = 0.02) were significant, but not AC–HC (p = 0.45). Mean T2 values in RT, AC, HC were: 51.6 ± 7.6, 40.0 ± 4.7, 45.9 ± 3.7 ms. Differences in T2 values between RT–AC (p = 0.0005), RT-HC (p = 0.04), and AC–HC (p = 0.004) were significant.
Conclusion
7 T MRI allows detection of differences in morphology and collagen architecture in: (1) cartilage repair tissue compared to adjacent cartilage and (2) cartilage repair tissue compared to cartilage from healthy controls. Although cartilage adjacent to repair tissue may be normal in thickness, it can demonstrate altered collagen composition.
- Single spin-echo T
2 relaxation times of cerebral metabolites at 14.1 T in the in vivo rat brain
- Abstract: Object
To determine the single spin-echo T
2 relaxation times of uncoupled and J-coupled metabolites in rat brain in vivo at 14.1 T and to compare these results with those previously obtained at 9.4 T.
Materials and methods
Measurements were performed on five rats at 14.1 T using the SPECIAL sequence and TE-specific basis-sets for LCModel analysis.
Results and conclusion
The T
2 of singlets ranged from 98 to 148 ms and T
2 of J-coupled metabolites ranged from 72 ms (glutamate) to 97 ms (myo-inositol). When comparing the T
2s of the metabolites measured at 14.1 T with those previously measured at 9.4 T, a decreasing trend was found (p < 0.0001). We conclude that the modest shortening of T
2 at 14.1 T has a negligible impact on the sensitivity of the 1H MRS when performed at TE shorter than 10 ms.
PubDate: 2013-04-21
- Abstract: Object
To determine the single spin-echo T
2 relaxation times of uncoupled and J-coupled metabolites in rat brain in vivo at 14.1 T and to compare these results with those previously obtained at 9.4 T.
Materials and methods
Measurements were performed on five rats at 14.1 T using the SPECIAL sequence and TE-specific basis-sets for LCModel analysis.
Results and conclusion
The T
2 of singlets ranged from 98 to 148 ms and T
2 of J-coupled metabolites ranged from 72 ms (glutamate) to 97 ms (myo-inositol). When comparing the T
2s of the metabolites measured at 14.1 T with those previously measured at 9.4 T, a decreasing trend was found (p < 0.0001). We conclude that the modest shortening of T
2 at 14.1 T has a negligible impact on the sensitivity of the 1H MRS when performed at TE shorter than 10 ms.
- 3T Renal 23Na-MRI: effects of desmopressin in patients with central diabetes insipidus
- Abstract: Purpose
The purpose of this prospective study was to assess physiologic changes in the renal corticomedullary 23Na-concentration ([23Na]) gradient with 23Na-MRI at 3.0T in patients with central diabetes insipidus (CDI) before and after intranasal administration of 20 μg desmopressin (DDAVP).
Methods and materials
Four patients with CDI (all male, mean age 60.2 years) were included in this IRB-approved study. For 23Na-imaging, a 3D density adapted, radial GRE-sequence (TE = 0.55 ms; TR = 120 ms; projections = 8,000; spatial resolution = 5 × 5 × 5 mm3) was used in combination with a dedicated 23Na-coil and reference phantoms. The corticomedullary [23Na] gradient (in mmol/L/mm) was calculated pixel-by-pixel along a linear region-of-interest (ROI) spanning from the renal cortex in the direction of the medulla. Mean ± SDs of [23Na] were calculated for each patient as well as for the entire group.
Results
Mean [23Na] increased along the corticomedullary gradient from the cortex (pre-DDAVP 38.0 ± 6.3 mmol/L vs. post-DDAVP 30.7 ± 3.5 mmol/L) to the medulla (pre-DDAVP 71.6 ± 14.8 mmol/L vs. post-DDAVP 59.7 ± 10.8 mmol/L). The overall mean decrease of [23Na] after DDAVP administration was 17.1 ± 1.1 %.
Conclusion
23Na-MRI with state-of-the-art techniques at 3T depicts the physiologic renal response to the administration of desmopressin in patients with central diabetes insipidus.
PubDate: 2013-04-06
- Abstract: Purpose
The purpose of this prospective study was to assess physiologic changes in the renal corticomedullary 23Na-concentration ([23Na]) gradient with 23Na-MRI at 3.0T in patients with central diabetes insipidus (CDI) before and after intranasal administration of 20 μg desmopressin (DDAVP).
Methods and materials
Four patients with CDI (all male, mean age 60.2 years) were included in this IRB-approved study. For 23Na-imaging, a 3D density adapted, radial GRE-sequence (TE = 0.55 ms; TR = 120 ms; projections = 8,000; spatial resolution = 5 × 5 × 5 mm3) was used in combination with a dedicated 23Na-coil and reference phantoms. The corticomedullary [23Na] gradient (in mmol/L/mm) was calculated pixel-by-pixel along a linear region-of-interest (ROI) spanning from the renal cortex in the direction of the medulla. Mean ± SDs of [23Na] were calculated for each patient as well as for the entire group.
Results
Mean [23Na] increased along the corticomedullary gradient from the cortex (pre-DDAVP 38.0 ± 6.3 mmol/L vs. post-DDAVP 30.7 ± 3.5 mmol/L) to the medulla (pre-DDAVP 71.6 ± 14.8 mmol/L vs. post-DDAVP 59.7 ± 10.8 mmol/L). The overall mean decrease of [23Na] after DDAVP administration was 17.1 ± 1.1 %.
Conclusion
23Na-MRI with state-of-the-art techniques at 3T depicts the physiologic renal response to the administration of desmopressin in patients with central diabetes insipidus.
- Combination of tagging and tissue phase mapping to accelerate myocardial motion measurements in three directions
- Abstract: Object
Until now, a three-directional velocity field has mostly been obtained by velocity encoding in three directions, which is very time-consuming and hence not usually used in clinical routine. We show the feasibility of combining in-plane tagging with through-plane tissue phase mapping (TPM) to encode a three-directional velocity field at 3 T with reduced overall acquisition time.
Materials and methods
Assessment of a three-directional velocity field was performed for 10 healthy volunteers. The motion patterns obtained by use of five different sequences including three-directional TPM, TPM in the through-plane direction, TPM in the through-plane direction with horizontal or vertical tagging lines, and TPM in the through-plane direction combined with a tagging grid were evaluated and compared.
Results
A three-dimensional velocity field can be obtained in approximately half the acquisition time by combining through-plane TPM with in-plane tagging. Although the velocity information is derived by different means, differences between the information obtained by three-directional TPM encoding and the suggested technique are only minor.
