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T M Grist

Publications and source records attributed to T M Grist.

15 recordsLinked to original sources
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↗
PubMed1987

Quadrature detection surface coil.

A surface coil assembly consisting of two interleaved coplanar resonators that are intrinsically decoupled from each other is described for imaging at 1.5 T. Vector reception fields on-axis at 3 cm depth are orthogonal and of equal magnitude. Both components of magnetization were received and combined resulting in a 2 1/2 improvement in signal-to-noise for temporomandibular joint images.

Humans↗
PubMed1986

The sectorial loop-gap resonator for 31P NMR of the adult human liver at 1.5 T with surface tissue suppression.

A new type of loop-gap resonator, which because of its topology is called a "sectorial resonator," is described in considerable analytical detail. It was used for 31P NMR of the livers of two human adult normal volunteers. An effective strategy for suppression of signals from surface tissue was developed. Depth selection was achieved using a separate excitation coil, permitting a reasonable degree of control of the shape of the sensitive region by adjustment of the dimensions of the excitation coil and of the type of pulse delivered. In addition a new principle for cancellation is described: the topology of the sectorial resonator leads to cancellation signals from tissue masses at the two openings of the structure. Using a phase-alternated repeated 90 degrees FID pulse sequence, good spectra were achieved in 2.5 min that are free from contamination by the phosphocreatine peak.

Adult↗
PubMed1986

1.5 T in vivo 31P NMR spectroscopy of the human liver using a sectorial resonator.

A novel resonator for receiving 31P NMR signals at 1.5 T, which because of its topology has been named a "sectorial resonator," exhibits good radio frequency coupling to the adult human liver. Minimal contamination from nonliver tissue (i.e., muscle) was achieved by the use of depth selective pulses and also because the geometry results in cancellation of signals arising from tissues that are close to the two openings of the structure.

Humans↗
PubMed1986

High-resolution MR imaging using loop-gap resonators. Work in progress.

Representative results using loop-gap resonator (LGR) configurations for local high-resolution MR imaging (0.6 mm X 0.6 mm pixel size with 3-mm section thickness) are presented. Four geometries were employed: flat single loops, coplanar (planar pair) loops, coaxial (axial pair) loops, and symmetrically angled (butterfly pair) loops. The LGRs were used as receiver coils with the body coil as transmitter. Butterfly pairs and planar pairs are intrinsically decoupled from a transmitted field of arbitrary orientation, whereas the single loops and axial pairs are decoupled from a field that is linearly polarized and orthogonal to its axis. The coils were evaluated using computer-generated and experimentally determined field distributions and by the imaging of human subjects on a 1.5-T MR system. Matching the region of sensitivity of the coil to the region of anatomic interest is emphasized.

Image Enhancement↗
PubMedDate not supplied

Carotid body tumor: flow sensitive pulse sequences and MR angiography.

The magnetic resonance (MR) appearance of carotid body tumors has been previously described. Classically, these lesions are well circumscribed and highly vascular and cause a characteristic separation of the external and internal carotid arteries. Hypointense foci within these tumors are seen on T1-weighted images. The T2-weighted images snow a "salt and pepper" pattern. We report a carotid bifurcation glomus tumor that exhibited the above features on conventional MR. Multiplanar gadolinium enhanced MR as well as flow-sensitive MR techniques including small flip angle gradient refocused images and MR angiography were performed. Magnetic resonance angiography demonstrated the abnormal morphology of the carotid bifurcation. Axial gradient refocused images revealed evidence of flow within the lesion. The findings were then correlated with postcontrast CT, arterial digital subtraction angiography, and histopathology.

Carotid Body Tumor↗
PubMedDate not supplied

The cervical nerves and foramina: local-coil MR imaging.

A detailed study of the magnetic resonance (MR) appearance of the cervical nerves and neural foramina is presented. Using a "butterfly" local coil and a 1.5-T GE Signa MR system, the authors obtained 3-mm-thick axial, parasagittal, and 45 degrees oblique MR images of the cervical neural foramina in four normal volunteers. For the oblique images, subjects were rotated 45 degrees toward the right posterior oblique. Neural and vascular structures in the foramina were identified by correlation of the MR images with corresponding cryomicrotomic sections from four cadavers. The 45 degrees oblique images were more useful than axial or parasagittal images for demonstrating anatomic relations in the neural foramina. MR imaging with local coils demonstrates the complex anatomy of cervical neural foramina in various planes.

Adult↗