Heidi A. Ward

Imaging artifacts at 3.0T

Clinical MRI at a field strength of 3.0T is finding increasing use. However, along with the advantages of 3.0T, such as increased SNR, there can be drawbacks, including increased levels of imaging artifacts. Although every imaging artifact observed at 3.0T can also be present at 1.5T, the intensity level is often higher at 3.0T and thus the artifact is more objectionable. This review describes some of the imaging artifacts that are commonly observed with 3.0T imaging, and their root causes. When possible, countermeasures that reduce the artifact level are described. J. Magn. Reson. Imaging 2006.

Prospective multiaxial motion correction for fMRI

Corruption of the image time series due to interimage head motion limits the clinical utility of functional MRI. This paper presents a method for real‐time prospective correction of rotation and translation in all six degrees of rigid body motion. By incorporating an orbital navigator (ONAV) echo for each of the sagittal, axial, and coronal planes into the fMRI pulse sequence, rotation and translation can be measured and the spatial orientation of the image acquisition sequence that follows can be corrected prospectively in as little as 160 msec. Testing of the method using a computerized motion phantom capable of performing complex multiaxial motion showed subdegree rotational and submillimeter translational accuracy over a range of ±8° and ±8 mm of motion. In vivo images demonstrate correction of simultaneous through‐plane and in‐plane motion and improved detection of fMRI activation in the presence of head motion. Magn Reson Med 43:459–469, 2000.

Real-time autoshimming for echo planar timecourse imaging†

Head motion within an applied magnetic field alters the effective shim within the brain, causing geometric distortions in echo planar imaging (EPI). Even if subtle, change in shim can lead to artifactual signal changes in timecourse EPI acquisitions, which are typically performed for functional MRI (fMRI) or diffusion tensor imaging. Magnetic field maps acquired before and after head motions of clinically realistic magnitude indicate that motion‐induced changes in magnetic field may cause translations exceeding 3 mm in the phase‐encoding direction of the EPI images. The field maps also demonstrate a trend toward linear variations in shim changes as a function of position within the head, suggesting that a real‐time, first‐order correction may compensate for motion‐induced changes in magnetic field. This article presents a navigator pulse sequence and processing method, termed a “shim NAV,” for real‐time detection of linear shim changes, and a shim‐compensated EPI pulse sequence for dynamic correction of linear shim changes. In vivo and phantom experiments demonstrate the detection accuracy of shim NAVs in the presence of applied gradient shims. Phantom experiments demonstrate reduction of geometric distortion and image artifact using shim‐compensated EPI in the presence of applied gradient shims. In vivo experiments with intentional interimage subject motion demonstrate improved alignment of timecourse EPI images when using the shim NAV‐detected values to update the shim‐compensated EPI acquisition in real time. Magn Reson Med 48:771–780, 2002.

Rapid T2 estimation with phase-cycled variable nutation steady-state free precession.

Variable nutation SSFP (DESPOT2) permits rapid, high‐resolution determination of the transverse (T2) relaxation constant. A limitation of DESPOT2, however, is the presence of T2 voids due to off‐resonance banding artifacts associated with SSFP images. These artifacts typically occur in images acquired with long repetition times (TR) in the presence of B0 inhomogeneities, or near areas of magnetic susceptibility difference, such that the transverse magnetization experiences a net phase shift during the TR interval. This places constraints on the maximum spatial resolution that can be achieved without artifact. Here, a novel implementation of DESPOT2 is presented incorporating RF phase‐cycling which acts to shift the spatial location of the bands, allowing reconstruction of a single, reduced artifact‐image. The method is demonstrated in vivo with the acquisition of a 0.34 mm3 isotropic resolution T2 map of the brain with high precision and accuracy and significantly reduced artifact. Magn Reson Med 52:435–439, 2004.

Functional inferences vary with the method of analysis in fMRI.