Conclusion
The combination of tagging and TPM enables assessment of the three-directional velocity field in nearly half the time taken when the conventional three-directional TPM sequence is used.
PubDate: 2013-04-01
- Abstract: Object
Until now, a three-directional velocity field has mostly been obtained by velocity encoding in three directions, which is very time-consuming and hence not usually used in clinical routine. We show the feasibility of combining in-plane tagging with through-plane tissue phase mapping (TPM) to encode a three-directional velocity field at 3 T with reduced overall acquisition time.
Materials and methods
Assessment of a three-directional velocity field was performed for 10 healthy volunteers. The motion patterns obtained by use of five different sequences including three-directional TPM, TPM in the through-plane direction, TPM in the through-plane direction with horizontal or vertical tagging lines, and TPM in the through-plane direction combined with a tagging grid were evaluated and compared.
Results
A three-dimensional velocity field can be obtained in approximately half the acquisition time by combining through-plane TPM with in-plane tagging. Although the velocity information is derived by different means, differences between the information obtained by three-directional TPM encoding and the suggested technique are only minor.
Conclusion
The combination of tagging and TPM enables assessment of the three-directional velocity field in nearly half the time taken when the conventional three-directional TPM sequence is used.
- Is NMR metabolic profiling of spent embryo culture media useful to assist in vitro human embryo selection'
- Abstract: Object
The prediction of embryo viability by usual morphological analysis is currently unsatisfactory. New non-invasive techniques such as high-resolution nuclear magnetic resonance (1H-NMR) spectroscopy that allows assessment of metabolic profiling in spent culture media might help embryologists to predict embryo development.
Materials and methods
Individual microdrops of culture media were analysed after 24 h of embryo culture (from day 3 to day 4) by spectroscopy using a 1 mm microliter probe allowing analysis without sample dilution. Embryos were divided into two groups on day 5: non-arrested embryos (n = 19) and arrested embryos unable to reach the blastocyst stage (n = 20). Multivariate analysis techniques such as Principal Component Analysis (PCA) and Orthogonal Partial Least Square Discriminant Analysis (OPLS-DA) were performed to compare extracellular metabolite balance.
Results
1H-NMR used in combination with a 1 mm probe suggested that in vitro cultured human embryos that have a high developmental potential modify their environment slightly compared to embryos that cease to develop. However, differences between the two groups did not reach statistical significance and multivariate statistical analysis did not allow clustering of the two groups.
Conclusion
This study indicated that this technique would not be sufficiently powerful alone to provide information that might help to assess the developmental potential of individual embryos for in vitro fertilisation (IVF).
PubDate: 2013-04-01
- Abstract: Object
The prediction of embryo viability by usual morphological analysis is currently unsatisfactory. New non-invasive techniques such as high-resolution nuclear magnetic resonance (1H-NMR) spectroscopy that allows assessment of metabolic profiling in spent culture media might help embryologists to predict embryo development.
Materials and methods
Individual microdrops of culture media were analysed after 24 h of embryo culture (from day 3 to day 4) by spectroscopy using a 1 mm microliter probe allowing analysis without sample dilution. Embryos were divided into two groups on day 5: non-arrested embryos (n = 19) and arrested embryos unable to reach the blastocyst stage (n = 20). Multivariate analysis techniques such as Principal Component Analysis (PCA) and Orthogonal Partial Least Square Discriminant Analysis (OPLS-DA) were performed to compare extracellular metabolite balance.
Results
1H-NMR used in combination with a 1 mm probe suggested that in vitro cultured human embryos that have a high developmental potential modify their environment slightly compared to embryos that cease to develop. However, differences between the two groups did not reach statistical significance and multivariate statistical analysis did not allow clustering of the two groups.
Conclusion
This study indicated that this technique would not be sufficiently powerful alone to provide information that might help to assess the developmental potential of individual embryos for in vitro fertilisation (IVF).
- Eddy current compensation for delta relaxation enhanced MR by dynamic reference phase modulation
- Abstract: Object
Eddy current compensation by dynamic reference phase modulation (eDREAM) is a compensation method for eddy current fields induced by B
0 field-cycling which occur in delta relaxation enhanced MR (dreMR) imaging. The presented method is based on a dynamic frequency adjustment and prevents eddy current related artifacts. It is easy to implement and can be completely realized in software for any imaging sequence.
Materials and methods
In this paper, the theory of eDREAM is derived and two applications are demonstrated. The theory describes how to model the behavior of the eddy currents and how to implement the compensation. Phantom and in vivo measurements are carried out and demonstrate the benefits of eDREAM.
Results
A comparison of images acquired with and without eDREAM shows a significant improvement in dreMR image quality. Images without eDREAM suffer from severe artifacts and do not allow proper interpretation while images with eDREAM are artifact free. In vivo experiments demonstrate that dreMR imaging without eDREAM is not feasible as artifacts completely change the image contrast.
Conclusion
eDREAM is a flexible eddy current compensation for dreMR. It is capable of completely removing the influence of eddy currents such that the dreMR images do not suffer from artifacts.
PubDate: 2013-04-01
- Abstract: Object
Eddy current compensation by dynamic reference phase modulation (eDREAM) is a compensation method for eddy current fields induced by B
0 field-cycling which occur in delta relaxation enhanced MR (dreMR) imaging. The presented method is based on a dynamic frequency adjustment and prevents eddy current related artifacts. It is easy to implement and can be completely realized in software for any imaging sequence.
Materials and methods
In this paper, the theory of eDREAM is derived and two applications are demonstrated. The theory describes how to model the behavior of the eddy currents and how to implement the compensation. Phantom and in vivo measurements are carried out and demonstrate the benefits of eDREAM.
Results
A comparison of images acquired with and without eDREAM shows a significant improvement in dreMR image quality. Images without eDREAM suffer from severe artifacts and do not allow proper interpretation while images with eDREAM are artifact free. In vivo experiments demonstrate that dreMR imaging without eDREAM is not feasible as artifacts completely change the image contrast.
Conclusion
eDREAM is a flexible eddy current compensation for dreMR. It is capable of completely removing the influence of eddy currents such that the dreMR images do not suffer from artifacts.
- DQF-MT MRI of connective tissues: application to tendon and muscle
- Abstract: Object
The sequence combining DQF (double quantum filtering) with magnetisation transfer (DQF-MT) was tested as an alternative to the DQF sequence for characterising tendon and muscle by MR imaging.
Materials and methods
DQF-MT images of tendon–muscle phantoms were obtained at 4.7 T using ultra-short time to echo (UTE) methods in order to alleviate the loss of SNR due to the short T2 of the tissues. Two different sampling schemes of the k-space, Cartesian or radial, were employed. In vivo images of the human ankle on a clinical 1.5 T scanner are also presented. Parameters providing optimal tendon signal as well as optimal contrast between this tissue and muscle were determined.
