Detrending phase drift: a preprocessing step to improve the effectiveness of the UNFOLD technique

Purpose

To improve the effectiveness of the UNFOLD technique in removing aliasing artifacts by detrending temporal phase drift in each voxel before the implementation of the UNFOLD technique. This is because linear and quadratic phase trends in temporal signal can shift and broaden aliasing peaks in the spectrum and cause the removal of aliasing artifacts by spectral filtering to be insufficient or even fail when minimal filtering of the spectrum is used.

Materials and Methods

A functional magnetic resonance imaging (fMRI) study with a visual stimulus was performed on normal subjects to test the hypothesis. A 2D spiral‐in/out sequence was used to acquire k‐space data. Undersampled k‐space trajectories were used to improve temporal resolution. Aliasing artifacts were removed by the UNFOLD technique. For comparison, two image sets, with and without phase trends removal, were obtained from each set of functional data.

Results

After detrending temporal phase drift, residual aliasing artifacts that were not suppressed by the standard UNFOLD technique could be successfully removed. Better image quality, temporal stability, and activation maps could be achieved by the proposed method.

Conclusion

The proposed method can improve the effectiveness of the UNFOLD technique in removing aliasing artifacts when spectral filtering is kept minimal to preserve temporal resolution. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.     

MR venography of the brain with enhanced vessel contrast using image-domain high-pass filtering of the susceptibility phase shift

Purpose:

To develop a postprocessing algorithm that enhances the visibility of intracranial venous vasculature and reduces the artifacts in the display of susceptibility‐weighted images (SWI).

Materials and Methods:

Image‐domain high‐pass filters based on second‐order phase difference were applied to the complex 3D SWI data to enhance the susceptibility phase shift of the veins and suppress background signal in SWI. A multivariant statistical parameter was used to suppress the noise in air.

Results:

Magnetic resonance (MR) venography with enhanced susceptibility phase shift and reduced off‐resonance artifacts was obtained using the proposed filters. The background signal in the 3D MR venography data was well suppressed. Venous vasculature in the peripheral regions of the brain was well depicted and the adverse effect of noise in air in the maximum‐intensity projection display of the 3D SWI data was well suppressed.

Conclusion:

Image‐domain high‐pass filtering with second‐order phase difference provides an alternative display of 3D SWI data with enhanced visibility of the venous vasculature and effective suppression of artifacts. J. Magn. Reson. Imaging 2011;. © 2011 Wiley Periodicals, Inc.     

Combined magnitude and phase-based segmentation of the cerebral cortex in 7T MR images of the elderly

Purpose:

To propose a new method that integrates both magnitude and phase information obtained from magnetic resonance (MR) T*₂‐weighted scans for cerebral cortex segmentation of the elderly.

Materials and Methods:

This method makes use of K‐means clustering on magnitude and phase images to compute an initial segmentation, which is further refined by means of transformation with reconstruction criteria. The method was evaluated against the manual segmentation of 7T in vivo MR data of 20 elderly subjects (age = 67.7 ± 10.9). The added value of combining magnitude and phase was also evaluated by comparing the performance of the proposed method with the results obtained when limiting the available data to either magnitude or phase.

Results:

The proposed method shows good overlap agreement, as quantified by the Dice Index (0.79 ± 0.04), limited bias (average relative volume difference = 2.94%), and reasonable volumetric correlation (R = 0.555, p = 0.011). Using the combined magnitude and phase information significantly improves the segmentation accuracy compared with using either magnitude or phase.

Conclusion:

This study suggests that the proposed method is an accurate and robust approach for cerebral cortex segmentation in datasets presenting low gray/white matter contrast. J. Magn. Reson. Imaging 2012;36:99–109. © 2012 Wiley Periodicals Inc.     

Minimizing the acquisition phase in coronary CT angiography using the second generation 320-row CT

Purpose

We aimed to compare the radiation dose and image quality of a minimal phase window centered at 77 % compared with a wide phase window in coronary CT angiography using the second-generation 320-row CT.

