Biomedical subjects

T L Chenevert

Publications and source records attributed to T L Chenevert.

67 records · Page 4Linked to original sources

PubMed1988

Mediastinal lymph nodes: relaxation time/pathologic correlation and implications in staging of lung cancer with MR imaging.

The authors measured the T1 and T2 relaxation times of freshly excised human mediastinal lymph nodes to determine whether the times are clinically useful in distinguishing benign from malignant nodes. All measurements were performed at 20 MHz and 40 degrees C, within 45 minutes of lymph node excision. Mean T1 and T2 relaxation times of 99 benign nodes were 566 msec (standard deviation [SD], 117 msec) and 92 msec (SD, 29 msec), respectively. For the 16 malignant nodes, these times were 640 msec (SD, 138 msec) and 105 msec (SD, 26 msec), respectively (P less than .05 for difference in T1 times, P greater than .05 for difference in T2 times). Histograms showed considerable overlap in the relaxation times of benign and malignant nodes such that absolute measurement of these times will likely be of limited clinical value.

Humans↗

PubMed1988

In vivo and in vitro MR imaging of renal tumors: histopathologic correlation and pulse sequence optimization.

Magnetic resonance (MR) imaging has given mixed results in the detection of renal masses. To identify the reasons for this and to determine the optimal pulse sequences for evaluating renal tumors, the authors imaged 12 primary renal tumors in vivo and 17 in vitro at 0.35 T. Histopathologic findings for each specimen were closely correlated with the MR images. Four of seven solid tumors imaged in vivo were isointense with surrounding normal renal parenchyma at all pulse sequences. The other three tumors were hyperintense in vivo at T2-weighted sequences. At heavily T2-weighted sequences eight solid tumors were hyperintense in vitro and four were hypointense. There was no correlation between signal intensity and specific tissue type or histologic pattern for solid tumors. The five cystic tumors were well seen both in vivo and in vitro on T2-weighted images. However, the signal intensity of the cyst fluid was an unreliable indicator of benignancy. SE MR imaging at 0.35 T has significant limitations in the detection of solid renal masses.

Carcinoma, Renal Cell↗

PubMed1987

Chemical shift-based true water and fat images: regional phase correction of modified spin-echo MR images.

The process of separating water and fat signal in magnetic resonance images with the Dixon pulse sequence is hindered by phase errors in the water-fat opposed image. These errors arise from static field inhomogeneities, varying magnetic susceptibilities of different body tissues, and other causes. Phase correction performed with data from phantom imaging can compensate for static field inhomogeneities but not for the other effects. A regional phase-correction algorithm is presented that removes non-chemical-shift phase effects and produces true water and fat images. The technique has been applied with good results to 21 patients and healthy volunteers. The images include ones of the abdomen, knees, hips, spine, and head. This method of regional phase correction is an efficacious way of producing separate water and fat images with the Dixon pulse sequence.

Adipose Tissue↗

PubMed1987

Pheochromocytoma and paraganglioma: comparison of MR imaging with CT and I-131 MIBG scintigraphy.

To ascertain the magnetic resonance (MR) imaging characteristics of pheochromocytomas and paragangliomas and to compare MR with computed tomography (CT) and iodine-131 metaiodobenzylguanidine (I-131 MIBG), 19 patients (18 with pheochromocytomas, one with a paraganglioma) were studied. The 18 patients with pheochromocytomas had had positive findings with I-131 MIBG scintigraphy. Abdominal pheochromocytomas were generally hypointense compared with normal liver on T1-weighted MR images and extremely hyperintense on T2-weighted MR images. MR imaging was preferable to CT in the evaluation of primary pheochromocytomas due to superior tissue characterization, particularly in the patient with hypertension and borderline catecholamine levels. For patients with recurrent or metastatic disease, the data suggest that I-131 MIBG scintigraphy is the examination of choice.

3-Iodobenzylguanidine↗

PubMed1986

Tissue characterization of the testes using ultrasonic CT. Work in progress.

Ultrasonic computed tomography (UCT) can aid in characterizing tissue for the detection and diagnosis of leukemic infiltration of the testes. Preliminary studies in 6 healthy adults and 26 patients (3-20 years old) with leukemia or non-Hodgkin lymphoma suggest that elevated speed of sound in the testis may be an indicator of leukemic infiltration. UCT may become an important screening method for detecting testicular involvement. In long-term follow-up, UCT can be performed more frequently and easily than biopsy, which is the current screening method.

Adult↗

PubMed1984

Ultrasonic computed tomography of the breast. Improvement of image quality by use of cross-correlation time-of-flight and phase-insensitive attenuation measurements.

The authors present two methods of reducing refraction artifacts in ultrasonic computed tomography of the breast. Measuring the time of flight of sonic pulses by cross-correlation instead of leading-edge detection significantly reduces distortion and improves resolution in speed-of-sound images. Phase-insensitive reception of pulse energy across a large aperture array is shown to be superior to a conventional single-element transducer for attenuation imaging.

Breast Diseases↗

PubMedDate not supplied

Feasibility of MR diffusion studies in the kidney.

Renal apparent diffusion coefficients (ADCs) were anisotropic within and significantly different between cortex and medulla using a relatively motion-insensitive one-dimensional technique in 20 volunteers. ADC values ranged from 1.79 +/- .39 to 2.95 +/- .58 (x 10(-3)mm2/sec), relatively high but similar to other reports. Further investigation may help clarify this data, and determine whether the findings result from diffusion properties (and/or radially oriented parenchymal architecture), or artifacts due to factors such as bulk motion.