Results
Two sets of parameters resulting in different contrasts between the tissues were found. For the first set (short creation time τ = 10 μs and magnetisation exchange time t
LM = 100 ms), DQF-MT signals in muscle and tendon were detected, with that of the tendon being the larger one. For the second set (long creation time τ = 750 μs and magnetisation exchange time 10 μs < t
LM < 100 ms), the DQF-MT signal was detected only in the tendon, and the decay of the double quantum coherence was slower than that observed for the first one, which allowed us to acquire DQF-MT MR images on a clinical 1.5 T MR scanner with minimal software interventions. In favourable conditions, the DQF-MT signal in the tendon could represent up to 10 % of the single-quantum signal.
Conclusion
Dipolar interaction within macromolecules such as collagen and myosin is at the origin of the DQF-MT signal observed in the first parameter set. This should enable the detection of muscle fibrosis.
PubDate: 2013-04-01
- Abstract: Object
The sequence combining DQF (double quantum filtering) with magnetisation transfer (DQF-MT) was tested as an alternative to the DQF sequence for characterising tendon and muscle by MR imaging.
Materials and methods
DQF-MT images of tendon–muscle phantoms were obtained at 4.7 T using ultra-short time to echo (UTE) methods in order to alleviate the loss of SNR due to the short T2 of the tissues. Two different sampling schemes of the k-space, Cartesian or radial, were employed. In vivo images of the human ankle on a clinical 1.5 T scanner are also presented. Parameters providing optimal tendon signal as well as optimal contrast between this tissue and muscle were determined.
Results
Two sets of parameters resulting in different contrasts between the tissues were found. For the first set (short creation time τ = 10 μs and magnetisation exchange time t
LM = 100 ms), DQF-MT signals in muscle and tendon were detected, with that of the tendon being the larger one. For the second set (long creation time τ = 750 μs and magnetisation exchange time 10 μs < t
LM < 100 ms), the DQF-MT signal was detected only in the tendon, and the decay of the double quantum coherence was slower than that observed for the first one, which allowed us to acquire DQF-MT MR images on a clinical 1.5 T MR scanner with minimal software interventions. In favourable conditions, the DQF-MT signal in the tendon could represent up to 10 % of the single-quantum signal.
Conclusion
Dipolar interaction within macromolecules such as collagen and myosin is at the origin of the DQF-MT signal observed in the first parameter set. This should enable the detection of muscle fibrosis.
- Dynamic MR imaging of a minipig’s knee using a high-density multi-channel receive array and a movement device
- Abstract: Object
To construct an optimised, high-density receive array and a movement device to achieve dynamic imaging of the knee in orthopedic large animal models (e.g., minipigs) at 1.5 T.
Materials and methods
A 13-channel RF receive array was constructed, and the crucial choice of the array element size (based on considerations like region of interest, geometry of the minipig’s knee, achievable signal-to-noise ratio, applicability of parallel imaging, etc.) was determined using the Q factors of loops with different sizes. A special movement device was constructed to guide and produce a reproducible motion of the minipig’s knee during acquisition.
Results
The constructed array was electrically characterised and the reproducibility of the cyclic motion was validated. Snapshots of dynamic in vivo images taken at a temporal resolution (308 ms) are presented. Some of the fine internal structures within the minipig’s knee, like cruciate ligaments, are traced in the snapshots.
Conclusion
This study is a step towards making dynamic imaging which can give additional information about joint injuries when static MRI is not able to give sufficient information, a routine clinical application. There, the combination of a high-density receive array and a movement device will be highly helpful in the diagnosis and therapy monitoring of knee injuries in the future.
PubDate: 2013-04-01
- Abstract: Object
To construct an optimised, high-density receive array and a movement device to achieve dynamic imaging of the knee in orthopedic large animal models (e.g., minipigs) at 1.5 T.
Materials and methods
A 13-channel RF receive array was constructed, and the crucial choice of the array element size (based on considerations like region of interest, geometry of the minipig’s knee, achievable signal-to-noise ratio, applicability of parallel imaging, etc.) was determined using the Q factors of loops with different sizes. A special movement device was constructed to guide and produce a reproducible motion of the minipig’s knee during acquisition.
Results
The constructed array was electrically characterised and the reproducibility of the cyclic motion was validated. Snapshots of dynamic in vivo images taken at a temporal resolution (308 ms) are presented. Some of the fine internal structures within the minipig’s knee, like cruciate ligaments, are traced in the snapshots.
Conclusion
This study is a step towards making dynamic imaging which can give additional information about joint injuries when static MRI is not able to give sufficient information, a routine clinical application. There, the combination of a high-density receive array and a movement device will be highly helpful in the diagnosis and therapy monitoring of knee injuries in the future.
- Proton CSI without solvent suppression with strongly reduced field gradient related sideband artifacts
- Abstract: Object
Non-water-suppressed MRSI (magnetic resonance spectroscopy imaging) offers a number of advantages; however, spectra are hampered by the sideband artifacts. The origin of those is associated with the vibration of the gradient coils, and most of the sidebands are assumed to be related to the crusher gradients. The aim was to examine the dependency between the physical direction of the crushers and the sidebands. Additionally, the possibilities of optimization of the point resolved spectroscopy sequence (PRESS) were investigated.
Materials and methods
For the assessment of the sidebands, spectra at short echo time (TE) were collected at 3 T from standard water phantom. A homemade agar phantom was used to test the optimal strength of the crusher gradients. Optimized PRESS sequence was tested in vivo.
Results
The greatest sidebands were found to be associated with the crusher gradient in x-direction. Agar phantom and in vivo measurements revealed that reduction of the crusher’s strength to 5 mT/m could provide a significant minimization of the sidebands without raising the unwanted signals produced by volume selection.
Conclusion
This study demonstrates that crusher gradients in different directions produce a unique pattern of the sidebands. Moreover, optimization of the strength of crushers has been found to decrease sidebands so, the remaining part could be reduced in postprocessing.
PubDate: 2013-04-01
- Abstract: Object
Non-water-suppressed MRSI (magnetic resonance spectroscopy imaging) offers a number of advantages; however, spectra are hampered by the sideband artifacts. The origin of those is associated with the vibration of the gradient coils, and most of the sidebands are assumed to be related to the crusher gradients. The aim was to examine the dependency between the physical direction of the crushers and the sidebands. Additionally, the possibilities of optimization of the point resolved spectroscopy sequence (PRESS) were investigated.
