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Biomedical subjects

T M Grist

Publications and source records attributed to T M Grist.

At least 55 records · Page 3Linked to original sources
PubMed1998

Contrast-enhanced 3D MR DSA of the carotid artery bifurcation: preliminary study of comparison with unenhanced 2D and 3D time-of-flight MR angiography.

PURPOSE: To compare the delineation of stenosis at the carotid artery bifurcation on three-dimensional (3D) magnetic resonance (MR) digital subtraction angiographic (DSA) images with that on two-dimensional (2D) and 3D time-of-flight (TOF) MR angiographic images. MATERIALS AND METHODS: Twenty-six patients with 29 carotid artery bifurcations and symptoms of cerebral ischemia underwent 3D MR DSA. A time-resolved series was generated with 3D MR DSA after the bolus injection of gadodiamide. The resolution for a carotid artery examination was 0.4 x 0.4 x 1.0 mm, with volumes reconstructed at 4.5-second intervals. The 3D MR DSA images were compared with contemporaneously acquired unenhanced 2D and 3D TOF images. Two observers ranked the 2D and 3D TOF MR angiographic and 3D MR DSA images according to the following: (a) stenosis delineation, (b) internal carotid artery delineation, (c) intravascular signal intensity, and (d) diagnostic confidence. RESULTS: The mean ranking for diagnostic confidence was 1.10 (1 = best technique, 3 = worst technique) for 3D MR DSA. Compared with the pooled 2D TOF and 3D TOF ranks, the 3D MR DSA rank was significantly better (P < .01). Similar levels of statistical significance were found for the other criteria. CONCLUSION: Three-dimensional MR DSA improves the delineation of carotid arterial stenosis by virtually eliminating saturation effects and reducing intravoxel dephasing. Surface morphology and nearly occluded vessels ("string sign") were easily identified. Confidence in identifying carotid arterial occlusions was also very high with this technique.

Angiography, Digital Subtraction↗
PubMed1998

MR angiography of the abdominal aorta.

MR angiography has become a powerful tool for the examination of abdominal aortic pathology, including aortic aneurysm, dissection, and occlusion. This article reviews the techniques, indications, and imaging findings obtained from MR angiography of the abdominal aorta. In addition, the role of MR angiography relative to existing modalities is discussed.

Aorta, Abdominal↗
PubMed1997

Dispersion in magnetization transfer contrast at a given specific absorption rate due to variations of RF pulse parameters in the magnetization transfer preparation.

The effects of RF pulse parameters on magnetization transfer contrast (MTC) were investigated using a magnetization prepared segmented fast gradient echo sequence. MTC was found not to be uniquely determined by the specific absorption rate (SAR). RF pulse parameters (RF amplitude, number of RF pulses and RF duration) also affect MTC. There can be 40% variation in MTC due to differences in RF parameters at SAR = 1 W/kg for a 70-kg subject. Increasing the number of RF pulses is a more efficient way to increase MTC than increasing RF amplitude. This phenomenon is likely caused by the fact that the time scale for magnetization transfer between the free and restricted proton pools is on the same order or longer than the duration of the MT pulse. Accordingly, increase in the MT pulse duration by increasing the number of RF pulses in the MT pulse allows more effective magnetization transfer. Such information can be used as a guide to select RF pulse parameters for a magnetization transfer (MT) pulse. An off-resonance MT pulse designed under this guide for coronary MR angiography improved the depiction of distal vessels.

Animals↗
PubMed1997

Vasodilatory responses and magnetic resonance angiography. Extracranial and intracranial intravascular flow data.