Materials and methods

Eighty patients with heart rate ≤75 bpm were retrospectively included. The first 40 patients underwent scanning with a wide phase window (65–85 %), while the last 40 patients underwent scanning with a minimal phase window centered at 77 %. Subjective image quality was graded using a 4-point scale (4 = excellent). Image noise and contrast-to-noise ratio at the proximal segments were also analyzed. The mean effective dose was derived from the dose length product multiplied by a chest conversion coefficient (κ = 0.014 mSv mGy^?1 cm^?1).

Results

Minimal phase window scanning centered at 77 % reduced the radiation dose by 30 % compared with wide phase window scanning (1.7 vs 2.4 mSv, p = 0.0009). The subjective image quality showed no significant difference (3.75 vs 3.76, p = 0.77). No significant difference was observed in the image noise, CT number, and contrast-to-noise ratio.

Conclusions

Radiation dose could be reduced while maintaining image quality by use of a minimal phase window centered at 77 % compared with a wide phase window in coronary CT angiography using the second generation 320-row CT.     

Automatic quantification of subcutaneous and visceral adipose tissue from whole‐body magnetic resonance images suitable for large cohort studies

Purpose:

To develop an automated method with which to distinguish metabolically different adipose tissues in a large number of subjects using whole‐body magnetic resonance imaging (MRI) datasets for improving the understanding of chronic disease risk predictions associated with distinct adipose tissue compartments.

Materials and Methods:

In all, 314 participants were scanned using a 1.5T MRI‐scanner with a 2‐point Dixon whole‐body sequence. Image segmentation was automated using standard image processing techniques and knowledge‐based methods. Abdominal adipose tissue was separated into subcutaneous adipose tissue (SAT) and visceral adipose tissue (VAT) by statistical shape models. Bone marrow was removed to provide a more accurate measurement of adipose tissue. To assess segmentation accuracy, ground‐truth segmentations in 52 images were performed manually by one operator. Due to the high effort of manual delineation, manual segmentation was limited to seven slices per volume.

Results:

Volumetric differences were 3.30 ± 2.97% and 6.22 ± 5.28% for SAT and VAT, respectively. The systematic error shows an overestimation of 4.22 ± 7.01% for VAT and 0.37 ± 4.45% for SAT. Coefficients‐of‐variation from repeated measurements were: 3.50 ± 2.93% for VAT and 0.35 ± 0.26% for SAT. The approach of removing bone marrow worked well in most body regions. Only occasionally the method failed for knees and/or shinbone, which resulted in an overestimation of SAT by 3.14 ± 1.45%.

Conclusion:

We developed a fully automatic process to assess SAT and VAT in whole‐body MRI data. The method can support epidemiological studies investigating the relationship between excess body fat and chronic diseases. J. Magn. Reson. Imaging 2012; 36:1421–1434. © 2012 Wiley Periodicals, Inc.     

Assessment of the liver strain among cirrhotic and normal livers using tagged MRI

Purpose:

To use magnetization tagged magnetic resonance imaging (MRI) (tag‐MRI) to quantify cardiac induced liver strain and compare strain of cirrhotic and normal livers.

Materials and Methods:

Tag‐MRI was performed at 1.5T on eight subjects with no history of liver disease and 10 patients with liver cirrhosis. A breath‐hold peripheral pulse‐gated (PPG) conventional tag‐MRI cine sequence was performed with planes to include the left lobe of the liver and the inferior wall of the heart. Commercially available software HARP (Diagnosoft, Palo Alto, CA) was used for image analysis and strain calculation. Three regions‐of‐interest (ROIs) were selected: segment II of the liver near the heart (A), right liver lobe far from the heart (B), and the left ventricular wall (C). The average and maximal (max) strain were measured in A, B, and C. The maximum strains were used to generate a cardiac‐corrected strain gradient: (maxA‐maxB)/maxC. Results were compared with Student's t‐test (SPSS, Chicago, IL).

Results:

In subjects with no history of liver disease vs. cirrhotic patients, the average strain was 22% ± 7% vs. 4% ± 3% (P < 0.001), the max strain was 63% ± 15% vs. 17% ± 5% (P < 0.001), and the corrected strain gradient was 0.52 ± 0.16 vs. 0.11% ± 0.08%.

Conclusion:

There is a significant difference in liver strain measured with tag‐MRI between subjects with no history of liver disease and patients with cirrhosis. J. Magn. Reson. Imaging 2012; 36:1490–1495. © 2012 Wiley Periodicals, Inc.     