Materials and methods
For the assessment of the sidebands, spectra at short echo time (TE) were collected at 3 T from standard water phantom. A homemade agar phantom was used to test the optimal strength of the crusher gradients. Optimized PRESS sequence was tested in vivo.
Results
The greatest sidebands were found to be associated with the crusher gradient in x-direction. Agar phantom and in vivo measurements revealed that reduction of the crusher’s strength to 5 mT/m could provide a significant minimization of the sidebands without raising the unwanted signals produced by volume selection.
Conclusion
This study demonstrates that crusher gradients in different directions produce a unique pattern of the sidebands. Moreover, optimization of the strength of crushers has been found to decrease sidebands so, the remaining part could be reduced in postprocessing.
- A scale space based algorithm for automated segmentation of single shot tagged MRI of shearing deformation
- Abstract: Object
This study proposes a scale space based algorithm for automated segmentation of single-shot tagged images of modest SNR. Furthermore the algorithm was designed for analysis of discontinuous or shearing types of motion, i.e. segmentation of broken tag patterns.
Materials and methods
The proposed algorithm utilises non-linear scale space for automatic segmentation of single-shot tagged images. The algorithm's ability to automatically segment tagged shearing motion was evaluated in a numerical simulation and in vivo. A typical shearing deformation was simulated in a Shepp-Logan phantom allowing for quantitative evaluation of the algorithm's success rate as a function of both SNR and the amount of deformation. For a qualitative in vivo evaluation tagged images showing deformations in the calf muscles and eye movement in a healthy volunteer were acquired.
Results
Both the numerical simulation and the in vivo tagged data demonstrated the algorithm’s ability for automated segmentation of single-shot tagged MR provided that SNR of the images is above 10 and the amount of deformation does not exceed the tag spacing. The latter constraint can be met by adjusting the tag delay or the tag spacing.
Conclusion
The scale space based algorithm for automatic segmentation of single-shot tagged MR enables the application of tagged MR to complex (shearing) deformation and the processing of datasets with relatively low SNR.
PubDate: 2013-04-01
- Abstract: Object
This study proposes a scale space based algorithm for automated segmentation of single-shot tagged images of modest SNR. Furthermore the algorithm was designed for analysis of discontinuous or shearing types of motion, i.e. segmentation of broken tag patterns.
Materials and methods
The proposed algorithm utilises non-linear scale space for automatic segmentation of single-shot tagged images. The algorithm's ability to automatically segment tagged shearing motion was evaluated in a numerical simulation and in vivo. A typical shearing deformation was simulated in a Shepp-Logan phantom allowing for quantitative evaluation of the algorithm's success rate as a function of both SNR and the amount of deformation. For a qualitative in vivo evaluation tagged images showing deformations in the calf muscles and eye movement in a healthy volunteer were acquired.
Results
Both the numerical simulation and the in vivo tagged data demonstrated the algorithm’s ability for automated segmentation of single-shot tagged MR provided that SNR of the images is above 10 and the amount of deformation does not exceed the tag spacing. The latter constraint can be met by adjusting the tag delay or the tag spacing.
Conclusion
The scale space based algorithm for automatic segmentation of single-shot tagged MR enables the application of tagged MR to complex (shearing) deformation and the processing of datasets with relatively low SNR.
- Spatial analysis of diffusion tensor tractography statistics along the inferior fronto-occipital fasciculus with application in progressive supranuclear palsy
- Abstract: Background
The purpose of the study was to develop a method for analysis of diffusion parameters along white matter (WM) tracts, using spatial normalization based on anatomical landmarks, and to introduce the apparent area coefficient (AAC). The method’s applicability was tested in the inferior fronto-occipital fasciculus (IFO) in patients with progressive supranuclear palsy (PSP) and healthy controls (HCs).
Methods
A framework for analysis of diffusion parameters was developed. Spatial normalization of the tracts was performed using anatomical landmarks, to avoid deformations caused by cerebral atrophy. Initially, 38 HCs were used to optimize a threshold for the minimal size of regions that differ between groups. The fractional anisotropy, mean diffusivity, AAC, and the hemispheric asymmetry index (AI), were compared between 11 PSP patients and 15 HCs.
Results
The method was feasible for analysis of PSP patients and HCs. The AI showed that the observed hemispheric asymmetry of AAC was significantly larger in PSP patients compared with HCs in small regions of the IFO.
Conclusions
The method was successfully employed for analysis of diffusion parameters along the IFO in a patient group. This method can be potentially useful in studies of WM diseases, with or without cerebral atrophy.
PubDate: 2013-03-30
- Abstract: Background
The purpose of the study was to develop a method for analysis of diffusion parameters along white matter (WM) tracts, using spatial normalization based on anatomical landmarks, and to introduce the apparent area coefficient (AAC). The method’s applicability was tested in the inferior fronto-occipital fasciculus (IFO) in patients with progressive supranuclear palsy (PSP) and healthy controls (HCs).
Methods
A framework for analysis of diffusion parameters was developed. Spatial normalization of the tracts was performed using anatomical landmarks, to avoid deformations caused by cerebral atrophy. Initially, 38 HCs were used to optimize a threshold for the minimal size of regions that differ between groups. The fractional anisotropy, mean diffusivity, AAC, and the hemispheric asymmetry index (AI), were compared between 11 PSP patients and 15 HCs.
Results
The method was feasible for analysis of PSP patients and HCs. The AI showed that the observed hemispheric asymmetry of AAC was significantly larger in PSP patients compared with HCs in small regions of the IFO.
Conclusions
The method was successfully employed for analysis of diffusion parameters along the IFO in a patient group. This method can be potentially useful in studies of WM diseases, with or without cerebral atrophy.
- 3D gradient system for two B
0 field directions in earth’s field MRI
- Abstract: Object
A new gradient system for earth’s field magnetic resonance imaging (EFMRI) is presented that can be rotated relatively to the earth’s field direction while maintaining the ability to encode images. Orthogonal components of the gradient field are exploited to reduce the number of gradient coils.
Materials and methods
Two favorable orientations of the gradient system relative to the earth’s magnetic field (parallel and perpendicular) are discussed. We introduce the theory for the magnetic fields of the new gradient system and illustrate the design of the coil geometries which were worked out with the help of simulations and a numerical optimization algorithm. Field mapping measurements and imaging experiments in the two different orientations of the gradient system were carried out.
Results
Orthogonal components of the gradient field take over the role of the additionally needed gradient fields when the gradient system is rotated relative to the earth’s magnetic field. The results from the field mapping and imaging experiments verify the presented theory and show the functionality of the new gradient system.
