Reduction of artifacts in susceptibility-weighted MR venography of the brain

PURPOSE: To reduce the off-resonance artifact in susceptibility-weighted imaging (SWI)-based MR venography (MRV) in the brain regions with severe field inhomogeneity and to reduce the signal loss in the minimum-intensity projection (mIP) display of the 3D MRV. MATERIALS AND METHODS: A novel postprocessing approach was presented to map the local field gradients (LFGs) using the 3D SWI data without phase-unwrapping. LFG measurements were used to assess the severity of field inhomogeneity and suppress the residual phase in the phase mask induced by the off-resonance effect. Volume segmentation of brain tissue was used to reduce the signal loss in the peripheral regions of the brain in the through-plane mIP images and enable in-plane mIP display of MRV. RESULTS: Off-resonance artifact in the brain regions with severe field inhomogeneity was effectively reduced by the LFG-based phase suppression approach. Signal loss was reduced in the through-plane mIP of MRV using volume segmentation of brain tissue prior to projection. In-plane mIP of MRV also became feasible with volume segmentation. CONCLUSION: Off-resonance artifacts and signal loss in mIP display of MRV can be effectively reduced through postprocessing.     

High-resolution 7T MRI of the human hippocampus in vivo

Purpose

To describe an initial experience imaging the human hippocampus in vivo using a 7T magnetic resonance (MR) scanner and a protocol developed for very high field neuroimaging.

Materials and Methods

Six normal subjects were scanned on a 7T whole body MR scanner equipped with a 16‐channel head coil. Sequences included a full field of view T1‐weighted 3D turbo field echo (T1W 3D TFE: time of acquisition (TA) = 08:58), T2*‐weighted 2D fast field echo (T2*W 2D FFE: TA = 05:20), and susceptibility‐weighted imaging (SWI: TA = 04:20). SWI data were postprocessed using a minimum intensity projection (minIP) algorithm. Total imaging time was 23 minutes.

Results

T1W 3D TFE images with 700 μm isotropic voxels provided excellent anatomic depiction of macroscopic hippocampal structures. T2*W 2D FFE images with 0.5 mm in‐plane resolution and 2.5 mm slice thickness provided clear discrimination of the Cornu Ammonis and the compilation of adjacent sublayers of the hippocampus. SWI images (0.5 mm in‐plane resolution, 1.0 mm slice thickness) delineated microvenous anatomy of the hippocampus.

Conclusion

In vivo 7T MR imaging can take advantage of higher signal‐to‐noise and novel contrast mechanisms to provide increased conspicuity of hippocampal anatomy. J. Magn. Reson. Imaging 2008;28:1266–1272. © 2008 Wiley‐Liss, Inc.     

Reliability in detection of hemorrhage in acute stroke by a new three-dimensional gradient recalled echo susceptibility-weighted imaging technique compared to computed tomography: a retrospective study

Purpose

To compare the sensitivity of magnetic resonance (MR) susceptibility‐weighted imaging (SWI) with conventional MR sequences and computed tomography (CT) in the detection of hemorrhage in an acute infarct.

Materials and Methods

A series of 84 patients suspected of having acute strokes had both CT and MR imaging (MRI) scans with diffusion‐weighted imaging (DWI) and SWI. The SWI sequence is a new high‐resolution three‐dimensional (3D) imaging technique that amplifies phase to enhance the magnitude contrast.

Results

Thirty‐eight of 84 cases showed abnormal DWI consistent with acute infarct. Of the 38, SWI showed evidence of hemorrhage in 16 cases, compared to eight cases with spin echo (SE) T2, seven cases with fluid attentuated inversion recovery (FLAIR), and only five cases with CT. In a subset of 17 cases of acute infarct who had both two‐dimensional gradient recalled echo (2D‐GRE) T2*‐weighted imaging and SWI, in addition to conventional MRI, evidence of hemorrhage was seen in 10 cases using SWI, compared to seven cases with 2D‐GRE T2*.

Conclusion

SWI proved to be a powerful new approach for visualizing hemorrhage in acute stroke compared to CT and conventional MRI methods. J. Magn. Reson. Imaging 2004;20:372–377. © 2004 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.

