Biomedical subjects

T M Grist

Publications and source records attributed to T M Grist.

At least 19 recordsLinked to original sources

PubMed2006

High-resolution, time-resolved MRA provides superior definition of lower-extremity arterial segments compared to 2D time-of-flight imaging.

PURPOSE: To evaluate a novel time-resolved contrast-enhanced (CE) projection reconstruction (PR) magnetic resonance angiography (MRA) method for identifying potential bypass graft target vessels in patients with Class II-IV peripheral vascular disease. MATERIALS AND METHODS: Twenty patients (M:F = 15:5, mean age = 58 years, range = 48-83 years), were recruited from routine MRA referrals. All imaging was performed on a 1.5 T MRI system with fast gradients (Signa LX; GE Healthcare, Waukesha, WI). Images were acquired with a novel technique that combined undersampled PR with a time-resolved acquisition to yield an MRA method with high temporal and spatial resolution. The method is called PR hyper time-resolved imaging of contrast kinetics (PR-hyperTRICKS). Quantitative and qualitative analyses were used to compare two-dimensional (2D) time-of-flight (TOF) and PR-hyperTRICKS in 13 arterial segments per lower extremity. Statistical analysis was performed with the Wilcoxon signed-rank test. RESULTS: Fifteen percent (77/517) of the vessels were scored as missing or nondiagnostic with 2D TOF, but were scored as diagnostic with PR-hyperTRICKS. Image quality was superior with PR-hyperTRICKS vs. 2D TOF (on a four-point scale, mean rank = 3.3 +/- 1.2 vs. 2.9 +/- 1.2, P < 0.0001). PR-hyperTRICKS produced images with high contrast-to-noise ratios (CNR) and high spatial and temporal resolution. 2D TOF images were of inferior quality due to moderate spatial resolution, inferior CNR, greater flow-related artifacts, and absence of temporal resolution. CONCLUSION: PR-hyperTRICKS provides superior preoperative assessment of lower limb ischemia compared to 2D TOF.

Aged↗

PubMed2004

Contrast-enhanced peripheral magnetic resonance angiography using time-resolved vastly undersampled isotropic projection reconstruction.

PURPOSE: To investigate the application of time-resolved vastly undersampled isotropic projection reconstruction (VIPR) in contrast-enhanced magnetic resonance angiography of the distal extremity (single station), and peripheral run-off vasculature in the abdomen, thigh, and calf (three stations). MATERIALS AND METHODS: Time-resolved distal extremity imaging was performed using VIPR sequence through the comparison of two acquisition matrix sizes: 256 with TR/TE=3.7/1.4 msec and 320 with TR/TE=4.5/1.8 msec under the same scan time of two minutes. VIPR acquisition was combined with a bolus-chase technique to image the peripheral run-off vasculature. The time-resolved images were reconstructed using a revised sliding window reconstruction filter whose temporal aperture remained narrow for low spatial frequencies and increased quadratically to include all the projection data for high spatial frequencies. RESULTS: The new temporal filter significantly suppressed the undersampling streak artifacts and venous contamination, while maintaining a high temporal resolution. Both high spatial resolution (ranging from 1.56 x 1.56 x 1.56 mm to 1.25 x 1.25 x 1.25 mm) and high temporal resolution (three seconds per frame) distal extremity images and peripheral run-off images were generated using time-resolved VIPR acquisition, which provides isotropic spatial resolution and isotropic coverage. CONCLUSION: Time-resolved VIPR acquisition was demonstrated to be well suited for distal extremity imaging by providing isotropic spatial resolution, isotropic coverage, and high temporal resolution. The combination of time-resolved VIPR and bolus chase technique provided a novel approach for peripheral run-off examinations.

Abdomen↗

PubMed2004

Artifact reduction in undersampled projection reconstruction MRI of the peripheral vessels using selective excitation.

