Biomedical subjects

T M Grist

Publications and source records attributed to T M Grist.

At least 73 records · Page 4Linked to original sources

PubMed1995

Basilar artery dolichoectasia. Review of the literature and six patients studied with magnetic resonance angiography.

Six patients for whom computed tomography revealed a curvilinear calcific mass anterior to their brainstem were evaluated and magnetic resonance imaging and magnetic resonance angiography were performed on each. Magnetic resonance studies confirmed the suspicion of basilar artery dolichoectasia, and demonstrated a partial thrombus in the basilar artery in 1 patient. The patients' clinical features were combined with those of basilar artery dolichoectasia patients reported in the literature (n = 122) who had case histories sufficiently detailed enough to determine each person's mode of clinical presentation. Basilar artery dolichoectasia patients were more often men (95/128, 74%) and had a mean age of 59 +/- 11 years. Of the 128 patients studied, there were cranial nerve compressive signs in 74 (58%), especially facial spasm (29/74, 39%) and trigeminal neuralgia (20/74, 27%); vertebral basilar insufficiency or vertebral basilar stroke or both in 61 (48%); hydrocephalus in 40 (31%); compressive brainstem symptoms and signs that progressed clinically in 31 (24%); and arterial hypertension in 31 (24%). Magnetic resonance imaging and magnetic resonance angiography safely diagnose this interesting arterial abnormality. The modes of clinical presentation of this disorder are reviewed.

Aged↗

PubMed1995

Musculoskeletal neoplasms: preoperative evaluation with MR angiography.

PURPOSE: To assess the ability of magnetic resonance (MR) angiography to depict vascularity of musculoskeletal neoplasms. MATERIALS AND METHODS: Two-dimensional (2D) time-of-flight (TOF) MR angiography was compared with conventional arteriography during staging of musculoskeletal tumors in 23 prospective examinations. Phase-contrast (PC) MR angiography was also performed in 19 cases and evaluated as a possible supplement to 2D TOF imaging. RESULTS: Of named vessels, 92% in proximity to tumor were noted by blinded readers. The PC technique provided supplemental data in 47% of cases, usually related to better delineation of in-plane feeder vessels and areas with pulsatile blood flow. Of the 28 branch feeder vessels, 23 were noted on both conventional arteriograms and MR angiograms in a nonblinded review, but 16 were difficult to distinguish as feeders because of lack of associated tumor blush. CONCLUSION: MR angiography has promise to replace conventional arteriography for orthopedic preoperative planning.

Adolescent↗

PubMed1994

Magnetic resonance angiography of renal artery stenosis.

Magnetic resonance angiography is a new noninvasive technique capable of imaging the renal arteries without nephrotoxic contrast agents. Therefore, it may play an important role in patients with suspected ischemic nephropathy. The objective of this discussion is to review the current state of the art of renal magnetic resonance angiography, including a description of the methods for performing magnetic resonance angiography, applications of the techniques specifically to the renal arteries, and a review of the results of several validation studies. These validation studies have reported sensitivities ranging from 83% to 100% and specificities from 92% to 97% for detecting significant (> 50%) renal artery stenoses. In addition, magnetic resonance techniques capable of evaluating renal function, including direct measurement of renal blood flow and glomerular filtration rate in individual kidneys, are described.

Clinical Trials as Topic↗

PubMed1994

MR angiography of the pelvis with variable velocity encoding and a phased-array coil.

PURPOSE: To evaluate the feasibility of an automated variable velocity-encoding sequence and improve the signal-to-noise ratio (S/N) on magnetic resonance angiograms with use of phase contrast and a pelvic phased-array coil. MATERIALS AND METHODS: Three cardiac-gated, two-dimensional (2D), phase-contrast (PC) sequences were evaluated in 10 healthy subjects. A 2D gated PC sequence with variable velocity encoding (velocity-optimized phase contrast [VOPC]) was compared with gated 2D PC sequences performed with high or low constant velocity encoding. S/Ns in VOPC images obtained with a pelvic phased-array coil were compared with those in VOPC images obtained with a body coil. RESULTS: Two blinded readers preferred VOPC for simultaneous display of large and small blood vessels in one acquisition compared with constant low (P = .0105) and high (P = .0067) velocity encoding and for overall image interpretation. VOPC images obtained with the pelvic coil had a 68%-100% better S/N compared with those obtained with the body coil. CONCLUSION: Use of a phased-array coil and variable velocity encoding improves depiction of segmental vascular anatomic structures of the pelvis.