MR angiography of the cerebral perforating arteries with magnetization prepared anatomical reference at 7T: Comparison with time‐of‐flight

Purpose

To develop a magnetic resonance imaging (MRI) protocol that visualizes both the perforating arteries and the related anatomy in a single acquisition at 7T.

Material and Methods

T₁‐weighted magnetization prepared imaging (MPRAGE) was empirically modified for use as angiography method at 7T. The resulting sequence depicts the vasculature simultaneously with the surrounding anatomical structures, and is referred to as “magnetization prepared anatomical reference MRA” (MPARE‐MRA). The method was compared to time‐of‐flight (TOF) MRA in seven healthy subjects. The conspicuity of the perforating arteries and the contrast between gray and white matter were evaluated both quantitatively by contrast‐to‐noise (CNR) measurements, and qualitatively by two radiologists who scored the images.

Results

The contrast‐to‐noise ratio (CNR) between blood and background was 28 ± 9 for MPARE‐MRA and 35 ± 16 for TOF‐MRA, indicating good conspicuity of the vessels. CNR values were: internal capsule (IC) vs. caudate head (CH): 4.2 ± 0.7; IC vs. putamen: 3.5 ± 0.6; white matter vs. gray matter: 9.7 ± 2.5.

Conclusion

The benefits of ultra‐high‐field MRI can transform MPRAGE into a new angiography method to image small vessels and associated parenchyma at the same time. This technique can be used to study the correlation between tissue damage and vascular pathology. J. Magn. Reson. Imaging 2008;28:1519–1526. © 2008 Wiley‐Liss, Inc.     

Detecting lesions in multiple sclerosis at 4.7 tesla using phase susceptibility‐weighting and T2‐weighting

Purpose

To demonstrate 4.7 Tesla (T) imaging methods for visualizing lesions in multiple sclerosis in the human brain using phase susceptibility‐weighting and T2 weighting.

Materials and Methods

Seven patients with relapsing‐remitting multiple sclerosis were imaged at 4.7T using three‐dimensional (3D) susceptibility‐weighted imaging (SWI) with 0.90 mm³ voxel volumes, and with 2D T2‐weighted fast spin echo (T2WFSE) with 0.34 mm³ voxels and 1.84 mm³ voxels. The visibility of MS lesions at 4.7T with phase SWI and T2WFSE was assessed by independent lesion counts made by an experienced neuroradiologist, and by quantitative measures.

Results

High resolution T2WFSE at 4.7T provided excellent depiction of hyperintense lesions. When combined with phase SWI, 124 total lesions were identified of which 18% were only visible on phase SWI and not on T2WFSE. The phase lesions had a mean phase shift relative to local background of ?11.15 ± 5.97 parts per billion.

Conclusion

Imaging at 4.7T can provide both high quality, high resolution T2WFSE and SWI for visualization of lesions in multiple sclerosis. Phase susceptibility‐weighting can identify additional lesions that are not visible with high resolution T2WFSE. J. Magn. Reson. Imaging 2009;30:737–742. © 2009 Wiley‐Liss, Inc.

     

Iron deposition in multiple sclerosis lesions measured by susceptibility-weighted imaging filtered phase: a case control study

Purpose:

To investigate phase lesions identified on susceptibility‐weighted imaging (SWI)‐filtered phase images in patients with multiple sclerosis (MS), clinically isolated syndrome (CIS) and healthy controls (HC). To relate phase lesion characteristics to other clinical and MRI outcomes.

Materials and Methods:

95 relapsing‐remitting (RR), 40 secondary‐progressive (SP) MS patients, as well as 19 CIS patients and 49 age‐ and sex‐matched HC, were scanned on a 3T scanner. Phase‐, T1‐, and T2‐lesion characteristics were determined. Overlap of T1‐ and T2‐weigthed imaging (WI) lesions with phase lesions (T1P and T2P), as well as brain atrophy outcomes, was assessed.

Results:

MS patients showed significantly greater numbers and larger volume of phase lesions, compared with HC (P < 0.001). 23.6% of T2 lesions overlapped with phase lesions, whereas the same figure for T1 lesions was 37.3%. Conversely, 33.4% and 69.7% of phase lesions were not visible on T2‐ or T1‐WI, respectively. Phase, T1P and T2P lesions were not related to clinical outcomes, but phase lesions were related to ventricular enlargement.