The projection reconstruction (PR)-HyperTRICKS (time resolved imaging of contrast kinetics) acquisition integrates the benefits of through-plane Cartesian slice encoding and in-plane undersampled PR. It provides high spatial resolution both in-plane (about 1 mm(2)) and through-plane (1-2 mm), as well as relatively high temporal resolution (about 0.25 frames per second). However, undersampling artifacts that originate from anatomy superior or inferior to a coronal imaging FOV may severely degrade the image quality. In coronal MRA acquisitions, the slice coverage is limited in order to achieve high temporal resolution. In this report we describe an artifact reduction method that uses selective excitation in PR-HyperTRICKS. This technique significantly reduces undersampling streak artifacts while it increases the slice coverage.

Artifacts↗

PubMed2004

High-resolution multistation peripheral MR angiography using undersampled projection reconstruction imaging.

A novel protocol for three-station MR angiography (MRA) of the lower extremities is described. A time-resolved undersampled projection reconstruction (PR) acquisition was used to image the calf station during a first injection, and non-time-resolved PR acquisitions were used with the bolus-chase technique to image the abdomen and thigh stations during a second injection. The streak artifacts resulting from undersampling the PR data were reduced with the use of a spatial Fermi filter based on the sensitivity of each coil element in a peripheral vascular phased-array coil. This novel technique provided high spatial resolution and a broad range of coverage, and depicted the contrast dynamics in the most distal station of the lower extremities.

Abdomen↗

PubMed2003

SNR improvement for multiinjection time-resolved high-resolution CE-MRA of the peripheral vasculature.

Peripheral MR angiography (MRA) should ideally provide images over a large field of view with high spatial resolution and adequate temporal resolution to accommodate differences in regional filling times. Image subtraction is usually used to remove background signals. In examination protocols involving multiple injections at multiple sites, previously injected contrast present in the mask image provides a substantial decrease in the subtraction image signal. Bolus chase methods avoid this problem but provide limited time for acquisition of high-resolution images at each station. We present here a technique applied to peripheral angiography that provides high spatial and temporal resolution while maintaining high SNR in multiple injection examinations. Undersampled projection imaging was used to increase spatial resolution relative to a previously reported technique using a Cartesian acquisition technique. Late acquisition of high spatial frequencies and temporal matched-filtering were used to increase spatial resolution and SNR, respectively. Temporal correlation analysis was applied to permit multistation examinations without mask subtraction, thus providing an additional gain in SNR relative to multistation subtraction methods. Quantitative analysis is provided to evaluate the signal and noise behavior in the matched-filtering process due to multiinjection and mask subtraction.

Computer Simulation↗

PubMed2002

Time-resolved, undersampled projection reconstruction imaging for high-resolution CE-MRA of the distal runoff vessels.

Imaging of the blood vessels below the knee using contrast-enhanced (CE) MRI is challenging due to the need to coordinate image acquisition and arrival of the contrast in the targeted vessels. Time-resolved acquisitions have been successful in consistently capturing images of the arterial phase of the bolus of contrast agent in the distal extremities. Although time-resolved exams are robust in this respect, higher spatial resolution for the depiction of tight stenoses and the small vessels in the lower leg is desirable. A modification to a high-spatial-resolution T(1)-weighted pulse sequence (projection reconstruction-time resolved imaging of contrast kinetics (PR-TRICKS)) that improves the through-plane spatial resolution by a factor of 2 and maintains a high frame rate is presented. The undersampled PR-TRICKS pulse sequence has been modified to double the spatial resolution in the slice direction by acquiring high-spatial-frequency slice data only after first pass of the bolus of contrast agent. The acquisition reported in the present work (PR-hyperTRICKS) has been used to image healthy volunteers and patients with known vascular disease. The temporal resolution was found to be beneficial in capturing arterial phase images in the presence of asymmetric filling of vessels.

Contrast Media↗

PubMed2001

The effect of injection rate on time-resolved contrast-enhanced peripheral MRA.