Blood Flow Velocity↗

PubMed1994

Comparison of magnetic resonance volume flow rates, angiography, and carotid Dopplers. Preliminary results.

BACKGROUND AND PURPOSE: We compared the results of conventional angiography, carotid Doppler, and magnetic resonance angiography volume flow rates to determine the clinical utility of volume flow rate assessment of blood flow to the anterior circulation in patients with carotid occlusive disease. METHODS: From 11 symptomatic patients, a total of 22 extracranial carotid arteries were studied with all three techniques. The studies were independently read, and regression analysis was used to compare the measurements. RESULTS: Carotid Doppler measurements of the distal extracranial carotid arteries were proportional to the inverse of the extracranial carotid volume flow rate (r = .53, R2 = 29%, P < .01), volume flow rates were proportional to the inverse of measured percent stenosis on angiography (r = .84, R2 = 71%, P < .01), and Dopplers were proportional to angiography (r = .94, R2 = 90%, P < .01). Symptomatic Doppler systolic velocity was significantly higher (P < .002), symptomatic measured stenosis was significantly higher (P < .002), and symptomatic volume flow rate was significantly lower (P < .01) than their respective asymptomatic-side values. These preliminary observations, however, may well change once a large data set, especially one in which more patients with high-grade carotid stenosis are included, is studied. CONCLUSIONS: Assessment of carotid volume flow rates by magnetic resonance angiography quantifies flow reduction secondary to atherosclerotic occlusive disease. The easily obtained flow data add both documentation of arterial flow characteristics related to internal carotid stenosis and information regarding the adequacy of collateral pathways.

Blood Flow Velocity↗

PubMed1993

Generalized matched filtering for time-resolved MR angiography of pulsatile flow.

Generating flow-specific images (arteriograms, venograms) with optimal signal-to-noise ratios for time-resolved MR angiography is a conditional maximum problem, and its solutions are generalized matched filters. We have investigated six matched filters, corresponding to all possible combinations of three flow suppression conditions and two signal-to-noise ratio maximization procedures. Four of these matched filters correspond to previously described methods: the subtractive matched filter, the standard deviation, the global venous eigenimage and the global arterial eigenimage. The two others are referred to here as the local venous eigenimage and the local arterial eigenimage. These six matched filters have been applied to 2D time-resolved phase contrast angiographic data. The local arterial eigenimage is found to be the most effective in suppressing undesired venous flow and preserving desired arterial flow.

Blood Flow Velocity↗

PubMed1993

MR angiography using velocity-selective preparation pulses and segmented gradient-echo acquisition.

We describe a cardiac-gated MR angiographic imaging method that employs velocity-selective preparation (VSP) pulses in conjunction with segmented gradient-echo acquisition and subtraction to produce images that, ideally, contain no signal from stationary tissues and display vessels with a signal intensity that is dependent on the velocity of the blood in the vessels. The novel features of this method are a) it acquires several phase-encoding values/application of a single VSP pulse, b) it uses subtraction to eliminate signal that is not sufficiently suppressed by the VSP pulses, and c) it uses VSP pulses that are synchronized with the cardiac cycle so it can be used to produce ghost-free images of pulsatile blood. An advantage of this sequence is that it detects a signal that, after preparation, is relatively unaffected by changes in blood velocity. This leads to a large signal-to-noise ratio for all the phase-encoding values, a reduction of ghosting artifacts, and the ability to visualize blood that is in motion for only a short time during the cardiac cycle. Because the signal is prepared during peak flow, venous signal can be suppressed by making the sequence sensitive to high velocities. An additional advantage of this sequence is that it permits sampling with a short TE because the velocity-encoding gradient can be applied in a preparatory interval. Signal loss that results from dephasing during the longer TE preparation interval can be reduced or eliminated by allowing the dephased spins to flow out of the region of complex flow, and perhaps out of the field-of-view, by introducing a delay between the finish of the VSP pulse and the beginning of data acquisition.