     

Brain iron detected by SWI high pass filtered phase calibrated with synchrotron X‐ray fluorescence

Purpose:

To test the ability of susceptibility weighted images (SWI) and high pass filtered phase images to localize and quantify brain iron.

Materials and Methods:

Magnetic resonance (MR) images of human cadaver brain hemispheres were collected using a gradient echo based SWI sequence at 1.5T. For X‐ray fluorescence (XRF) mapping, each brain was cut to obtain slices that reasonably matched the MR images and iron was mapped at the iron K‐edge at 50 or 100 μm resolution. Iron was quantified using XRF calibration foils. Phase and iron XRF were averaged within anatomic regions of one slice, chosen for its range of iron concentrations and nearly perfect anatomic correspondence. X‐ray absorption spectroscopy (XAS) was used to determine if the chemical form of iron was different in regions with poorer correspondence between iron and phase.

Results:

Iron XRF maps, SWI, and high pass filtered phase data in nine brain slices from five subjects were visually very similar, particularly in high iron regions. The chemical form of iron could not explain poor matches. The correlation between the concentration of iron and phase in the cadaver brain was estimated as c_Fe [μg/g tissue] = 850Δ? + 110.

Conclusion:

The phase shift Δ? was found to vary linearly with iron concentration with the best correspondence found in regions with high iron content. J. Magn. Reson. Imaging 2010;31:1346–1354. © 2010 Wiley‐Liss, Inc.

     

ToF‐SWI: Simultaneous time of flight and fully flow compensated susceptibility weighted imaging

Purpose

To perform systematic investigations on parameter selection of a dual‐echo sequence (ToF‐SWI) for combined 3D time‐of‐flight (ToF) angiography and susceptibility weighted imaging (SWI).

Materials and Methods

ToF‐SWI was implemented on 1.5 T and 3 T MR scanners with complete 3D first‐order flow compensation of the second echo. The efficiency of flow compensating the SWI echo was studied based on phantom and in vivo examinations. Arterial and venous contrasts were examined in volunteers as a function of flip angle and compared with additionally acquired single‐echo ToF and single‐echo SWI data.

Results

Complete flow compensation is required to reduce arterial contamination in the SWI part caused by signal voids. A ramped flip angle of 20° depicted arteries best while venous contrast was preserved. Comparing ToF‐SWI with single‐echo ToF demonstrated arteries with similar quality and delineated all major arteries equally well. Venous delineation was degraded due to lower SNR associated with the thinner slabs used with ToF‐SWI compared to single‐echo SWI acquisition.

Conclusion

A dual‐echo sequence (ToF‐SWI) with full flow compensation of the second echo in a single scan is feasible. This sequence allows simultaneous visualization of intrinsically coregistered arteries and veins without spatial mis‐registration of vessels caused by oblique flow and with minimal signal loss in arteries. J. Magn. Reson. Imaging 2009;29:1478–1484. © 2009 Wiley‐Liss, Inc.     

Multilayer appearance on contrast‐enhanced susceptibility‐weighted images on patients with brain abscesses: Possible origins and effects of postprocessing

Purpose:

To demonstrate the presence of a multilayer appearance of the capsule on contrast‐enhanced (CE) susceptibility‐weighted imaging (SWI) in patients with pyogenic brain abscesses. Possible origins for the appearance and effects of postprocessing settings are discussed.

Materials and Methods:

Fourteen patients with pyogenic brain abscesses underwent post gadolinium‐enhanced SWI at 1.5 T. All SWI images were postprocessed with various filter and mask settings to compare the image appearance. Computer simulations using a paramagnetic spherical shell model were performed to verify the clinical findings.

Results:

Pyogenic brain abscesses demonstrated a multilayer appearance with a darkened ring within the enhanced capsule on CE‐SWI in all patients. The multilayer appearance was slice‐orientation‐dependent, decreased with larger widths of the high‐pass filter, and increased with larger numbers of phase mask multiplication operations, consistently on both simulation results and the clinical images.