Conclusion:

Phase lesions were present in both MS and CIS patients, and showed partial overlap with lesions observed using conventional MRI. The role of phase lesions in clinical progression remains unclear and should be further explored. J. Magn. Reson. Imaging 2012;36:73–83. © 2012 Wiley Periodicals, Inc.     

Comparison of two different injection rates of gadoxetic acid for arterial phase MRI of the liver

Purpose:

To compare two different injection rates for gadoxetic acid‐enhanced hepatic arterial phase images on hepatic dynamic MRI.

Materials and Methods:

Hepatic arterial phase images were obtained after an intravenous bolus injection of gadoxetic acid at a rate of 1 mL/second in 62 patients and 2 mL/second in 64 patients on a 3 Tesla MR scanner using a test‐bolus injection method. The signal‐to‐noise ratios (SNR) of the liver, portal vein, hepatic vein, aorta, spleen and pancreas were measured. The contrast‐to‐noise ratio (CNR) of hypervascular hepatic tumors was calculated. Two radiologists independently scored items to evaluate image quality of hepatic arterial phase and detected hypervascular hepatocellular carcinoma (HCC).

Results:

The SNR of the aorta on the arterial phase images was significantly higher in the 1 mL/second group (235.43 ± 82.59) than in the 2 mL/second group (190.94 ± 96.90, P < 0.05). The SNRs of the liver, spleen and pancreas, the CNRs of hypervascular hepatic tumors, the detection rate of hypervascular HCC and subjective ratings for the optimal arterial enhancement were comparable between the two groups.

Conclusion:

Injection rates of 2 mL/second and 1 mL/second provided comparable image qualities on arterial phase images of hepatic dynamic MRI using gadoxetic acid. J. Magn. Reson. Imaging 2010; 31: 365–372. © 2010 Wiley‐Liss, Inc.     

Comparison of pulsed and pseudocontinuous arterial spin-labeling for measuring CO2 -induced cerebrovascular reactivity

Purpose:

To compare the performance of pulsed and pseudocontinuous arterial spin‐labeling (PASL and pCASL) methods in measuring CO₂‐induced cerebrovascular reactivity (CVR).

Materials and Methods:

Subjects were scanned using both ASL sequences during a controlled hypercapnia procedure and visual stimulation. CVR was computed as the percent CO₂‐induced increase in cerebral blood flow (Δ%CBF) per mmHg increase in end‐tidal PCO₂. Visually evoked responses were expressed as Δ%CBF. Resting CBF and temporal signal‐to‐noise ratio were also computed. Regionally averaged values for the different quantities were compared in gray matter (GM) and visual cortex (VC) using t‐tests.

Results:

Both PASL and pCASL yielded comparable respective values for resting CBF (56 ± 3 and 56 ± 4 mL/min/100g) and visually evoked responses (75 ± 5% and 81 ± 4%). Values of CVR determined using pCASL (GM 4.4 ± 0.2, VC 8 ± 1 Δ%CBF/mmHg), however, were significantly higher than those measured using PASL (GM 3.0 ± 0.6, VC 5 ± 1 Δ%CBF/mmHg) in both GM and VC. The percentage of GM voxels in which statistically significant hypercapnia responses were detected was also higher for pCASL (27 ± 5% vs. 16 ± 3% for PASL).

Conclusion:

pCASL may be less prone to underestimation of CO₂‐induced flow changes due to improved label timing control. J. Magn. Reson. Imaging 2012;36:312–321. © 2012 Wiley Periodicals, Inc.     

Optimized inversion-prepared gradient echo imaging

Purpose:

To implement a method using an extended phase graph (EPG)‐based simulation to optimize inversion‐prepared gradient echo sequences with respect to signal and contrast within the shortest acquisition time.

Materials and Methods:

A critical issue in rapid gradient‐echo imaging is the effect of residual transverse magnetization between consecutive data acquisition windows. Various spoiling schemes have been proposed to mitigate this problem, and while spoiling is often considered to be perfect, imaging can be more truthfully described using the EPG. An EPG‐based simulation is used to analyze and predict the image signal and contrast to serve as a basis for sequence optimization.