Neuroanatomic substrates of specific cognitive functions have been inferred from anatomic distributions of activated pixels during fMRI studies. With declarative memory tasks, interest has focused on the extent to which various medial temporal lobe anatomic structures are activated while subjects encode new information. The aim of this project was to examine how commonly used variations in fMRI data processing methods affect the distribution of activation in anatomically defined medial temporal lobe regions of interest (ROIs) during a complex scene-encoding task. ROIs were drawn on an MRI anatomic template formed from 3D SPGR scans of eight subjects combined in Talairach space. Separate ROIs were drawn for the posterior and anterior hippocampal formation, parahippocampal gyrus, and entorhinal cortex. Twelve different activation maps were created for each subject by using four correlation coefficients and three cluster volumes. Friedmans two-way ANOVA by ranks was used to test the hypothesis that the distribution of activated pixels among defined anatomic ROIs varied as a function of the data processing method. By simply varying the combination of correlation coefficient and cluster volume, significantly different distributions of activation within named medial temporal lobe structures were obtained from the same fMRI datasets (P < 0.015; P < 0.001). The number of subjects studied (n = 8) is in a range commonly found in the literature yet this clearly resulted in spurious associations between processing parameter variations and activation distribution. Using data processing methods that are independent of the arbitrary selection of cutoff values for thresholding activation maps may reduce the likelihood of obtaining spurious results.

Calorimetric calibration of head coil SAR estimates displayed on a clinical MR scanner

Calorimetric measurements were performed to determine the average specific absorption rates (SAR) resulting from MRI head examinations. The data were compared with average head coil SAR estimates displayed by the MR scanner in order to refine the imaging protocols used in imaging patients with implanted deep brain stimulators (DBS). The experiments were performed using transmit-receive (TR) head coil on clinical 1.5 T General Electric MR scanners running 11.0 M4 revision software. The average applied SAR was derived from temperature increases measured inside a head phantom, due to deposition of RF energy during MRI scanning with a spin echo imaging sequence. The measurements were repeated for varied levels of RF transmit gain (TG) and analyzed with a range of entered patient weights. The measurements demonstrate that the ratio of the actual average head SAR to the scanner-displayed value (coil correction factor) decreases for decreasing TG or for increasing patient weight and may vary between 0.3 and 2.1. An additional retrospective patient study, however, shows that not all combinations of TG and patient weight are encountered clinically and, instead, TG generally increases with the patient weight. As a result, a much narrower range of coil correction factors (e.g., typically 0.5-1.0) will be encountered in practice. The calorimetric method described in this work could aid the physicians and technologists in refinement of the model-dependent SAR estimates displayed by the MR scanner, and in selection of imaging parameters for MR head examinations within allowable SAR safety levels.

Spherical navigator echoes for full 3D rigid body motion measurement in MRI

We are developing a 3-D spherical navigator (SNAV) echo technique for MRI that can measure rigid body motion in all six degrees of freedom simultaneously, in a single echo, by sampling a spherical shell in k-space. MRI pulse sequences were developed to acquire varying amounts of data on such a shell. 3-D rotations of an imaged object simply rotate the data on this shell, and can be detected by registration of magnitude values. 3-D translations add phase shifts to the data on the shell, and can be detected with a weighted least squares fit to the phase differences at corresponding points. Data collected with a computer controlled motion phantom with known rotational and translational motions was used to evaluate the technique. The accuracy and precision of the technique depend on the sampling density, with roughly 1000 sample points necessary for accurate detection to within the error limits of the motion phantom. This number of samples can be captured in a single SNAV echo with a 3-D helical spiral trajectory. Motion detection in MRI with spherical navigator echoes is thus feasible and practical. Accurate motion measurements about all three axes, suitable for retrospective or prospective correction, can be obtained in a single pulse sequence.