This study measured the responses of both extracranial (internal carotid arteries) and intracranial (middle cerebral/angular, basilar arteries) intravascular arterial volume flow rates to acetazolamide using phase-contrast magnetic resonance angiography. Twenty-eight newly studied patients were subdivided into four groups: Group I--Nonocclusive, asymptomatic (n = 7, or 14 carotid and middle cerebral/angular artery sides); Group II--unilateral carotid transient ischemic attacks, nonhemodynamic (embolic), varying stenoses (n = 11); Group III--unilateral carotid transient ischemic attacks, hemodynamic, varying stenoses (n = 5); and Group IV--unilateral carotid occlusion, asymptomatic (n = 5). The data were separated into nonischemic and ischemic sides so as to illustrate group differences based on vasodilatory responses to acetazolamide. For example, the percent change in volume flow rates over baseline values for the ischemic-side middle cerebral arteries of Group III was significantly the lowest of all of the vasodilatory responses (-25 +/- 11% vs 40 +/- 14% for group II ischemic middle cerebral/angular artery sides, p = 0.008). Group III patients also had significantly lower standing blood pressures (p = 0.012), higher number of transient ischemic attacks (p = 0.008), and shorter duration of events (p = 0.013). Determinations of volume flow rate continue to assist in determining the degree of hemodynamic compromise of a particular vascular territory.

Acetazolamide↗
PubMed1997

Rapid measurement of Gd-DTPA extraction fraction in a dialysis system using echo-planar imaging.

Gd-DTPA (Magnevist, Berlex, Wayne, NJ) extraction fractions (EF) have been measured for three dialysis filter types using an echo-planar imaging (EPI) Look-Locker T1 measurement technique under conditions of fast and slow flow. The mean EF measured in Fresenius (Bad Homburg, Germany) F3, F6, and F8 dialysis filters were 0.015 +/- 0.005, 0.053 +/- 0.004, and 0.084 +/- 0.003, respectively, under conditions of fast flow which provided complete refreshment of spins in the intervals between read-out pulse samples of the T1 relaxation recovery. Data acquisition and post-processing techniques were developed to extend the accuracy of the LL technique to systems with slow flow which did not provide complete refreshment of spins between samples of the T1 recovery. A multi-shot EPI LL interleaved acquisition of relaxation recovery space (IRRS) provided T1 measurement accuracy comparable to the refreshed system, +/- 10, but at the expense of increased scan times (factor of 2 or 3). Discarding the first few non-equilibrium relaxation recovery samples from the T1 fit allowed accurate T1 estimation (+/- 10) with a single-shot EPI LL method under conditions of slow flow. These EPI LL EF measurement methods may provide useful techniques for evaluating renal function in vivo.

Dialysis↗
PubMed1996

Frequency response of multi-phase segmented k-space phase-contrast.

A theoretical analysis of the temporal frequency response of multi-phase segmented k-space phase-contrast was developed. This includes the effects of both segment duration and the number of cardiac phases that are reconstructed. An increase in the number of views per segment and the corresponding increase in segment duration results in an increased smoothing or low-pass filtering of the time-resolved flow waveform. Reconstruction of all intermediate cardiac phases makes the Nyquist sampling frequency independent of the number of views per segment. This analysis was verified experimentally using a multi-phase phase-contrast segmented k-space MR pulse sequence. This sequence reconstructs all intermediate cardiac phases and uses fractional segments at the end of the cardiac cycle if an entire segment does not fit. The use of fractional segments increases the portion of the cardiac cycle over which data are acquired.

Coronary Circulation↗
PubMed1996

Time-resolved contrast-enhanced 3D MR angiography.

An MR angiographic technique, referred to as 3D TRICKS (3D time-resolved imaging of contrast kinetics) has been developed. This technique combines and extends to 3D imaging several previously published elements. These elements include an increased sampling rate for lower spatial frequencies, temporal interpolation of k-space views, and zero-filling in the slice-encoding dimension. When appropriately combined, these elements permit reconstruction of a series of 3D image sets having an effective temporal frame rate of one volume every 2-6 s. Acquiring a temporal series of images offers advantages over the current contrast-enhanced 3D MRA techniques in that it I) increases the likelihood that an arterial-only 3D image set will be obtained. II) permits the passage of the contrast agent to be observed, and III) allows temporal-processing techniques to be applied to yield additional information, or improve image quality.

Contrast Media↗
PubMed1996

Measurement of Gd-DTPA dialysis clearance rates by using a look-locker imaging technique.