Conclusion
The presented system demonstrates that gradient coils can be used for image encoding in multiple directions. This fact can be exploited to realize an EFMRI setup for parallel and perpendicular prepolarization with a single set of gradient coils.
PubDate: 2013-03-23
- Abstract: Object
A new gradient system for earth’s field magnetic resonance imaging (EFMRI) is presented that can be rotated relatively to the earth’s field direction while maintaining the ability to encode images. Orthogonal components of the gradient field are exploited to reduce the number of gradient coils.
Materials and methods
Two favorable orientations of the gradient system relative to the earth’s magnetic field (parallel and perpendicular) are discussed. We introduce the theory for the magnetic fields of the new gradient system and illustrate the design of the coil geometries which were worked out with the help of simulations and a numerical optimization algorithm. Field mapping measurements and imaging experiments in the two different orientations of the gradient system were carried out.
Results
Orthogonal components of the gradient field take over the role of the additionally needed gradient fields when the gradient system is rotated relative to the earth’s magnetic field. The results from the field mapping and imaging experiments verify the presented theory and show the functionality of the new gradient system.
Conclusion
The presented system demonstrates that gradient coils can be used for image encoding in multiple directions. This fact can be exploited to realize an EFMRI setup for parallel and perpendicular prepolarization with a single set of gradient coils.
- Magnetic resonance imaging of solid dental restoration materials using 3D UTE sequences: visualization and relaxometry of various compounds
- Abstract: Object
Due to an increasing scientific interest in MR-imaging of carious lesions and teeth, an accurate signal characterization of dental restoration materials is necessary for optimization of MR sequence protocols and evaluation of material degradation. Therefore, signal yield and relaxation behavior of common dental restoration materials in comparison to those of dentine of extracted human teeth were assessed in vitro by ultrashort echo time (UTE) sequences.
Materials and methods
Eighteen material samples and dentine of two freshly extracted human teeth were investigated on a 3T whole-body clinical MR-scanner. Transverse (T2*) and longitudinal relaxation times (T1) were quantified using a recently published modified Ernst equation that takes relevant in-pulse relaxation effects into account.
Results
All investigated samples could be successfully visualized but maximum signal yield was highly variable between samples. T1-values of the investigated dental restoration materials ranged between 28 and 365 ms, whereas T2*-values ranged between 96 and 917 μs. In contrast, T1-values of dentine (T1 = 545 ms ± 299 ms) were higher, while T2*-values (T2* = 478 μs ± 271 μs) showed similar values.
Conclusions
Dental restoration materials and dentine of extracted human teeth can be visualized by UTE sequences and show a broad range of signal yield and relaxation times.
PubDate: 2013-03-23
- Abstract: Object
Due to an increasing scientific interest in MR-imaging of carious lesions and teeth, an accurate signal characterization of dental restoration materials is necessary for optimization of MR sequence protocols and evaluation of material degradation. Therefore, signal yield and relaxation behavior of common dental restoration materials in comparison to those of dentine of extracted human teeth were assessed in vitro by ultrashort echo time (UTE) sequences.
Materials and methods
Eighteen material samples and dentine of two freshly extracted human teeth were investigated on a 3T whole-body clinical MR-scanner. Transverse (T2*) and longitudinal relaxation times (T1) were quantified using a recently published modified Ernst equation that takes relevant in-pulse relaxation effects into account.
Results
All investigated samples could be successfully visualized but maximum signal yield was highly variable between samples. T1-values of the investigated dental restoration materials ranged between 28 and 365 ms, whereas T2*-values ranged between 96 and 917 μs. In contrast, T1-values of dentine (T1 = 545 ms ± 299 ms) were higher, while T2*-values (T2* = 478 μs ± 271 μs) showed similar values.
Conclusions
Dental restoration materials and dentine of extracted human teeth can be visualized by UTE sequences and show a broad range of signal yield and relaxation times.
- A modified oddball paradigm for investigation of neural correlates of attention: a simultaneous ERP–fMRI study
- Abstract: Introduction
The objective of the presented study was to develop and evaluate a P300 experimental protocol for simultaneous registration of event-related potentials (ERPs) and functional MRI (fMRI) data with continuous imaging. It may be useful for investigating attention and working memory processes in specific populations, such as children and neuropsychiatric patients.
Materials and methods
Eleven children were investigated with simultaneous ERP–fMRI. To fulfill requirements of both BOLD and electroencephalographic signal registration, a modified oddball task was used. To verify the ERP–fMRI protocol we also performed a study outside the scanner using a typical two-stimuli oddball paradigm.
Results
Localization of the P300 component of ERPs partially corresponded with fMRI results in the frontal and parietal brain regions. FMRI activations were found in: middle frontal gyrus, insula, SMA, parietal lobule, thalamus, and cerebellum. Our modified oddball task provided ERP–fMRI results with high level of significance (EEG SNR = 35, fMRI p < 0.05–Bonf.). ERPs obtained in the scanner were comparable with those registered outside the scanner, although some differences in the amplitude were noticed, mainly in the N100 component.
Conclusion
In our opinion the presented paradigm may be successfully applied for simultaneous ERP–fMRI registration of neural correlates of attention in vulnerable populations.
PubDate: 2013-03-17
- Abstract: Introduction
The objective of the presented study was to develop and evaluate a P300 experimental protocol for simultaneous registration of event-related potentials (ERPs) and functional MRI (fMRI) data with continuous imaging. It may be useful for investigating attention and working memory processes in specific populations, such as children and neuropsychiatric patients.
Materials and methods
Eleven children were investigated with simultaneous ERP–fMRI. To fulfill requirements of both BOLD and electroencephalographic signal registration, a modified oddball task was used. To verify the ERP–fMRI protocol we also performed a study outside the scanner using a typical two-stimuli oddball paradigm.
Results
Localization of the P300 component of ERPs partially corresponded with fMRI results in the frontal and parietal brain regions. FMRI activations were found in: middle frontal gyrus, insula, SMA, parietal lobule, thalamus, and cerebellum. Our modified oddball task provided ERP–fMRI results with high level of significance (EEG SNR = 35, fMRI p < 0.05–Bonf.). ERPs obtained in the scanner were comparable with those registered outside the scanner, although some differences in the amplitude were noticed, mainly in the N100 component.
Conclusion
In our opinion the presented paradigm may be successfully applied for simultaneous ERP–fMRI registration of neural correlates of attention in vulnerable populations.
- Quantitative liver ADC measurements using diffusion-weighted MRI at 3 Tesla: evaluation of reproducibility and perfusion dependence using different techniques for respiratory compensation
- Abstract: Object
Diffusion weighted imaging (DWI) of the liver suffers from low signal to noise making 3 Tesla (3 T) an attractive option, but 3 T data is scarce. It was the aim to study the influence of different b values and respiratory compensation methods (RCM) on the apparent diffusion coefficient (ADC) level and on ADC reproducibility at 3 T.