Results:

Fourteen biological phantom experiments and five brain imaging experiments on each of five healthy volunteers was performed to validate and verify the accuracy of the EPG‐based simulation. In addition, two experiments on an in‐cranial cadaver brain were performed to show the ability of the proposed method for improving overall image quality.

Conclusion:

From the experiment results, it is demonstrated that optimization of 3D magnetization‐prepared rapid gradient‐echo imaging sequences can be performed with an EPG‐based simulation to manipulate the sequence parameters for generating images with highly specific signal and contrast characteristics for quantitative T1‐weighted human brain imaging. J. Magn. Reson. Imaging 2012;36:748–755. © 2012 Wiley Periodicals, Inc.     

Dual‐echo arteriovenography imaging with 7T MRI

Purpose:

To implement a dual‐echo sequence MRI technique at 7T for simultaneous acquisition of time‐of‐flight (TOF) MR angiogram (MRA) and blood oxygenation level‐dependent (BOLD) MR venogram (MRV) in a single MR acquisition and to compare the image qualities with those acquired at 3T.

Materials and Methods:

We implemented a dual‐echo sequence with an echo‐specific k‐space reordering scheme to uncouple the scan parameter requirements for MRA and MRV at 7T. The MRA and MRV vascular contrast was enhanced by maximally separating the k‐space center regions acquired for the MRA and MRV and by adjusting and applying scan parameters compatible between the MRA and MRV. The same imaging sequence was implemented at 3T. Four normal subjects were imaged at both 3T and 7T. MRA and MRV at 7T were reconstructed both with and without phase‐mask filtering and were compared quantitatively and qualitatively with those at 3T with phase‐mask filtering.

Results:

The depiction of small cortical arteries and veins on MRA and MRV at 7T was substantially better than that at 3T, due to about twice higher contrast‐to‐noise ratio (CNR) for both arteries (164 ±57 vs. 77 ± 26) and veins (72 ± 8 vs. 36 ± 6). Even without use of the phase‐masking filtering, the venous contrast at 7T (65 ± 7) was higher than that with the filtering at 3T (36 ± 6).

Conclusion:

The dual‐echo arteriovenography technique we implemented at 7T allows the improved visualization of small vessels in both the MRA and MRV because of the greatly increased signal‐to‐noise ratio (SNR) and susceptibility contrast, compared to 3T. J. Magn. Reson. Imaging 2010;31:255–261. © 2009 Wiley‐Liss, Inc.     

Automated segmentation of the aorta from phase contrast MR images: Validation against expert tracing in healthy volunteers and in patients with a dilated aorta

Purpose:

To assess if segmentation of the aorta can be accurately achieved using the modulus image of phase contrast (PC) magnetic resonance (MR) acquisitions.

Materials and Methods:

PC image sequences containing both the ascending and descending aorta of 52 subjects were acquired using three different MR scanners. An automated segmentation technique, based on a 2D+t deformable surface that takes into account the features of PC aortic images, such as flow‐related effects, was developed. The study was designed to: 1) assess the variability of our approach and its robustness to the type of MR scanner, and 2) determine its sensitivity to aortic dilation and its accuracy against an expert manual tracing.

Results:

Interobserver variability in the lumen area was 0.59 ± 0.92% for the automated approach versus 10.09 ± 8.29% for manual segmentation. The mean Dice overlap measure was 0.945 ± 0.014. The method was robust to the aortic size and highly correlated (r = 0.99) with the manual tracing in terms of aortic area and diameter.

Conclusion:

A fast and robust automated segmentation of the aortic lumen was developed and successfully tested on images provided by various MR scanners and acquired on healthy volunteers as well as on patients with a dilated aorta. J. Magn. Reson. Imaging 2010;31:881–888. ©2010 Wiley‐Liss, Inc.     

Diffusion-weighted whole-body MRI with background body signal suppression: technical improvements at 3.0 T

Purpose:

To improve image quality of diffusion‐weighted body magnetic resonance imaging (MRI) with background body signal suppression (DWIBS) at 3.0 T.