Conclusion:

CE‐SWI shows the multilayer appearance of the capsule in pyogenic brain abscesses, which may arise from postprocessing procedures originally designed to enhance susceptibility contrast. Although SWI may provide additional information valuable in the diagnosis of pyogenic brain abscesses, image interpretation should be exercised with caution, particularly for CE‐SWI. J. Magn. Reson. Imaging 2012; 36:1353–1361. © 2012 Wiley Periodicals, Inc.     

Removing background phase variations in susceptibility‐weighted imaging using a fast,forward‐field calculation

Purpose

To estimate magnetic field variations induced from air–tissue interface geometry and remove their effects from susceptibility‐weighted imaging (SWI) data.

Materials and Methods

A Fourier transform–based field estimation method is used to calculate the field deviation arising from air–tissue interface geometry. This is accomplished by manually drawing or automatically detecting the sinuses, the mastoid cavity, and the head geometry. The difference in susceptibility, Δχ, between brain tissue and air spaces is then calculated using a residual‐phase minimization approach. SWI data are corrected by subtracting the predicted phase from the original phase images. Resultant phase images are then used to perform the SWI postprocessing.

Results

Significant improvement in the postprocessed SWI data is demonstrated, most notably in the frontal and midbrain regions and to a lesser extent at the boundary of the brain. Specifically, there is much less dropout of signal after phase correction near air–tissue interfaces, making it possible to see vessels and structures that were often incorrectly removed by the conventional SWI postprocessing.

Conclusion

The Fourier transform–based field estimation method is a powerful 3D background phase removal method for improving SWI images, providing clearer images of the forebrain and the midbrain regions. J. Magn. Reson. Imaging 2009;29:937–948. © 2009 Wiley‐Liss, Inc.     

New look at renal vasculature: 7 tesla nonenhanced T1-weighted FLASH imaging

Purpose:

To investigate the feasibility of 7 Tesla (T) nonenhanced high field MR imaging of the renal vasculature and to evaluate the diagnostic potential of various nonenhanced T1‐weighted (T1w) sequences.

Materials and Methods:

Twelve healthy volunteers were examined on a 7T whole‐body MR system (Magnetom 7T, Siemens Healthcare Sector) using a custom‐built eight‐channel radiofrequency (RF) transmit/receive body coil. Subsequent to RF shimming, the following sequences were acquired (i) fat‐saturated two‐dimensional (2D) FLASH, (ii) fat‐saturated 3D FLASH, and a (iii) fat‐saturated 2D time‐of‐flight MR angiography (TOF MRA). SNR and CNR were measured in the aorta and both renal arteries. Qualitative analysis was performed with regard to vessel delineation (5‐point scale: 5 = excellent to 1 = nondiagnostic) and presence of artifacts (5‐point scale: 5 = no artifact present to 1 = strong impairment).

Results:

The inherently high signal intensity of the renal arterial vasculature in T1w imaging enabled moderate to excellent vessel delineation in all sequences. Qualitative (mean, 4.7) and quantitative analysis (SNR_mean: 53.9; CNR_mean: 28.0) demonstrated the superiority of TOF MRA, whereas 2D FLASH imaging provided poorest vessel delineation and was most strongly impaired by artifacts (overall impairment 3.7). The 3D FLASH MRI demonstrated its potential for fast high quality imaging of the nonenhanced arterial vasculature, providing homogeneous hyperintense vessel signal.

Conclusion:

Nonenhanced T1w imaging in general and, TOF MRA in particular, appear to be promising techniques for good quality nonenhanced renal artery assessment at 7 Tesla. J. Magn. Reson. Imaging 2012;36:714–721. © 2012 Wiley Periodicals, Inc.     

Analysis of multiple sclerosis lesions using a fusion of 3.0 T FLAIR and 7.0 T SWI phase: FLAIR SWI

Purpose

To improve multiple sclerosis (MS) research by introducing a new type of contrast, namely, the combination of fluid‐attenuated inversion recovery (FLAIR) data acquired at 3.0 T and 7.0 T susceptibility‐weighted imaging (SWI) phase data. The approach of this new contrast is whole‐brain coverage with 3.0 T‐FLAIR data for lesion detection—currently limited at 7.0 T due to specific absorption rate (SAR) limits—overlaid with high‐resolution, small vessel, and iron‐related 7.0 T SWI contrast. Lesion analysis in terms of penetrating veins and local iron depositions were performed.