Blood Flow Velocity↗

PubMed1993

MR imaging of adrenal lymphoma.

A case of lymphoma of the adrenal glands is presented. Both computed tomography (CT) and magnetic resonance imaging (MRI) demonstrated large bilateral adrenal masses with no other associated abdominal abnormalities. MR was better able to demonstrate inferior vena caval patency than CT, and effectively exclude pheochromocytoma as a differential diagnostic consideration due to signal characteristics of the masses.

Adrenal Gland Neoplasms↗

PubMed1993

Pulmonary angiography with MR imaging: preliminary clinical experience.

PURPOSE: To study the accuracy of magnetic resonance (MR) pulmonary angiography in 20 patients in whom pulmonary embolism (PE) was clinically suspected. MATERIALS AND METHODS: Fourteen patients (group 1) were recruited for the MR pulmonary angiography study before they underwent conventional pulmonary angiography (CPA) based on clinical findings. Six patients (group 2) did not undergo CPA but were considered to have PE on the basis of findings in other studies. MR venography was performed at the time of MR pulmonary angiography in 13 patients. RESULTS: MR pulmonary angiography had a sensitivity of 92%-100% and specificity of 62% for detection of PE. Performance of MR pulmonary arteriography and MR venography in a single examination to demonstrate thrombus in both the arterial and deep venous systems was proved feasible. CONCLUSION: This report describes an early clinical implementation of new MR pulmonary angiographic techniques. Further advances to improve specificity by enhancing sensitivity to slow flow and increasing spatial resolution are necessary before routine clinical use of MR pulmonary angiography is justified.

Adult↗

PubMed1993

Magnetic resonance angiography of the aorta and renal arteries.

Recently developed MR angiographic techniques currently provide useful information regarding arterial vascular abnormalities within the abdomen. With further improvements in technique, MR angiography in the abdomen is expected to provide the radiologist and clinician with extensive information regarding vascular pathology without the need for iodinated contrast or intravascular catheters. Further work is needed to evaluate the role of MR angiography with respect to other noninvasive imaging techniques.

Aorta↗

PubMed1992

Peripheral MR angiography with variable velocity encoding. Work in progress.

An electrocardiographically triggered two-dimensional phase-contrast (PC) magnetic resonance angiographic pulse sequence was developed in which velocity encoding (VENC) was varied throughout an acquisition in response to changes in blood velocity during the cardiac cycle. This was done to better capture signal in the peripheral vasculature, where pulsatile flow degrades images. After reconstruction, a matched filter addition technique was applied to the cardiac phase images to obtain a single high-quality static image. Images were obtained of six healthy volunteers--with and without varying VENC--and contrast-to-noise ratio (C/N) calculations were performed for the added images. Varying VENC significantly improved vascular signal from small and large vessels (P less than .02), but it was most helpful for small vessels, for which the C/N increased by as much as 260% (average increase, 149%). These preliminary findings suggest that variable VENC can enhance the signal from the small and large peripheral blood vessels in cardiac-gated PC acquisitions.

Angiography↗

PubMed1992

Magnetic resonance imaging--cardiac ejection fraction measurements. Phantom study comparing four different methods.