Materials and methods
Ten healthy volunteers and 12 patients with malignant liver lesions underwent repeated (2–22 days) breathhold, free-breathing and respiratory triggered DWI at 3 T using b values between 0 and 1,000 s/mm2.
Results
The ADCs changed up to 150 % in healthy livers and up to 48 % in malignant lesions depending on b value combinations. Best ADC reproducibility in healthy livers were obtained with respiratory triggering (95 % limits of agreement: ±0.12) and free-breathing (±0.14). In malignant lesions equivalent reproducibility was obtained with less RCM dependence. The use of a lower maximum b value (b = 500) decreased reproducibility (±0.14 to ±0.32) in both normal liver and malignant lesions.
Conclusion
Large differences in absolute ADC values and reproducibility caused by varying combinations of clinically realistic b values were demonstrated. Different RCMs caused smaller differences. Lowering maximum b value to 500 increased limits of agreement up to a factor of two. Serial ADC changes larger than approximately 15 % can be detected confidently on an individual basis in both malignant lesions and normal liver parenchyma at 3 T using appropriate b values and respiratory compensation.
PubDate: 2013-03-13
- Abstract: Object
Diffusion weighted imaging (DWI) of the liver suffers from low signal to noise making 3 Tesla (3 T) an attractive option, but 3 T data is scarce. It was the aim to study the influence of different b values and respiratory compensation methods (RCM) on the apparent diffusion coefficient (ADC) level and on ADC reproducibility at 3 T.
Materials and methods
Ten healthy volunteers and 12 patients with malignant liver lesions underwent repeated (2–22 days) breathhold, free-breathing and respiratory triggered DWI at 3 T using b values between 0 and 1,000 s/mm2.
Results
The ADCs changed up to 150 % in healthy livers and up to 48 % in malignant lesions depending on b value combinations. Best ADC reproducibility in healthy livers were obtained with respiratory triggering (95 % limits of agreement: ±0.12) and free-breathing (±0.14). In malignant lesions equivalent reproducibility was obtained with less RCM dependence. The use of a lower maximum b value (b = 500) decreased reproducibility (±0.14 to ±0.32) in both normal liver and malignant lesions.
Conclusion
Large differences in absolute ADC values and reproducibility caused by varying combinations of clinically realistic b values were demonstrated. Different RCMs caused smaller differences. Lowering maximum b value to 500 increased limits of agreement up to a factor of two. Serial ADC changes larger than approximately 15 % can be detected confidently on an individual basis in both malignant lesions and normal liver parenchyma at 3 T using appropriate b values and respiratory compensation.
- Quantitative in vivo 23Na MR imaging of the healthy human kidney: determination of physiological ranges at 3.0T with comparison to DWI and BOLD
- Abstract: Objectives
The purpose of this prospective study was to assess the normal physiologic ranges of the renal corticomedullary 23Na-concentration ([23Na]) gradient at 3.0T in healthy volunteers. The corticomedullary [23Na] gradient was correlated with other functional MR imaging parameters—blood oxygenation level dependent (BOLD) and diffusion-weighted imaging (DWI)—and to individual and physiologic parameters—age, gender, estimated glomerular filtration rate (eGFR), body mass index (BMI), and blood serum sodium concentration ([23Na]serum).
Methods and materials
50 healthy volunteers (30 m, 20 w; mean age: 29.2 years) were included in this IRB-approved study, without a specific a priori preparation in regard to water or food intake. For 23Na-imaging a 3D density adapted, radial gradient echo (GRE)-sequence (spatial resolution = 5 × 5 × 5 mm3) was used in combination with a dedicated 23Na-coil and 23Na-reference phantoms. [23Na] values of the corticomedullary [23Na] gradient were measured by placement of a linear region of interest (20 × 1 mm2) from the renal cortex in the direction of the renal medulla. By using external standard reference phantoms, [23Na] was calculated in mmol/L of wet tissue volume (mmol/l WTV). Axial diffusion-weighted images (spatial resolution = 1.7 × 1.7 × 5.0 mm3) and 2D GRE BOLD images (spatial resolution = 1.2 × 1.2 × 4.0 mm3) were acquired. Mean values ± standard deviations for [23Na], apparent diffusion coefficient (ADC) values, and R2* values were computed for each volunteer. The corticomedullary 23Na-concentration gradient (in mmol/l/mm) was calculated along the area of linear concentration increase from the cortex in the direction of the medulla. Correlations between the [23Na] and DWI, BOLD, and the physiologic parameters were assessed with Pearson correlation coefficients.
Results
The mean corticomedullary [23Na] for all healthy volunteers increased from the renal cortex (58 ± 17 mmol/l WTV) in the direction of the medulla (99 ± 18 mmol/l WTV). The inter-individual differences ranged from respective cortical and medullary values of 27 and 63 mmol/L WTV to 126 and 187 mmol/L WTV. No statistically significant differences in renal [23Na] were found based on differences in individual or physiologic parameters (age, gender, [23Na]serum, BMI, GFR). No ADC or R2* gradients were identified, and [23Na] did not correlate with these parameters.
Conclusion
Renal corticomedullary [23Na] values increase from the cortex in the direction of the medullary pyramid, demonstrating wide inter-individual ranges and no significant correlations with age, gender, [23Na]serum, BMI, GFR, ADC, or R2* values. For future clinical evaluations, an approach relying on renal stimulation (e.g. pharmacologically induced diuresis) may be applicable to account for wide inter-individual ranges of normal [23Na].
PubDate: 2013-03-09
- Abstract: Objectives
The purpose of this prospective study was to assess the normal physiologic ranges of the renal corticomedullary 23Na-concentration ([23Na]) gradient at 3.0T in healthy volunteers. The corticomedullary [23Na] gradient was correlated with other functional MR imaging parameters—blood oxygenation level dependent (BOLD) and diffusion-weighted imaging (DWI)—and to individual and physiologic parameters—age, gender, estimated glomerular filtration rate (eGFR), body mass index (BMI), and blood serum sodium concentration ([23Na]serum).