In contrast-enhanced (CE) magnetic resonance (MR) angiography (MRA), lower injection rates of a fixed contrast agent dose provide longer contrast agent bolus at the expense of lower intravascular signal. This study evaluated the effect of different injection rates in imaging of the vasculature of the lower extremities with time-resolved, CE MRA. In three volunteers, injection rates of 0.5, 1.5 and 3.0 mL/second were administered in a randomized order and imaged in two separate sessions. Contrast agent bolus dynamics measured in volunteers were used in computer simulations to confirm variations in contrast agent concentration as a source of vessel ringing and blurring artifacts. To validate the effect of injection rate in pathologic vessels, 37 patients with peripheral vascular disease were imaged with a time-resolved technique using an injection rate of 0.5 mL/second or 1.5 mL/second and retrospectively divided into two groups. In volunteers, higher injection rates caused a stronger modulation of k-space and resulted in increased ringing artifacts in time-resolved CE MRA. These results were reproduced with computer simulations. In the qualitative patient study, significantly less vessel blurring was observed using a lower injection-rate, without a significant loss of vessel contrast.

Adult↗

PubMed2001

Effect of MR angiography on the diagnosis and treatment of patients with suspected renovascular disease.

PURPOSE: Although the diagnostic accuracy of renal magnetic resonance (MR) angiography is established, its effect on referring physicians is unknown. The authors prospectively measured the effect of MR angiography results on referring physicians' diagnosis and treatment (plans) of patients with suspected renovascular disease. MATERIALS AND METHODS: Referring physicians prospectively completed questionnaires before and after MR angiography was performed during evaluation of their patients with suspected renovascular disease. The questionnaires asked them to estimate the probability (0%-100%) of their most likely diagnosis before and after receiving the imaging information. They were also asked for their anticipated and final treatment plans. The authors calculated the mean gain in diagnostic percentage confidence and the proportion of patients with changed initial diagnoses or anticipated management. A paired t-test was used to assess significance of the gains in diagnostic percentage confidence. RESULTS: Physicians prospectively completed pre- and post-MR-angiography questionnaires for 30 patients. MR angiography improved mean diagnostic certainty by 35% (P < .0001). MR angiography changed physicians' initial diagnoses in 12 patients (40%). Anticipated treatment plans were changed in 20 patients (67%). Invasive procedures were avoided in eight patients (27%). CONCLUSION: MR angiography has a substantial effect on the diagnostic and therapeutic decision-making of physicians managing patients with suspected renovascular disease.

Adult↗

PubMed2001

Contrast-Enhanced magnetic resonance angiography of the carotid bifurcation using the time-resolved imaging of contrast kinetics (TRICKS) technique.

The time-resolved contrast-enhanced magnetic resonance (MR) angiographic technique TRICKS (time-resolved imaging of contrast kinetics) reconstructs a temporal series of three-dimensional (3D) images. The temporal resolution is increased by using a short TR (<8 ms) and TE (<2 ms), zero filling, partial echo sampling, view sharing, and temporally sampling k-space at variable rates. TRICKS allows reconstruction of multiple sequential 3D volumes following bolus injection of a gadolinium chelate (0.2 mmol/kg body weight up to 40 ml, injection rate -2 ml/s). The resulting temporally defined datasets are conceptually similar to a catheter-based intra-arterial digital subtraction angiographic series, except that they are 3D volumes and not projection images. Similar to other contrast-enhanced MR angiographic methods, TRICKS improves delineation of carotid artery stenosis by minimizing saturation effects. TRICKS and other contrast-enhanced MR angiographic techniques use short echo times and small voxels, thus reducing intravoxel dephasing. Surface morphology of atherosclerotic plaque and slow flow in nearly occluded vessels ("string sign") are well delineated. The major advantage of the TRICKS technique is that the timing of the acquisition in relation to the passage of the contrast bolus occurs automatically, allowing for consistent capture of the arterial phase. and eliminating the need for sophisticated synchronization methods.

Carotid Arteries↗

PubMed2001

Aorta and runoff vessels: single-injection MR angiography with automated table movement compared with multiinjection time-resolved MR angiography--initial results.