The exponential clearance rate constant, (kappa), and filtration fraction (FF) have been measured for dialysis of Gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA) (Magnevist, Berlex, Wayne, NJ) solutions by using a Look-Locker imaging technique under conditions of flow. The measured values of kappa for the Baxter CA-50, CA-110, and CA-210 filters were 0.0037 +/- 0.0003, 0.0057 +/- 0.0017, and 0.0092 +/- 0.0018 min-1, respectively, for dialysis of 4.0 liters of aqueous Gd-DTPA solutions. The measured values of FF for the Baxter CA-110 and CA-210 filters were 0.060 +/- 0.013 and 0.089 +/- 0.015, respectively, for dialysis of aqueous Gd-DTPA at 350 ml/min. The kappa and FF measurements agree with values that use inversion recovery (IR) on static samples obtained by drawing aliquots of solution during the course of dialysis. This in vitro experiment suggests that accurate in vivo measurements of filtration fraction and glomerular filtration rate (GFR) may be possible.

Filtration↗
PubMed1996

Late renal function in patients undergoing renal revascularization for control of hypertension and/or renal preservation.

Long-term outcome was studied in 233 patients who had undergone renal artery revascularization (51 with balloon angioplasty, 182 with surgery) between 1976 and 1992. Patients (excluding renal transplants) were treated for renal vascular hypertension without or with renal insufficiency (serum creatinine > 1.6 mg/dl. All patients still alive (n = 188) were contacted to determine current blood pressure, medications, serum creatinine, and subsequent significant medical events. In patients who had died the cause of death was determined and renal function status at the time of death noted from medical records. Some follow-up information was obtained on all 233 patients; follow-up serum creatinine data were obtained in 193 (82.8%) patients. Some 24 patients (10.3%) became dialysis-dependent. Using a multiple logistic regression analysis only, preoperative creatinine maintained significance (P < 0.001) for increased dialysis risk. There was no statistically significant association of dialysis for type of revascularization (percutaneous transluminal angioplasty, autogenous artery, saphenous vein, endarterectomy or synthetic material), simultaneous or previous aortic or other vascular surgery (carotid endarterectomy, femoropopliteal bypass, etc.), pathology (atherosclerosis or fibromuscular dysplasia), number of renal arteries stenosed or treated, length of follow-up, age, coronary artery disease, congestive heart failure, stroke, chronic lung disease or type II diabetes. It is concluded that, in patients with renal artery stenosis, the timing of renal artery revascularization relative to the level of renal function is the most important determinant for long-term renal salvage.

Adolescent↗
PubMed1996

Magnetic resonance imaging detection of extraluminal enterally administered gadopentetate dimeglumine in a rat model of intestinal ischemia.

RATIONALE AND OBJECTIVES: We assessed whether intestinal ischemia would result in transudation of orally administered gadopentetate dimeglumine into the peritoneal cavity. METHODS: Twenty-eight rats were anesthetized and midline laparotomy was performed. Animals were divided into four groups: control, ligation of a single mesenteric arcade, ligation of six consecutive arcades, and ligation of the anterior mesenteric artery (analogous to the superior mesenteric artery in humans). A 1.0-ml enteric bolus of gadopentetate dimeglumine diluted with sterile water (1:1) was given via gavage. Magnetic resonance imaging was performed 2 hr after laparotomy and reviewed for the presence of intraperitoneal gadopentetate dimeglumine by two experienced observers. Animals were sacrificed 24 hr after surgery for pathologic examination. RESULTS: Four animals died prior to sacrifice. The bladder had a grossly high signal in all cases, implying some degree of intravascular absorption of the contrast material. A correlation was found between increasing mean radiology scores and increasing numbers of ligated vessels. The intraperitoneal signal tended to be higher in experimental animals than in control animals. Histologic damage was more severe in experimental animals (ischemic changes extending deeper into the intestinal wall) than in control animals. CONCLUSION: Direct visualization of spilled gastrointestinal gadopentetate dimeglumine helped discriminate ischemic from control rats in this model.