Methods and materials
50 healthy volunteers (30 m, 20 w; mean age: 29.2 years) were included in this IRB-approved study, without a specific a priori preparation in regard to water or food intake. For 23Na-imaging a 3D density adapted, radial gradient echo (GRE)-sequence (spatial resolution = 5 × 5 × 5 mm3) was used in combination with a dedicated 23Na-coil and 23Na-reference phantoms. [23Na] values of the corticomedullary [23Na] gradient were measured by placement of a linear region of interest (20 × 1 mm2) from the renal cortex in the direction of the renal medulla. By using external standard reference phantoms, [23Na] was calculated in mmol/L of wet tissue volume (mmol/l WTV). Axial diffusion-weighted images (spatial resolution = 1.7 × 1.7 × 5.0 mm3) and 2D GRE BOLD images (spatial resolution = 1.2 × 1.2 × 4.0 mm3) were acquired. Mean values ± standard deviations for [23Na], apparent diffusion coefficient (ADC) values, and R2* values were computed for each volunteer. The corticomedullary 23Na-concentration gradient (in mmol/l/mm) was calculated along the area of linear concentration increase from the cortex in the direction of the medulla. Correlations between the [23Na] and DWI, BOLD, and the physiologic parameters were assessed with Pearson correlation coefficients.
Results
The mean corticomedullary [23Na] for all healthy volunteers increased from the renal cortex (58 ± 17 mmol/l WTV) in the direction of the medulla (99 ± 18 mmol/l WTV). The inter-individual differences ranged from respective cortical and medullary values of 27 and 63 mmol/L WTV to 126 and 187 mmol/L WTV. No statistically significant differences in renal [23Na] were found based on differences in individual or physiologic parameters (age, gender, [23Na]serum, BMI, GFR). No ADC or R2* gradients were identified, and [23Na] did not correlate with these parameters.
Conclusion
Renal corticomedullary [23Na] values increase from the cortex in the direction of the medullary pyramid, demonstrating wide inter-individual ranges and no significant correlations with age, gender, [23Na]serum, BMI, GFR, ADC, or R2* values. For future clinical evaluations, an approach relying on renal stimulation (e.g. pharmacologically induced diuresis) may be applicable to account for wide inter-individual ranges of normal [23Na].
- The role of tissue microstructure and water exchange in biophysical modelling of diffusion in white matter
- Abstract: Abstract
Biophysical models that describe the outcome of white matter diffusion MRI experiments have various degrees of complexity. While the simplest models assume equal-sized and parallel axons, more elaborate ones may include distributions of axon diameters and axonal orientation dispersions. These microstructural features can be inferred from diffusion-weighted signal attenuation curves by solving an inverse problem, validated in several Monte Carlo simulation studies. Model development has been paralleled by microscopy studies of the microstructure of excised and fixed nerves, confirming that axon diameter estimates from diffusion measurements agree with those from microscopy. However, results obtained in vivo are less conclusive. For example, the amount of slowly diffusing water is lower than expected, and the diffusion-encoded signal is apparently insensitive to diffusion time variations, contrary to what may be expected. Recent understandings of the resolution limit in diffusion MRI, the rate of water exchange, and the presence of microscopic axonal undulation and axonal orientation dispersions may, however, explain such apparent contradictions. Knowledge of the effects of biophysical mechanisms on water diffusion in tissue can be used to predict the outcome of diffusion tensor imaging (DTI) and of diffusion kurtosis imaging (DKI) studies. Alterations of DTI or DKI parameters found in studies of pathologies such as ischemic stroke can thus be compared with those predicted by modelling. Observations in agreement with the predictions strengthen the credibility of biophysical models; those in disagreement could provide clues of how to improve them. DKI is particularly suited for this purpose; it is performed using higher b-values than DTI, and thus carries more information about the tissue microstructure. The purpose of this review is to provide an update on the current understanding of how various properties of the tissue microstructure and the rate of water exchange between microenvironments are reflected in diffusion MRI measurements. We focus on the use of biophysical models for extracting tissue-specific parameters from data obtained with single PGSE sequences on clinical MRI scanners, but results obtained with animal MRI scanners are also considered. While modelling of white matter is the central theme, experiments on model systems that highlight important aspects of the biophysical models are also reviewed.
PubDate: 2013-02-27
- Abstract: Abstract
Biophysical models that describe the outcome of white matter diffusion MRI experiments have various degrees of complexity. While the simplest models assume equal-sized and parallel axons, more elaborate ones may include distributions of axon diameters and axonal orientation dispersions. These microstructural features can be inferred from diffusion-weighted signal attenuation curves by solving an inverse problem, validated in several Monte Carlo simulation studies. Model development has been paralleled by microscopy studies of the microstructure of excised and fixed nerves, confirming that axon diameter estimates from diffusion measurements agree with those from microscopy. However, results obtained in vivo are less conclusive. For example, the amount of slowly diffusing water is lower than expected, and the diffusion-encoded signal is apparently insensitive to diffusion time variations, contrary to what may be expected. Recent understandings of the resolution limit in diffusion MRI, the rate of water exchange, and the presence of microscopic axonal undulation and axonal orientation dispersions may, however, explain such apparent contradictions. Knowledge of the effects of biophysical mechanisms on water diffusion in tissue can be used to predict the outcome of diffusion tensor imaging (DTI) and of diffusion kurtosis imaging (DKI) studies. Alterations of DTI or DKI parameters found in studies of pathologies such as ischemic stroke can thus be compared with those predicted by modelling. Observations in agreement with the predictions strengthen the credibility of biophysical models; those in disagreement could provide clues of how to improve them. DKI is particularly suited for this purpose; it is performed using higher b-values than DTI, and thus carries more information about the tissue microstructure. The purpose of this review is to provide an update on the current understanding of how various properties of the tissue microstructure and the rate of water exchange between microenvironments are reflected in diffusion MRI measurements. We focus on the use of biophysical models for extracting tissue-specific parameters from data obtained with single PGSE sequences on clinical MRI scanners, but results obtained with animal MRI scanners are also considered. While modelling of white matter is the central theme, experiments on model systems that highlight important aspects of the biophysical models are also reviewed.
- Iterative separation of transmit and receive phase contributions and B
1
+
-based estimation of the specific absorption rate for transmit arrays
- Abstract: Object
The specific absorption rate (SAR) can be determined from radiofrequency transmit fields measured via magnetic resonance imaging.
Materials and methods
The proposed method estimates the SAR solely from the complex transmit field (B
1
+
) by taking into account the particular properties of the electromagnetic field generated by an 8-channel transmit array. It is further based on an iterative consistency check between the measured B
1
+
magnitude and an appropriate field estimate fulfilling Maxwell’s equations. For testing the method, simulations and phantom experiments were performed for a multi-transmit array at 3T using a cylindrical phantom.
Results
The method’s robustness with respect to the assumptions made about electric tissue properties as well as its stability under different initial conditions regarding the signal phase was shown. A high sensitivity to signal noise was found. Robust reconstruction results were achieved including information from more than two transmit elements. The validity of the experimental results was confirmed by a qualitative comparison to simulated electromagnetic fields.