Materials and Methods:

In 30 patients and eight volunteers, a diffusion‐weighted spin‐echo echo‐planar imaging sequence with short TI inversion recovery (STIR) fat suppression was applied and repeated using slice‐selective gradient reversal (SSGR) and/or dual‐source parallel radiofrequency (RF) transmission (TX). The quality of diffusion‐weighted images and gray scale inverted maximum intensity projections (MIP) were visually assessed by intraindividual comparison with respect to the level of fat suppression and signal homogeneity. Moreover, the contrast between lesions/lymph nodes and background (C_lb) was analyzed in the MIP reconstructions.

Results:

By combining STIR with SSGR, fat suppression was significantly improved (P < 0.001) and C_lb was increased two times. The use of TX allowed the reduction of acquisition time and improved image quality with regard to signal homogeneity (P < 0.001) and fat suppression (P = 0.005).

Conclusion:

DWIBS at 3.0 T can be improved by using SSGR and TX. J. Magn. Reson. Imaging 2012;456‐461. © 2011 Wiley Periodicals, Inc.     

Novel technique for MR elastography of the prostate using a modified standard endorectal coil as actuator

Purpose:

To present a novel method for MR elastography (MRE) of the prostate at 3 Tesla using a modified endorectal imaging coil.

Materials and Methods:

A commercial endorectal coil was modified to dynamically generate mechanical stress (contraction and dilation) in a prostate phantom with embedded phantom “lesions” (6 mm diameter) and in a porcine model. Resulting tissue displacements were measured with a motion‐sensitive EPI sequence at actuation frequencies of 50–200 Hz. Maps of shear modulus G were calculated from the measured phase‐difference shear‐wave patterns.

Results:

In the G maps of the phantom, “lesions” were easily discernible against the background. The average G values of regions of interest placed in the “lesion” (8.2 ± 1.9 kPa) were much higher than those in the background (3.6 ± 1.4 kPa) but systematically lower than values reported by the vendor (13.0 ± 1.0 and 6.7 ± 0.7 kPa, respectively). In the porcine model, shear waves could be generated and measured shear moduli were substantially different for muscle (7.1 ± 2.0 kPa), prostate (3.0 ± 1.4 kPa), and bulbourethral gland (5.6 ± 1.9 kPa).

Conclusion:

An endorectal MRE concept is technically feasible. The presented technique will allow for simultaneous MRE and MRI acquisitions using a commercial base device with minor, MR‐conditional modifications. The diagnostic value needs to be determined in further trials. J. Magn. Reson. Imaging 2013;37:1480–1485. © 2012 Wiley Periodicals, Inc.     

Free-breathing imaging of the heart using 2D cine-GRICS (generalized reconstruction by inversion of coupled systems) with assessment of ventricular volumes and function

Purpose:

To assess cardiac function by means of a novel free‐breathing cardiac magnetic resonance imaging (MRI) strategy.

Materials and Methods:

A stack of ungated 2D steady‐state free precession (SSFP) slices was acquired during free breathing and reconstructed as cardiac cine imaging based on the generalized reconstruction by inversion of coupled systems (GRICS). A motion‐compensated sliding window approach allows reconstructing cine movies with most motion artifacts cancelled. The proposed reconstruction uses prior knowledge from respiratory belts and electrocardiogram recordings and features a piecewise linear model that relates the electrocardiogram signal to cardiac displacements. The free‐breathing protocol was validated in six subjects against a standard breath‐held protocol.

Results:

Image sharpness, as assessed by the image gradient entropy, was comparable to that of breath‐held images and significantly better than in uncorrected images. Volumetric parameters of cardiac function in the left ventricle (LV) and right ventricle (RV) were similar, including end‐systolic volumes, end‐diastolic volumes and mass, stroke volumes, and ejection fractions (with differences of 3% ± 2.4 in the LV and 2.9% ± 4.4 in the RV). The duration of the free‐breathing protocol was nearly the same as the breath‐held protocol.

Conclusion:

Free‐breathing cine‐GRICS enables accurate assessment of volumetric parameters of cardiac function with efficient correction of motion. J. Magn. Reson. Imaging 2012;340‐351. © 2011 Wiley Periodicals, Inc.     

Evaluation of referenceless thermometry in MRI-guided focused ultrasound surgery of uterine fibroids

Purpose

To clinically assess a previously described method (Rieke et.al., Magn Reson Med 2004) to produce more motion‐robust MRI‐based temperature images using data acquired during MRI‐guided focused ultrasound surgery (MRgFUS) of uterine fibroids.