Administration, Oral↗
PubMed1996

MR angiography with three-dimensional MR digital subtraction angiography.

We have developed a time-resolved, contrast-enhanced, volume-imaging technique for magnetic resonance (MR) angiography, known as three-dimensional (3D) MR digital subtraction angiography (DSA). This technique greatly improves MR angiogram quality because it combines the injection of a contrast agent with the ability to image the temporal passage of this agent and, thereby, obviates the need for timing scans or other complicated synchronization schemes. Three-dimensional MR DSA also represents a potential improvement in the sense that, relative to DSA and computed tomography (CT) angiography, the contrast agent is less toxic. Additionally, unlike CT angiography, images may be acquired during the passage of the contrast agent. Therefore, 3D MR DSA shows the sequential passage of contrast through the arterial and venous system, followed by uptake in various organs. Unlike conventional DSA, 3D MR DSA imaging acquires full volume datasets, which allows subsequent reprojection and reformatting. Because images are obtained at approximately 2-6 s time intervals using a temporal aperture on the order of several seconds, motion (such as respiration) causes only a temporary disruption of image quality, similar to that observed in MR fluoroscopy. These temporal characteristics also make the proposed sequence insensitive to variations in the shape and timing of the contrast-pass curve. Although the individual time-resolved images will have somewhat decreased signal-to-noise ratio (SNR) relative to nontime-resolved scans collected in the same acquisition time, the SNR improvement due to the gadolinium appears to accommodate this trade-off. Additionally, if motion between successive images is small, then the full suite of temporal processing schemes, previously investigated in connection with DSA and time-resolved two-dimensional (2D) MR, such as mask mode subtraction, simple matched filtering and Eigen filtering, can be used to obtain composite images. These derived images generally have an increased SNR or negligible venous signal if an arterial-phase image is not obtained in the early time-resolved images. In summary, 3D MR DSA will significantly advance MR angiography because of the following intrinsic advantages: (1) improved signal-to-noise, (2) scan orientation may be chosen independently of the direction of blood flow, (3) uniform vascular signal, even from regions of complex flow, (4) minimization of motion artifacts, (5) greatly reduced sensitivity to variation in the shape and timing of the contrast bolus, (6) ability to be reformatted or reprojected, and (7) ability to apply a variety of temporal postprocessing techniques.

Angiography, Digital Subtraction↗
PubMed1995

Multicenter trial to evaluate vascular magnetic resonance angiography of the lower extremity. American College of Radiology Rapid Technology Assessment Group.

OBJECTIVES: To assess the value of magnetic resonance angiography (MRA) in presurgical evaluation of patients with severe lower limb atherosclerotic occlusive disease and to assess the feasibility of rapidly conducting rigorous technology assessment. DESIGN: Blinded, prospective study of consecutive patients with signs or symptoms of severe infrainguinal peripheral vascular disease who were candidates for percutaneous or surgical intervention. Using both descriptive statistics and multivariate logistic analyses, MRA was compared with contrast arteriography (CA) (the current technique) for imaging 15 arterial segments of the leg and foot. Intraoperative contrast angiography was the "gold" standard. Also studied was the effect of adding MRA to the information used in planning treatment. SETTING: Six US hospitals, one a community hospital. PATIENTS: A total of 155; 84% with either rest pain or tissue loss. RESULTS: Sensitivity in distinguishing patent segments from completely occluded segments was 83% for CA and 85% for MRA; both had 81% specificity. For distinguishing near-normal segments (suitable as bypass graft termini), CA was less sensitive than MRA (77% vs 82%), but more specific (92% vs 84%). After adjusting for same-reader effects, odds of correctly distinguishing patent segments were 1.6 times as great for MRA as for CA (P < .01); for distinguishing near-normal segments, the odds for CA were 1.5 times as great as for MRA (P < .05). The addition of MRA changed the treatment plan in 13% of patients; in 86% of these cases, the surgery actually performed indicated that the MRA-inclusive plan was superior. CONCLUSIONS: Individually, MRA and CA are approximately equivalent in diagnostic accuracy. The addition of MRA to treatment plans based only on CA and other diagnostic information clearly improves the plans. Completed in 15 months (as planned), our study demonstrates the feasibility of conducting rigorous technology assessment rapidly enough to be timely even in fields in which diagnostic and treatment techniques are rapidly changing.