Conclusions
The method allows the determination of the SAR as well as the transmit phase of the individual channels of a multi-transmit array. With additional B0 inhomogeneity measurements, a reconstruction of the receive phase is feasible independent of the receive coil type in use.
PubDate: 2013-02-27
- Abstract: Object
The specific absorption rate (SAR) can be determined from radiofrequency transmit fields measured via magnetic resonance imaging.
Materials and methods
The proposed method estimates the SAR solely from the complex transmit field (B
1
+
) by taking into account the particular properties of the electromagnetic field generated by an 8-channel transmit array. It is further based on an iterative consistency check between the measured B
1
+
magnitude and an appropriate field estimate fulfilling Maxwell’s equations. For testing the method, simulations and phantom experiments were performed for a multi-transmit array at 3T using a cylindrical phantom.
Results
The method’s robustness with respect to the assumptions made about electric tissue properties as well as its stability under different initial conditions regarding the signal phase was shown. A high sensitivity to signal noise was found. Robust reconstruction results were achieved including information from more than two transmit elements. The validity of the experimental results was confirmed by a qualitative comparison to simulated electromagnetic fields.
Conclusions
The method allows the determination of the SAR as well as the transmit phase of the individual channels of a multi-transmit array. With additional B0 inhomogeneity measurements, a reconstruction of the receive phase is feasible independent of the receive coil type in use.
- Alleviating artifacts in 1H MRI thermometry by single scan spatiotemporal encoding
- Abstract: Objective
Recent years have seen an increased interest in combining MRI thermometry with devices capable of destroying malignancies by heat ablation. Expected from the MR protocols are accurate and fast thermal characterizations, providing real time feedback on restricted tissue volumes and/or rapidly moving organs like liver. This article explores the potential advantages of relying on spatiotemporally encoded (SPEN) sequences for retrieving real-time thermometric images based on the water’s proton resonance frequency (PRF) shifts.
Materials and methods
Hybrid spatiotemporal/k-space encoding single-scan MRI experiments were implemented on animal and human scanners, and their abilities to deliver single- and multi-slice real-time thermometric measurements based on PRF-derived phase maps in phantoms and in vivo, were compared against echo planar imaging (EPI) and gradient-echo counterparts.
Results
Under comparable acquisition conditions, SPEN exhibited advantages vis-à-vis EPI in terms of dealing with inhomogeneous magnetic field distortions, with shifts arising due to changes in the central frequency offsets, with PRF distributions, and for zooming into restricted fields-of-view without special pulse sequence provisions.
Conclusion
This work confirms the ability of SPEN sequences, particularly when implemented under fully-refocused conditions, to exploit their built-in robustness to shift- and field-derived inhomogeneities for monitoring thermal changes in real-time under in vitro and in vivo conditions.
PubDate: 2013-02-27
- Abstract: Objective
Recent years have seen an increased interest in combining MRI thermometry with devices capable of destroying malignancies by heat ablation. Expected from the MR protocols are accurate and fast thermal characterizations, providing real time feedback on restricted tissue volumes and/or rapidly moving organs like liver. This article explores the potential advantages of relying on spatiotemporally encoded (SPEN) sequences for retrieving real-time thermometric images based on the water’s proton resonance frequency (PRF) shifts.
Materials and methods
Hybrid spatiotemporal/k-space encoding single-scan MRI experiments were implemented on animal and human scanners, and their abilities to deliver single- and multi-slice real-time thermometric measurements based on PRF-derived phase maps in phantoms and in vivo, were compared against echo planar imaging (EPI) and gradient-echo counterparts.
Results
Under comparable acquisition conditions, SPEN exhibited advantages vis-à-vis EPI in terms of dealing with inhomogeneous magnetic field distortions, with shifts arising due to changes in the central frequency offsets, with PRF distributions, and for zooming into restricted fields-of-view without special pulse sequence provisions.
Conclusion
This work confirms the ability of SPEN sequences, particularly when implemented under fully-refocused conditions, to exploit their built-in robustness to shift- and field-derived inhomogeneities for monitoring thermal changes in real-time under in vitro and in vivo conditions.
- A battery-driven, low-field NMR unit for thermally and hyperpolarized samples
- Abstract: Object
The design of a multinuclear low-field NMR unit with variable field strength <6 mT providing accurate spin manipulations and sufficient sensitivity for direct detection of samples in thermal equilibrium to aid parahydrogen-based hyperpolarization experiments.
Materials and methods
An optimized, resistive magnet connected to a battery or wall-power driven current source was constructed to provide a magnetic field <6 mT. A digital device connected to a saddle-shaped transmit- and solenoid receive-coil enabled MR signal excitation and detection with up to 106 samples/s, controlled by a flexible pulse-programming software.
Results
The magnetization of thermally polarized samples at 1.8 and 5.7 mT is detected in a single acquisition with a SNR ≈101 and ≈102 and a line width of 42 and 32 Hz, respectively. Nuclear spins are manipulated to an uncertainty of ±1° by means of pulses, which can be arranged in an arbitrary combination. As a demonstration, standard experiments for the measurement of relaxation parameters of thermally polarized samples were implemented. The detection of much stronger hyperpolarized signal was exemplified employing parahydrogen.
Conclusion
Direct detection of thermal and hyperpolarized 1H-MR signal in a single acquisition and accurate spin manipulations at 1.8 and 5.5 mT were successfully demonstrated.
PubDate: 2013-02-15
- Abstract: Object
The design of a multinuclear low-field NMR unit with variable field strength <6 mT providing accurate spin manipulations and sufficient sensitivity for direct detection of samples in thermal equilibrium to aid parahydrogen-based hyperpolarization experiments.
Materials and methods
An optimized, resistive magnet connected to a battery or wall-power driven current source was constructed to provide a magnetic field <6 mT. A digital device connected to a saddle-shaped transmit- and solenoid receive-coil enabled MR signal excitation and detection with up to 106 samples/s, controlled by a flexible pulse-programming software.
Results
The magnetization of thermally polarized samples at 1.8 and 5.7 mT is detected in a single acquisition with a SNR ≈101 and ≈102 and a line width of 42 and 32 Hz, respectively. Nuclear spins are manipulated to an uncertainty of ±1° by means of pulses, which can be arranged in an arbitrary combination. As a demonstration, standard experiments for the measurement of relaxation parameters of thermally polarized samples were implemented. The detection of much stronger hyperpolarized signal was exemplified employing parahydrogen.
Conclusion
Direct detection of thermal and hyperpolarized 1H-MR signal in a single acquisition and accurate spin manipulations at 1.8 and 5.5 mT were successfully demonstrated.