The authors compared two techniques for performing runoff, contrast material-enhanced magnetic resonance (MR) angiography. Multiinjection time-resolved imaging of contrast kinetics (TRICKS) and single-injection bolus-chase MR angiographic examinations were performed in 10 volunteers and 10 patients. Image quality and venous overlay of the major blood vessels of the abdomen, thigh, and calf were evaluated. Significantly more (P <.05) vessels were depicted with diagnostic quality on multiinjection TRICKS than on single-injection bolus-chase MR angiographic images.

Adult↗

PubMed2001

Carotid MR angiography: phase II study of safety and efficacy for MS-325.

PURPOSE: To evaluate the safety and efficacy of MS-325 in patients suspected of having carotid arterial disease. MATERIALS AND METHODS: Fifty carotid arteries in 26 patients were imaged with three-dimensional spoiled gradient-recalled-echo magnetic resonance (MR) angiography at 5 and 50 minutes after injection of MS-325. MS-325 was administered intravenously as a single dose of 0.01, 0.03, or 0.05 mmol per kilogram of body weight as determined with a dose randomization scheme for four, nine, and 13 patients, respectively. Safety, including clinical laboratory changes and electrocardiographic monitoring, was assessed until approximately 3 days after injection. Conventional contrast agent-enhanced angiography was used as the standard of reference. Independent readers blinded to the dose interpreted the MR angiographic and conventional images. Images were assessed for location and extent of carotid arterial stenosis. RESULTS: There were no severe or serious adverse events. For the determination of clinically significant stenosis (>70%) on the 5-minute images, sensitivity, specificity, and accuracy (P =.07, three-way comparison) were 100%, 100%, and 100%; 63%, 100%, and 88%; and 40%, 75%, and 55% at 0.01, 0.03, and 0.05 mmol/kg, respectively. Sensitivity and specificity for images at 50 minutes after MS-325 administration showed the same trends as the 5-minute images. CONCLUSION: Overall accuracy for MS-325-enhanced carotid MR angiography performed during steady-state conditions of circulating contrast agent approximately 5 minutes after injection was high (88%-100%) at 0.03 and 0.01 mmol/kg. MS-325 was well tolerated at all evaluated doses.

Carotid Stenosis↗

PubMed2001

Carotid bifurcation: evaluation of time-resolved three-dimensional contrast-enhanced MR angiography.

A magnetic resonance (MR) angiographic protocol was evaluated in the carotid bifurcation with use of a pulse sequence for time-resolved three-dimensional imaging of contrast material kinetics. The enhancement ratio, a quantitative measure of contrast enhancement, indicated that all studies included an image obtained near the peak of the intraarterial concentration of contrast agent (enhancement ratio, 90% +/- 9 [standard deviation]). Studies acquired at a higher frame rate (4.1-4.9 seconds) exhibited less venous enhancement (enhancement ratio, 25% +/- 16) than studies acquired with slower (6.0-9.6-second) frame rates (enhancement ratio, 46% +/- 25).

Adult↗

PubMed2000

MRA of the abdominal aorta and lower extremities.

Atherosclerotic involvement of the aorta and lower extremity vessels is a common clinical problem, especially in developed countries. While x-ray angiography has been the method of choice for preoperative evaluation of patients with atherosclerotic disease, magnetic resonance angiography (MRA) is emerging as a powerful noninvasive tool that is capable of providing information critical to the care of these patients. The objective of this manuscript is to review the current state-of-the-art of MRA of the abdominal aorta and lower extremity vessels. The techniques are described, the clinical indications for MRA are discussed, and the diagnostic accuracy and pitfalls of the various methods are presented.

Aorta, Abdominal↗

PubMed2000

Undersampled projection reconstruction applied to MR angiography.

Undersampled projection reconstruction (PR) is investigated as an alternative method for MRA (MR angiography). In conventional 3D Fourier transform (FT) MRA, resolution in the phase-encoding direction is proportional to acquisition time. Since the PR resolution in all directions is determined by the readout resolution, independent of the number of projections (Np), high resolution can be generated rapidly. However, artifacts increase for reduced Np. In X-ray CT, undersampling artifacts from bright objects like bone can dominate other tissue. In MRA, where bright, contrast-filled vessels dominate, artifacts are often acceptable and the greater resolution per unit time provided by undersampled PR can be realized. The resolution increase is limited by SNR reduction associated with reduced voxel size. The hybrid 3D sequence acquires fractional echo projections in the k(x)-k(y) plane and phase encodings in k(z). PR resolution and artifact characteristics are demonstrated in a phantom and in contrast-enhanced volunteer studies.