Materials and Methods

The method (“referenceless thermometry”) uses surface fitting in nonheated regions of individual phase images to extrapolate and then remove background phase variations that are unrelated to temperature changes. We tested this method using images from 100 sonications selected from 33 patient MRgFUS treatments. Temperature measurements and thermal dose contours estimated with the referenceless method were compared with those produced with the standard phase‐difference technique. Fitting accuracy and noise level were also measured.

Results

In 92/100 sonications, the difference between the two measurements was less than 3°C. The average difference in the measurements was 1.5 ± 1.4°C. Small motion artifacts were observed in the phase‐difference imaging when the difference was greater than 3°C. The method failed in two cases. The mean absolute error in the surface fit in baseline images corresponded to a temperature error of 0.8 ± 1.4°C. The noise level was approximately 40% lower than the phase‐difference method. Thermal dose contours calculated from the two methods agreed well on average.

Conclusion

Based on the small error when compared with the standard technique, this method appears to be adequate for temperature monitoring of MRgFUS in uterine fibroids and may prove useful for monitoring temperature changes in moving organs. J. Magn. Reson. Imaging 2008;28:1026–1032. © 2008 Wiley‐Liss, Inc.     

Quantification of aortic compliance in mice using radial phase contrast MRI

Purpose

To investigate the feasibility of radial phase contrast MR imaging to measure in vivo pulse wave velocity (PWV) and wall shear stress (WSS) in small animals on a 7 Tesla scanner.

Materials and Methods

The aortic compliance of 9‐month‐old ApoE deficient (ApoE‐KO) mice (n = 10) on a normal diet was studied in comparison to that of wild‐type (WT) mice (n = 10). An undersampled, asymmetric echo radial phase contrast MR technique was developed to measure through plane blood flow velocity at axial slices along the descending aorta. The PWV and the time averaged WSS was calculated from the spatiotemporal flow data. The reproducibility of PWV and WSS was evaluated by taking multiple measures on a separate cohort of WT (n = 4) mice.

Results

The mean percentage standard deviation among repeated measures was 10.1% for PWV and 24.8% for WSS. The PWV of ApoE‐KO mice (5.84 ± 2.15 m/s) was significantly higher (p = 0.02) than that of WT (3.55 ± 0.97 m/s), whereas WSS was lower in ApoE‐KO mice (1.44 ± 0.31Pa) compared with WT (1.55 ± 0.36Pa).

Conclusion

This study demonstrates that in vivo PWV derived from radial phase contrast MR imaging can be potentially used as a surrogate marker for impaired vascular function in mice. J. Magn. Reson. Imaging 2009;30:286–291. © 2009 Wiley‐Liss, Inc.     

Resolving arterial phase and temporal enhancement characteristics in DCE MRM at high spatial resolution with TWIST acquisition

Purpose:

To investigate the potential of a view‐sharing 3D fast gradient‐echo sequence using pseudo random trajectories (TWIST) to achieve very short acquisition times with high in‐plane resolution and good volume coverage and its application to dynamic contrast‐enhanced (DCE) breast magnetic resonance imaging (MRI).

Materials and Methods:

Two versions of a 3D fast gradient echo TWIST sequence were implemented and applied to patients: First, an ultrafast TWIST acquisition (TA = 5.7 sec) in combination with a routine DCE MRM protocol to allow the extraction of arterial input functions and to resolve the first pass of the contrast agent. Second, a dynamic full coverage TWIST DCE acquisition (TA = 10.6 sec) in a repeat examination, replacing the routine DCE MRM sequence.

Results:

The ultrafast acquisition enabled extraction of arterial input functions and the monitoring of the contrast agent's first pass through vessels and lesions. The dynamic full coverage TWIST acquisition captured the initial dynamic slope of the signal time curve of lesions accurately, in contrast to the routine protocol.

Conclusion:

TWIST acquisitions proved very robust and offer high flexibility in protocol timing. The ultrafast protocol achieved 5.7 seconds time resolution with good image quality and can be combined with any established routine protocol. The full dynamics TWIST DCE protocol offers improved time resolution of the CE dynamic time‐course and closely matches the image quality of the routine protocol. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.