Diffusion↗

PubMedDate not supplied

The effects of time varying intravascular signal intensity and k-space acquisition order on three-dimensional MR angiography image quality.

The optimum infusion timing and k-space ordering for obtaining gadolinium-enhanced three-dimensional MR angiograms was determined through computer modeling using temporal contrast characteristics obtained from patient gadolinium infusion data. The effects of bolus timing were evaluated by varying the relationship between peak intravascular gadolinium concentration and the time at which the center of k space was acquired (tck) for sequential and centric acquisition techniques. Flow phantom experiments were performed to validate the theoretical computations. Gadolinium concentration at the time of central k-space acquisition determines intravascular signal intensity. Artifacts, including vessel broadening and edge ringing, depend on the order in which k space is collected and on how rapidly the gadolinium concentration changes. Artifacts are greatest when the center of k space is acquired before the intravascular gadolinium peak. Application of the optimal infusion timing results in preferential arterial enhancement with a minimum of artifacts in patients undergoing MR angiography.

Algorithms↗

PubMedDate not supplied

MR angiography with an ultrasmall superparamagnetic iron oxide blood pool agent.

The purpose of the study was to investigate the use of a dextran-coated ultrasmall superparamagnetic iron oxide (USPIO) as a blood pool contrast agent for thoracic and abdominal MR angiography. Abdominal and thoracic MR angiography was performed in six healthy volunteers using two-dimensional and three-dimensional spoiled gradient echo (SPGR) sequences before and after intravenous administration of USPIO. Doses ranged from 1.1 to 2.6 mg Fe/kg. Flip angle was varied from 20 to 60 degrees. Subjective image quality, analysis of signal-to-noise ratio (SNR), and blood T1 relaxation times were measured. USPIO significantly lowered the T1 of blood (from 1,210 ms precontrast to 159 ms postcontrast at a dose of 2.6 mg Fe/kg) (P < .01). Image quality on coronal fast three-dimensional breath-hold SPGR images of the abdomen increased with increasing dose and was maximum at the highest dose, producing an aortic SNR of 9.6 compared to 1.8 precontrast. Axial two-dimensional time-of-flight (TOF) aortic SNR was reduced significantly from 13 on precontrast to 6 on the postcontrast images at the highest dose (P < .05) due to T2* shortening effects. There was little flip angle dependence on image quality. Due to the T1 shortening effect and long intravascular half-life, USPIO improved visualization of vascular anatomy using three-dimensional fast SPGR imaging. The echo time must be minimized to minimize signal loss from T2* shortening effects. The blood pool distribution of USPIO is useful for equilibrium-phase MR angiography.

Abdomen↗

PubMedDate not supplied

The effects of incomplete breath-holding on 3D MR image quality.

The purpose of this study was to investigate how fast three-dimensional (3D) MR image quality is affected by breath-holding and to develop an optimal breath-holding strategy that minimizes artifact in the event of an incomplete breath-hold. A computer model was developed to study variable-duration breath-holds during fast 3D imaging. Modeling was validated by 3D gradient-echo imaging performed on 10 volunteers. Signal-to-noise ratio (SNR) and image blur were measured for both simulated and clinical images. Insights gained were applied to clinical 3D gadolinium-enhanced MR angiography. Breath-holding significantly improved abdominal 3D MR image quality. Most of this benefit could be achieved with a breath-hold fraction of 50% if it occurred during acquisition of central k space. Breath-holding during peripheral k-space acquisition, however, had no significant benefit. Respiratory motion artifact on fast 3D MRI occurring when a patient fails to suspend respiration for the entire scan duration can be minimized by collecting central k space first (centric acquisition) so that premature breathing affects only the acquisition of peripheral k space.

Adult↗

PubMedDate not supplied

Contrast-enhanced MR angiography.

Gadolinium-enhanced MRA in the abdomen provides a safe, fast, and cost-efficient alternative to conventional diagnostic angiography. Numerous recent advances allow for high-resolution breath-hold imaging with optimal arterial-phase gadolinium bolus timing. As the technique is refined further, greater spatial and temporal resolution will become possible, thus increasing the usefulness of Gd-MRA.

Abdomen↗

PubMedDate not supplied

Calculation of pressure gradients from MR velocity data in a laminar flow model.

The MR velocity data obtained in a simple geometry and low Reynolds number are employed to calculate pressure gradients using the known solution of the Navier-Stokes equation. Calculations compare favorably with measured results for the simple model presented. The potential usefulness of noninvasive pressure gradient determination from quantitative analysis of imaging-derived velocity distributions is suggested.

Blood Flow Velocity↗

PubMedDate not supplied

Flow-enhanced magnetic resonance imaging.

A new technique for detecting blood flow in magnetic resonance imaging is proposed. This technique is tailored to enhance areas containing flow while suppressing static and nonsignal areas with the objective of optimizing the contrast of vascularized tumors. Unlike flow phase imaging, in-plane flow directionality (parallel versus antiparallel to applied flow gradient) is removed by the proposed method to reduce phase cancellation of flow signals. This signal loss is apt to occur in instances of complicated vascularity patterns consisting of many small vessels having multiple flow directions. The new flow-enhanced imaging method is compared to flow phase imaging by computer simulation of simple objects containing many small vessels. The results indicate that flow-enhanced imaging yields significantly greater detectability of regions of complicated small-vessel patterns than phase imaging.

Computers↗