Materials and Methods

Data from 10 MS patients were acquired at 3.0 T and at 7.0 T. FLAIR data, acquired at 3.0 T, were registered to 7.0 T SWI phase data and SWI image processing was performed using 3.0 T FLAIR data instead of SWI magnitude data.

Results

A total of 299 MS plaques were detected in eight MS patients. Penetrating veins were found in 75 MS plaques, iron depositions in 48 MS plaques, and veins accompanied with iron depositions in 44 MS plaques.

Conclusion

FLAIR‐SWI provides radiologically known, hyperintense definition of MS lesions overlaid with high‐resolution visualization of iron deposits and venous blood vessels and offers new insights into MS lesions. J. Magn. Reson. Imaging 2011;33:543–549. © 2011 Wiley‐Liss, Inc.     

Deep vein thrombosis using noncontrast‐enhanced MR venography with electrocardiographically gated three‐dimensional half‐fourier FSE: Preliminary experience

Three noncontrast‐enhanced MR venography techniques are presented for assessing deep vein thrombosis (DVT) at 0.5T in patients with metallic implants. Two cardiac‐gated 3D half‐Fourier FSE fresh blood imaging sequences with flow‐refocusing pulses (FR‐FBI) in the read‐out (RO) direction and without FR pulses (non‐FR‐FBI) were developed for slower‐flowing blood. For faster flowing blood, a swap phase‐encode arterial double‐subtraction elimination (SPADE) technique was developed. The three techniques were assessed both quantitatively using signal‐to‐noise (SNR) and contrast‐noise‐ratio (CNR) measurements and qualitatively by subjective image analysis in 15 volunteers. SPADE was compared to FR‐FBI in the pelvic veins and FR‐FBI was compared to non‐FR‐FBI in the thigh and calf veins. Both SPADE and FR‐FBI techniques produced significantly higher SNRs, CNRs, and image quality in each comparative study (P < 0.001). Five patients with metallic implants and confirmed DVT underwent SPADE (pelvic veins) and FR‐FBI (thigh and calf veins) examinations and the results were compared to conventional venography. The SPADE and FR‐FBI images showed all DVTs from all five patients without interference from implant susceptibility artifacts. The excellent image quality produced by both SPADE and FR‐FBI throughout peripheral vasculature demonstrates their promise for detecting DVT in postsurgery patients. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

HYPR TOF: time-resolved contrast-enhanced intracranial MR angiography using time-of-flight as the spatial constraint

Purpose

To investigate the feasibility of using time‐of‐flight (TOF) images as a constraint in the reconstruction of a series of highly undersampled time‐resolved contrast‐enhanced MR images (HYPR TOF), to allow simultaneously high temporal and spatial resolution and increased SNR.

Materials and Methods

Ten healthy volunteers and three patients with aneurysms underwent a HYPR TOF study, which includes a clinical routine TOF scan followed by a first pass time‐resolved contrast‐enhanced exam using an undersampled three‐dimensional (3D) projection trajectory (VIPR). Image quality, waveform fidelity and signal to background variation ratio measurements were compared between HYPR TOF images and VIPR images without HYPR reconstruction.

Results

Volunteer results demonstrated the feasibility of using the clinical routine TOF as the spatial constraint to reconstruct the first pass time‐resolved contrast‐enhanced MRA acquired using highly undersampled 3D projection trajectory (VIPR). All the HYPR TOF images are superior to the corresponding VIPR images with the same temporal reconstruction window on both spatial resolution and SNR.

Conclusion

HYPR TOF improves the spatial resolution and SNR of the rapidly acquired dynamic images without losing the temporal information. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.     

Adaptive noise cancellation to suppress electrocardiography artifacts during real-time interventional MRI

Purpose:

To develop a system for artifact suppression in electrocardiogram (ECG) recordings obtained during interventional real‐time magnetic resonance imaging (MRI).