The utility of pelvic coil SNR testing in the quality assurance of a clinical MRgFUS system

During MRI-guided focused ultrasound (MRgFUS) treatments of uterine fibroids using ExAblate 2000, tissue ablations are delivered by a FUS transducer while MR imaging is performed with a pelvic receiver coil. The consistency of the pelvic coil performance is crucial for reliable MR temperature measurements as well as detailed anatomic imaging in patients. Test sonications in a gel phantom combined with MR thermometry are used to test the performance of the FUS transducer prior to each treatment. As we show, however, these tests do not adequately evaluate receiver coil performance prior to clinical use. This could become a problem since the posterior part of the coil is frequently moved and can malfunction. The aim of this work is to demonstrate the utility of the signal-to-noise ratio (SNR) as a reliable indicator of pelvic coil performance. Slight modification of the vendor-provided coil support was accomplished to assure reproducible coil positioning. The SNR was measured in a gel phantom using axial acquisitions from the 3D-localizer scan. MR temperature and SNR measurements were obtained using a degraded receiver coil (with posterior element removed) and a known faulty coil, and compared to those obtained with a fully functioning coil. While the MR temperature-based tests were insensitive to change in pelvic coil performance, (degraded, p = 0.24; faulty, p = 0.28), the SNR tests were highly sensitive to coil performance, (degraded, p < 0.001; faulty, p < 0.001). Additional clinical data illustrate the utility of SNR testing of the receiver coil. These tests require minimal (or possibly no) additional scan time and have proven to be effective in our clinical practice.

Diffusion tensor imaging characteristics of dementia with Lewy bodies and Alzheimer's disease

Background: Diffusion tensor MR imaging (DTI) provides information on the integrity of tissue microstructure. The magnitude of diffusivity measured with the apparent diffusion coefficient (ADC) increases, and the directionality of diffusivity measured with fractional anisotropy (FA) decreases with neurodegeneration. The two most common neurodegenerative disorders associated with dementia are Alzheimer’s disease (AD) and dementia with Lewy bodies (DLB). Little is known about the DTI changes in DLB. Our objective was to determine the regional DTI characteristics of patients with DLB compared to patients with AD and cognitively normal subjects (CN). Methods: We studied clinically diagnosed age, gender and education matched patients with DLB (n 1⁄4 24), AD (n 1⁄4 24), and CN (n 1⁄4 24). DLB and AD subjects were further matched on dementia severity based on Clinical Dementia Rating scores. Parallel imaging was performed at 3 T using an acceleration factor of two. 3DMPRAGE was performed for anatomical segmentation and labeling. In order to avoid partial volume averaging of tissue diffusivity with CSF, we used an EPI-FLAIR-DTI sequence (which nulls CSF) with 21 diffusion sensitive gradient directions (b 1⁄4 1000 s/mm). We measured ADC from segmented cortical gray matter in regions derived from the automated anatomic labeling atlas. Color FA maps were used for measuring tract-based FA and ADC. Voxel-based analysis was performed to determine gray matter ADC differences among the clinical groups. Results: In the cortical gray matter, patients with DLB and AD had elevated ADC in the amygdala compared to CN subjects (p 1⁄4 0.01). In addition, patients with AD had elevated ADC in the hippocampus and other temporal lobe regions, compared to DLB and CN subjects (p < 0.01). The posterior cingulum white matter tract ADC was elevated (p < 0.01) and FA was decreased (p < 0.05) in patients with AD compared to DLB and CN subjects. SPM5 analysis showed additional ADC elevation in the head of caudate nucleus and putamen in patients with DLB. Conclusions: DLB is characterized by elevated ADC in the amygdala, consistent with the neurodegenerative pathological involvement during the limbic-transitional stage of Lewy body disease. DTI findings agree with the expected pattern of neurodegenerative pathological involvement in clinically diagnosed patients with DLB and AD, and may be useful in differential diagnosis and disease characterization.

Spherical Navigator Echoes for Full 3-D Rigid Body Motion Measurement in MRI

A 3-D spherical navigator (SNAV) echo technique for MRI that can measure rigid body motion in all six degrees of freedom simultaneously by sampling a spherical shell in k-space is under development. Rigid body 3-D rotations of an imaged object simply rotate the k-space data, while translations alter phase in a known way. A computer controlled motion phantom was used to execute known rotations and translations to evaluate the technique. Accurate detection was possible with a double SNAV echo acquisition following a 3-D helical spiral trajectory. Motion detection for retrospective or prospective correction in MRI with spherical navigator echoes is thus feasible and practical.