Artifacts↗

PubMed2000

Undersampled projection-reconstruction imaging for time-resolved contrast-enhanced imaging.

In time-resolved contrast-enhanced 3D MR angiography, spatial resolution is traded for high temporal resolution. A hybrid method is presented that attempts to reduce this tradeoff in two of the spatial dimensions. It combines an undersampled projection acquisition in two dimensions with variable rate k-space sampling in the third. Spatial resolution in the projection plane is determined by readout resolution and is limited primarily by signal-to-noise ratio. Oversampling the center of k-space combined with temporal k-space interpolation provides time frames with minimal venous contamination. Results demonstrating improved resolution in phantoms and volunteers are presented using angular undersampling factors up to eight with acceptable projection reconstruction artifacts.

Abdomen↗

PubMed2000

Phase-contrast with interleaved undersampled projections.

MR phase-contrast techniques provide velocity-sensitive angiograms and quantitative flow measurements but require long scan times. Recently it has been shown that undersampled projection reconstruction can acquire higher resolution per unit time than Fourier techniques with acceptable artifacts when used in contrast-enhanced MR angiography. Undersampled projection reconstruction has similar potential for phase-contrast acquisitions. Flow sensitization gradients are used with projection trajectories to acquire velocity-dependent phase information. An acquisition scheme that acquires three flow encoding directions on three sets of angular-interleaved projections is introduced. Depending on the resolution, acquisition times for 3D datasets can decrease by factors of two to four.

Artifacts↗

PubMed2000

3D MR DSA: effects of injection protocol and image masking.

The purpose of this study was to investigate the effect on three-dimensional (3D) magnetic resonance digital subtraction angiography (MR DSA) images of various injection protocol parameters (ie, injection order, volume, and rate), as well as image masking. The pelves of 10 normal volunteers were scanned using seven different contrast agent volume/injection rate combinations. Subtraction of a precontrast mask image resulted in vascular image contrast improvements of between 4.0 and 7.7 times. Image quality and smaller vessel image contrast in the masked data decreased with increasing injection number. Data acquired with a high (0.150 mmol kg(-1)) volume yielded the highest quality images, although only small nonsignificant differences in image quality and large vessel conspicuity were found between images obtained using the high and medium (0.075 mmol kg(-1)) volumes. Images acquired with a low (0. 038 mmol kg(-1)) volume, while of lower image contrast, were judged to be of reasonable quality, especially when acquired as the first or second injection. Injection rate (1 ml s(-1), 2 ml s(-1), and 4 ml s(-1)) was not found to affect the images significantly, although selection of an injection rate that gave an injection duration of approximately 10 seconds tended to give better vascular image contrast. Based on these data, a series of escalating volumes for multi-injection examination is proposed. J. Magn. Reson. Imaging 2000;12:476-487.

Adult↗

PubMed2000

Method for rapidly determining and reconstructing the peak arterial frame from a time-resolved CE-MRA exam.

A method that determines the information necessary to reconstruct a single vascular image from a time-resolved CE-MRA exam is presented. Raw k-space data are used to approximate the time course of the contrast passage prior to image reconstruction. The resulting k-space contrast curve is used to select the data corresponding to peak arterial enhancement. These data are reconstructed and immediately presented for physician review, with the entire time-series of images available at a later time for more detailed diagnosis. This approach dramatically reduces the latency between acquisition of large 4D (3D plus time) data sets and presentation of a diagnostic quality time frame. This algorithm has proven successful in the imaging of several anatomical regions and-in exams that do not require a breath hold-permits the use of an acquisition method that produces a contrast-enhanced angiogram without a timing scan.

Arteries↗