Materials and Methods:

We characterized ECG artifacts due to radiofrequency pulses and gradient switching during MRI in terms of frequency content. A combination of analog filters and digital least mean squares adaptive filters were used to filter the ECG during in vivo experiments and the results were compared with those obtained with simple low‐pass filtering. The system performance was evaluated in terms of artifact suppression and ability to identify arrhythmias during real‐time MRI.

Results:

Analog filters were able to suppress artifacts from high‐frequency radiofrequency pulses and gradient switching. The remaining pulse artifacts caused by intermittent preparation sequences or spoiler gradients required adaptive filtering because their bandwidth overlapped with that of the ECG. Using analog and adaptive filtering, a mean improvement of 38 dB (n = 11, peak QRS signal to pulse artifact noise) was achieved. This filtering system was successful in removing pulse artifacts that obscured arrhythmias such as premature ventricular complexes and complete atrioventricular block.

Conclusion:

We have developed an online ECG monitoring system employing digital adaptive filters that enables the identification of cardiac arrhythmias during real‐time MRI‐guided interventions. J. Magn. Reson. Imaging 2011;33:1184–1193. © 2011 Wiley‐Liss, Inc.     

Characterizing iron deposition in multiple sclerosis lesions using susceptibility weighted imaging

Purpose

To investigate whether the variable forms of putative iron deposition seen with susceptibility weighted imaging (SWI) will lead to a set of multiple sclerosis (MS) lesion characteristics different than that seen in conventional MR imaging.

Materials and Methods

Twenty‐seven clinically definite MS patients underwent brain scans using magnetic resonance imaging including: pre‐ and postcontrast T1‐weighted imaging, T2‐weighted imaging, FLAIR, and SWI at 1.5 T, 3 T, and 4 T. MS lesions were identified separately in each imaging sequence. Lesions identified in SWI were reevaluated for their iron content using the SWI filtered phase images.

Results

There were a variety of new lesion characteristics identified by SWI, and these were classified into six types. A total of 75 lesions were seen only with conventional imaging, 143 only with SWI, and 204 by both. From the iron quantification measurements, a moderate linear correlation between signal intensity and iron content (phase) was established.

Conclusion

The amount of iron deposition in the brain may serve as a surrogate biomarker for different MS lesion characteristics. SWI showed many lesions missed by conventional methods and six different lesion characteristics. SWI was particularly effective at recognizing the presence of iron in MS lesions and in the basal ganglia and pulvinar thalamus. J. Magn. Reson. Imaging 2009;29:537–544. © 2009 Wiley‐Liss, Inc.     

Can MR fluoroscopic triggering technique and slow rate injection provide appropriate arterial phase images with reducing artifacts on gadoxetic acid‐DTPA (Gd‐EOB‐DTPA)‐enhanced hepatic MR imaging?

Purpose:

To evaluate whether using MR fluoroscopic triggering technique and slow rate injection improves the quality of arterial phase images in gadoxetic acid‐DTPA‐enhanced (Gd‐EOB‐DTPA) MR imaging because of proper acquisition timing and reduction of artifacts.

Materials and Methods:

Two hundred sixteen patients undergoing examination for liver diseases were retrospectively reviewed. All MR images were obtained with two Gd‐EOB‐DTPA injection protocols: (i) a combination protocol, in which the MR fluoroscopic triggering technique and slow rate injection (1 mL/s) were used; and for comparison, (ii) a conventional protocol, in which adjusted fixed scan delay and ordinary rate injection (2 mL/s) were adopted. Signal‐to‐noise ratio (SNR) of aorta, portal vein, and liver parenchyma on arterial phase images were calculated. Two blinded readers independently evaluated the obtained arterial phase images in terms of acquisition timing and degree of artifacts.

Results:

The SNRs of aorta and portal vein on arterial phase images were significantly higher in the combination protocol group (aorta/portal: 221.9 ± 91.9/197.1 ± 89.8) than that in the conventional protocol group (aorta/portal: 169.8 ± 97.4/92.7 ± 48.5) (P < 0.05). The acquisition timing for arterial phase images with the combination protocol was significantly better than that with the conventional protocol (P < 0.01). The image quality of the combination protocol was significantly higher than that of the conventional protocol (P < 0.01). The occurrence rate of moderate or severe degree of artifacts in the conventional protocol (38.0%) was more prominent than that in the combination protocol (18.5%).