The accuracy of cardiac ejection fraction (EF) measurements with thin, contiguous cine-magnetic resonance imaging (MR) sections is well established. Still, faster imaging and measurement techniques would be desirable. The authors evaluated the accuracy of four different MR EF measurements methods in a biventricular, anthropomorphic, foam-latex rubber phantom which was connected via noncompliant fluid-filled tubing to a pulsatile flow pump. Nine contiguous 10 mm cine-MR sections (TR/TE, 25/13; flip angle, 45 degrees) were obtained through the heart in long and short cardiac axes at 16 frames per cardiac cycle at a pump rate of 60 beats/minute. EF measurements were based on either the multi-slice summation technique (nine contiguous 10-mm sections versus four 10-mm sections spaced 10 mm apart) or the area-length method (single largest long section versus combination of largest long- and short-axis section). Three replications were performed for each of the tested EFs (40.8%, 29.4%, and 13.4%), which were compared with actual EFs. EF measurements based on contiguous 1-cm sections correlated best with the actual EFs. Average relative errors ranged from 3.2% to 6.0%. EF measurements based on every other section were less accurate; average relative errors were between 5.2% and 10.2%. Single and biplane area-length algorithm EF measurements were significantly less accurate; average relative errors were as high as 59%. EF measurements based on multi-slice summation are more accurate than those based on the area-length algorithm. Contiguous 1-cm section acquisitions are most accurate and most time consuming. With slight decrease of accuracy, acquisition and processing times can be halved by skipping every other slice.

Humans↗

PubMed1991

1990 ARRS Executive Council Award. Renal transplant rejection: diagnosis with 31P MR spectroscopy.

We evaluated the role of 31P MR spectroscopy in the diagnosis of renal transplant allograft dysfunction. Thirty-six 31P MR spectroscopy examinations were prospectively performed in 35 patients with renal allografts. The study was performed in two phases. In the first phase, 12 transplant recipients with normal graft function were studied as normal controls. During phase two, 24 31P MR spectroscopy studies were performed in patients at the time of renal transplant biopsy for allograft dysfunction. Twenty-one of these studies were technically adequate. Pathologic analysis of the biopsy specimens showed evidence of allograft rejection in 14 and no rejection in seven. Various phosphorus metabolite ratios were calculated for each patient, including phosphodiesters/phosphomonoesters (PDE/PME), phosphomonoesters/inorganic phosphate (PME/Pi), and inorganic phosphate/adenosine triphosphate (Pi/ATP). The PDE/PME and Pi/ATP ratios in the allografts with rejection differed significantly from the corresponding metabolite ratios in patients without rejection (p = .017 and p = .024, respectively). A PDE/PME ratio exceeding 0.8 had a sensitivity of 100% and specificity of 86% for predicting rejection. A Pi/ATP ratio greater than 0.6 had a sensitivity of 72% and a specificity of 86% for predicting rejection. We conclude that 31P MR spectroscopy may be useful as a noninvasive method for evaluating renal metabolism during episodes of transplant allograft dysfunction.

Adenosine Triphosphate↗

PubMed1991

Imaging of the renal arteries: value of MR angiography.

We compared the efficacy of MR angiography with that of conventional angiography for visualizing the renal arteries and detecting renovascular disease. Thirty-three MR angiographic studies, consisting of axial two-dimensional (2-D) phase-contrast, coronal 2-D phase-contrast, and coronal 2-D time-of-flight acquisitions, were performed within 48 hr of conventional arteriography. The studies were done to evaluate possible renovascular hypertension (n = 25) or potential donor nephrectomy (n = 8). The three MR image sets were interpreted independently, in random order by three observers, with regard to the number of renal arteries, degree of vessel visualization, arteriovenous overlap, and presence of renovascular disease. A fourth interpretation was based on the combined axial and coronal phase-contrast image sets. Evaluation was limited to the proximal 35 mm of each renal artery. Renal artery visualization and detection of renovascular disease were more complete with coronal phase-contrast (80% sensitivity, 91% specificity) than with time-of-flight (53% sensitivity, 97% specificity) images. Combined axial and coronal phase-contrast images permitted visualization of the proximal 35 mm of all dominant renal arteries and detection of 13 of 15 stenoses (87% sensitivity, 97% specificity). Our data suggest that biplanar MR angiography has considerable potential as a noninvasive screening technique for the evaluation of renovascular disease.

Angiography↗

PubMed1991

Magnetic resonance angiography: technique and skull-base applications.