Adult↗
PubMed1995

Coronary MRI with a respiratory feedback monitor: the 2D imaging case.

The inability to return the heart to the same position for all breath-holds during 2D coronary MR imaging can result in imaging different locations than desired. This can lead to problems such as (i) missing a whole vessel, or a part of it, (ii) misaligning segments of vessels imaged in different breath-holds, and (iii) degrading image quality when a single slice is acquired in multiple breath-holds. To reduce inconsistencies in the breath-hold level, we designed a respiratory feedback monitor (RFM) that uses a bellows to monitor the circumference of the subject's chest. When the circumference of the subject's chest is within preset limits, an audio signal alerts subjects to hold their breath at that position. Use of the RFM significantly reduces the problems caused by inconsistent breath-holds and the number of breath-holds for an examination in 2D coronary MR imaging.

Adult↗
PubMed1995

Respiratory blur in 3D coronary MR imaging.

3D MR imaging of coronary arteries has the potential to provide both high resolution and high signal-to-noise ratio, but it is very susceptible to respiratory artifacts, especially respiratory blurring. Resolution loss caused by respiratory blurring in 3D coronary imaging is analyzed theoretically and verified experimentally. Under normal respiration, the width for any Gaussian point spread function is increased to a new value that is at least several millimeters (about 3-4 mm). In vivo studies were performed to compare respiratory pseudo-gated 3D acquisition with breath-hold 2D acquisition. On average, the overall quality of a pseudo-gated 3D image is worse than that of the corresponding breath-hold 2D image (P = 0.005). In most cases, respiratory blur caused coronary arteries in pseudo-gated 3D data to have lower resolution than in breath-hold 2D data.

Adult↗
PubMed1995

Development of a phased-array coil for the lower extremities.

A phased-array coil was developed to facilitate imaging the vasculature of the lower extremities. The array consists of four surface coils placed in a Plexiglas "I-beam" frame that are configured to allow bilateral studies with up to a 40-cm field of view (FOV). Data from phantoms indicate an increase in signal-to-noise ratio (SNR) in the regions of interest by an average factor of 2.8 +/- 0.9 over that of the body coil. Preliminary in vivo data have also been obtained from n = 8 subjects and demonstrate significant improvements in image quality. The coil design described here should lead to reduced scan times through the ability to image both legs simultaneously with less need for patient repositioning.

Angiography, Digital Subtraction↗
PubMed1995

Magnetic resonance angiography in progressive renal failure: a technology assessment.

The objective of this study was to assess the cost-effectiveness of magnetic resonance angiography (MRA) imaging for renal artery stenosis (RAS) in people with progressive renal failure (PRF). We created a simulation model to determine the incremental cost-effectiveness of MRA screening in PRF compared with the fallback strategy of not screening. Costs, probabilities, and utilities were estimated from the literature and from institutional data. A three-state Markov model was used to simulate the progression from PRF to end-stage renal disease and death. In our baseline analysis, assuming a sensitivity of 0.85 and a specificity of 0.8 of MRA for RAS, we obtained an incremental cost-effectiveness of MRA screening compared with no screening of $2,214 per quality-adjusted life year saved, which is less than many commonly performed procedures. Under our baseline assumptions, if the receiver-operating characteristic curve of MRA for RAS is better than the chance curve, then MRA screening would be cost-effective. The analysis was most sensitive to assumptions about renal function after correction of RAS and prevalence of RAS, although the results show that MRA remains cost-effective for reasonable ranges of these assumptions. The use of MRA in PRF would be a worthwhile investment of resources in comparison with many currently funded procedures. The expense and morbidity associated with end-stage renal disease make any reasonable way of delaying or preventing the disease worth examining in detail.

Cost-Benefit Analysis↗