Conclusion:

The combination of the MR fluoroscopic triggering technique and slow rate injection provides proper arterial phase images and reduces the artifacts in Gd‐EOB‐DTPA MR imaging. J. Magn. Reson. Imaging 2010;32:334–340. © 2010 Wiley‐Liss, Inc.     

Comparison of the high relaxivity Gd chelates P1152 and Gd‐BOPTA for contrast‐enhanced MR angiography in rabbits at 1.5 Tesla and 3.0 Tesla

Purpose:

To compare signal‐enhancing properties of the high relaxivity Gd chelates P1152 and Gd‐BOPTA for contrast‐enhanced MR angiography (CE‐MRA) in rabbits at 1.5 Tesla (T) and 3.0T.

Materials and Methods:

Three‐dimensional CE‐MRA of the abdominal vasculature was performed in six rabbits using both contrast agents at a dose of 0.1 mmol/kg. Data acquisition was carried out during first pass and up to 10 min after contrast material administration. CNR was determined in aorta, vena cava, and renal cortex. Image quality (5‐point scale, 5 = best) of first pass MR angiograms was rated by two radiologists.

Results:

During first pass CNR of the aorta was 55.1 ± 5.8 (P1152) and 40.3 ± 3.9 (Gd‐BOPTA) at 1.5T (P < 0.05), and 114.9 ± 9.9 (P1152) and 73.5 ± 8.1 (Gd‐BOPTA) at 3.0T (P < 0.05). Both contrast agents showed a comparable decline of CNR within 10 min. Image quality was rated 4.8 ± 0.40 (P1152) and 4.5 ± 0.50 (Gd‐BOPTA) at 1.5T (P = 0.17), and 4.8 ± 0.37 (P1152) and 4.7 ± 0.47 (Gd‐BOPTA) at 3.0T (P = 0.61).

Conclusion:

The high relaxivity Gd‐chelate P1152 offers potential to improve image contrast for CE‐MRA compared with a clinically approved high relaxivity contrast agent. J. Magn. Reson. Imaging 2010;32:459–465. © 2010 Wiley‐Liss, Inc.     

Visualization of external carotid artery and its branches: Non–contrast‐enhanced MR angiography using balanced steady‐state free‐precession sequence and a time‐spatial labeling inversion pulse

Purpose

To evaluate visibility of the external carotid artery (ECA) and its branches using three‐dimensional (3D) balanced steady‐state free‐precession (SSFP) MR angiography with a time‐spatial labeling inversion pulse (Time‐SLIP), and to provide an optimal value of the inversion time (TI).

Materials and Methods

Peripheral‐pulse‐wave‐gated 3D balanced SSFP images were obtained in 20 healthy volunteers. Images with a Time‐SLIP using four different TIs (600, 900, 1200, and 1500 ms) and without a Time‐SLIP, referred to as sequence A to E, were acquired for each subject and compared for visibility scores of ECA system and relative signal intensity (SI) of ECA.

Results

Average Friedman rank for overall visibility was 1.63, 3.01, 3.59, 3.58, and 3.20 for sequence A to E, respectively. Sequence C and D yielded significantly higher visibility than sequence A, B, and E. The mean relative SI value was 0.97, 0.87, 0.81, 0.76, and 0.67 for sequence A to E, respectively.

Conclusion

Balanced SSFP MR angiography with a Time‐SLIP is superior to that without a Time‐SLIP, showing excellent visualization of ECA system in approximately 3 min in average with sufficient background suppression including veins and salivary ducts. A TI of 1200 ms was considered to be optimal for this purpose. J. Magn. Reson. Imaging 2009;30:678–683. © 2009 Wiley‐Liss, Inc.     