Cerebral angiography has become essential in the evaluation and management of extensive skull-base lesions. However, with a 2.6 percent incidence of neurologic complications and a 0.3 percent incidence of permanent neurologic deficits associated with cerebral angiography, the procedure is not without risk. Recent advances in magnetic resonance angiography (MRA), allow one to visualize the skull-base vasculature without need of intravenous contrast injection or radiation exposure. Our initial experience in visualizing the normal vascular anatomy and pathology at the base of skull with this new imaging modality is presented. Two different MRA techniques were used: (1) thin-slice two-dimensional gradient echo pulse sequence acquisition with three-dimensional projection reconstruction, and (2) two-dimensional phase contrast angiography. The specifics of these techniques and their usefulness in the evaluation of lesions involving the skull base are discussed and demonstrated.

Adult↗

PubMed1990

Femur length in the US prediction of trisomy 21 and other chromosomal abnormalities.

After review of 461 consecutive amniocentesis examinations, 428 fetuses with normal karyotype and 13 with chromosomal abnormalities were analyzed. Six of the abnormal fetuses had trisomy 21, and seven had other chromosomal abnormalities. The ratio of measured-to-expected femur length (MFL/EFL) was determined for each fetus. The MFL/EFL values were 1.0 +/- 0.7 (mean +/- standard deviation) for the control group, 0.94 +/- 0.06 for trisomy 21 fetuses, 0.94 +/- 0.10 for non-trisomy 21 fetuses, and 0.94 +/- 0.08 for the entire group of fetuses with chromosomal abnormalities. The mean MFL/EFL value for the abnormal fetuses differed significantly from that for the control group (P less than .003). An MFL/EFL of 0.90 or less resulted in a sensitivity of 46.2% and a specificity of 94.1% for detection of all chromosomal abnormalities. For detecting trisomy 21, the sensitivity and specificity were 50% and 93.5%, respectively. Assuming a pretest probability for trisomy 21 of one in 250, an MFL/EFL of 0.90 or less resulted in a positive predictive value of 3%. It is concluded that an MFL/EFL of less than 0.90 modifies the relative risk for chromosomal abnormalities and may significantly influence the decision for amniocentesis.

Chromosome Aberrations↗

PubMed1989

Gated cardiac MR imaging and P-31 MR spectroscopy in humans at 1.5 T. Work in progress.

The authors describe a technique to acquire gated cardiac proton magnetic resonance images and localized hydrogen-1 and phosphorus-31 spectra with a doubly tuned surface coil probe. The probe reduces study time because the need to exchange imaging and spectroscopy coils is eliminated, while at the same time the sensitivity of singly tuned coils is retained. In addition, the probe enhances the ability of the investigator to localize cardiac spectra spatially and temporally during the cardiac cycle. Spectra were acquired from the left ventricular myocardium in five volunteers, and systolic and diastolic gated P-31 spectra were shown.

Humans↗

PubMed1988

Doubly tuned local coils for MRI and MRS at 1.5 T.

We describe a doubly tuned radiofrequency (RF) local coil probe designed specifically for performing in vivo image-localized spectroscopy. The probe was designed using principles developed in connection with the counter-rotating-current (CRC) and planar-pair loop gap resonators for magnetic resonance imaging (MRI). The probe design satisfies several criteria useful for in vivo 1H/31P experiments at 1.5 T. First, sensitivity on the low-frequency mode is preserved relative to a singly tuned coil. This result was confirmed by bench-test and in vitro MR experimental data. Second, through principles of intrinsic decoupling the probe is isolated from any externally applied uniform excitation field, which is desirable for in vivo 1H imaging and solvent suppression. Third, the regions of sensitivity of the high- and low-frequency modes of the coil are similar, and therefore spectroscopic volumes of interest identified on an image will reflect the same volumes as those selected during spectroscopy. Finally, interface to the MR system is such that the high- or low-frequency circuits may be selected entirely under software control, with no requirement for changing coils or cables or moving the subject.

Humans↗