Susceptibility-weighted MR imaging of radiation therapy-induced cerebral microbleeds in patients with glioma: a comparison between 3T and 7T

Introduction

Cerebral microbleeds have been observed in normal-appearing brain tissue of patients with glioma years after receiving radiation therapy. The contrast of these paramagnetic lesions varies with field strength due to differences in the effects of susceptibility. The purpose of this study was to compare 3T and 7T MRI as platforms for detecting cerebral microbleeds in patients treated with radiotherapy using susceptibility-weighted imaging (SWI).

Methods

SWI was performed with both 3T and 7T MR scanners on ten patients with glioma who had received prior radiotherapy. Imaging sequences were optimized to obtain data within a clinically acceptable scan time. Both T2*-weighted magnitude images and SWI data were reconstructed, minimum intensity projection was implemented, and microbleeds were manually identified. The number of microbleeds was counted and compared among datasets.

Results

Significantly more microbleeds were identified on SWI than magnitude images at both 7T (p?=?0.002) and 3T (p?=?0.023). Seven-tesla SWI detected significantly more microbleeds than 3T SWI for seven out of ten patients who had tumors located remote from deep brain regions (p?=?0.016), but when the additional three patients with more inferior tumors were included, the difference was not significant.

Conclusion

SWI is more sensitive for detecting microbleeds than magnitude images at both 3T and 7T. For areas without heightened susceptibility artifacts, 7T SWI is more sensitive to detecting radiation therapy-induced microbleeds than 3T SWI. Tumor location should be considered in conjunction with field strength when selecting the most appropriate strategy for imaging microbleeds.     

A population‐specific symmetric phase model to automatically analyze susceptibility‐weighted imaging (SWI) phase shifts and phase symmetry in the human brain

Purpose:

To create a population‐specific symmetric phase model and to evaluate the susceptibility‐weighted imaging (SWI) phase in terms of phase shift using different segmentation methods (manual and automatic) and phase shift symmetry, which is expected as a marker for lateralized Parkinson's disease (PD) symptoms.

Materials and Methods:

SWI and T₁‐weighted data from 25 PD patients and five healthy controls were acquired on a 3T MRI system. A population‐specific, symmetric phase model was developed. Regions of interest (ROIs) were defined manually on the phase model, manually on each individual data set, and automatically using model‐based segmentation (MBS). Manually‐ and MBS‐defined ROIs were compared using kappa values, and left‐right phase symmetry was evaluated using correlation analysis.

Results:

Independent of the analysis method, a phase increase from the anterior to the posterior putamen, and the average phase value relationship substantia nigra > globus pallidus > red nucleus was found. Phase symmetry analysis shows a difference between lateralized and symmetric PD.

Conclusion:

The symmetric phase model helps to analyze phase data with similar accuracy, but a greatly reduced tracing effort compared to individual tracing and also allows evaluating left‐right phase symmetries. J. Magn. Reson. Imaging 2010;31:215–220. © 2009 Wiley‐Liss, Inc.     

Susceptibility weighted imaging with multiple echoes

Purpose:

To extend susceptibility weighted imaging (SWI) to multiple echoes with an adapted homodyne filtering of phase images for the computation of venograms with improved signal to noise ratio (SNR) and contrast to noise ratio (CNR) and to produce high resolution maps of R₂* relaxation.

Materials and Methods:

Three‐dimensional multi echo gradient echo data were acquired with five equidistant echoes ranging from 13 to 41 ms. The phase images of each echo were filtered with filter parameters adjusted to the echo time, converted into a phase mask, and combined with the corresponding magnitude images to obtain susceptibility weighted images. The individual images were then averaged. Conventional single echo data were acquired for comparison. Maps of R₂* relaxation rates were computed from the magnitude data. Field maps derived from the phase data were used to correct R₂* for the influences from background inhomogeneities of the static magnetic field.

Results:

Compared with the single echo images, the combined images had an increase in SNR by 46% and an improvement in CNR by 34 to 80%, improved visibility of small venous vessels and reduced blurring along the readout direction. The R₂* values of different tissue types are in good agreement with values from the literature.

Conclusion:

Acquisition of SWI with multiple echoes leads to an increase in SNR and CNR and it allows the computation of high resolution maps of R₂* relaxation. J. Magn. Reson. Imaging 2010;31:185–191. © 2009 Wiley‐Liss, Inc.