Biomedical subjects

J A Tkach

Publications and source records attributed to J A Tkach.

At least 19 recordsLinked to original sources

PubMed1999

Development and characterization of an adult model of obstructive hydrocephalus.

While hydrocephalus is common in adults its pathophysiology is not fully understood and its treatment remains problematic. Previous animal models have been acute, developmental, or involved non-specific blockage or inflammation and are not suitable for study of chronic adult-onset hydrocephalus. In this study, we describe the development of a canine model which allows basic physiological studies along with diagnostic and treatment procedures via surgical occlusion of the fourth ventricle with a bolus injection of cyanoacrylic gel glue. A total of 26 adult male canine mongrels were used for the induction of chronic hydrocephalus and were monitored for 1-12 weeks post-induction using magnetic resonance imaging (MRI), intracranial pressure measurements, and neurological fitness assessments. Of these, 81% (21/26) developed hydrocephalus that was mild (N = 6), moderate (N = 7), or severe (N = 8). Pressures were mild and transiently elevated, and brain compliance decreased. Clinical symptoms were also mild and transient. This model is unique in its focal obstruction without local compression or general inflammation and should facilitate the study of the pathophysiology and treatment of chronic adult-onset hydrocephalus.

Animals↗

PubMed1999

MR characteristics of oxidized cellulose (Surgicel).

OBJECTIVE: Oxidized regenerated cellulose (Surgicel) is one of the most commonly used bioabsorbable topical hemostatic agents. Surgicel may mimic an abscess on both CT and sonography when a patient undergoes imaging early in the postoperative period. The objective of our study was to describe the appearance of Surgicel on postoperative MR imaging. CONCLUSION: Surgicel has a short relaxation time on T2-weighted images, resulting in low signal intensity in the early postoperative period. MR imaging may be helpful in differentiating Surgicel from an abscess and therefore in preventing unnecessary attempts at aspiration.

Abscess↗

PubMed1998

Differential ventricular expansion in hydrocephalus.

In the large canine model of acquired obstructive hydrocephalus that we have developed recently, computer-assisted 3-dimensional morphometry has been performed on T1-weighted Spin Echo MRI images from adult dogs before and after the induction of hydrocephalus. To date, 7 hydrocephalic animals have been analyzed that survived 7-83 days (median = 54) after receiving injections of cyanoacrylate glue into the anterior fourth ventricle. Measurements were obtained from lateral, 3rd, and 4th ventricles. The volumes of the left and right lateral ventricles were symmetrical before and after induction. Mean lateral ventricle volume increased 424% from a baseline of 0.63 cc to a post-induction value of 3.30 cc (p < 0.01 with unpaired t-test). In contrast, the 3rd ventricle expanded 187% from a mean of 0.15 cc to 0.43 cc (p < 0.05). The combined volume of the lateral and 3rd ventricles increased 369% from a mean of 0.78 cc to 3.69 cc (p < 0.01). Evans' ratios, which are used routinely in the clinical setting, were also obtained from linear measurements of the lateral ventricle width divided by brain width at the level of the foramen of Monro. These values exhibited only a 94% increase from mean baseline ratios of 0.17 to post-induction ratios of 0.33 (p < 0.05). These findings indicate that in mechanically-induced obstructive hydrocephalus the relative expansion of the lateral ventricles is greater than that of the 3rd ventricle. In addition, volumetric measurements of the lateral and 3rd ventricles suggest that the extent of ventriculomegaly is 3-4 times greater than estimated by Evans' ratios.

Animals↗

PubMed1997

Segmented three-dimensional echo-planar flow imaging of the cervical carotid arteries.

PURPOSE: To implement and assess the application of segmented three-dimensional echo-planar MR imaging time-of-flight flow sequences for studying the anatomy of the cervical carotid arteries at 1.5 T. METHODS: The 3-D echo-planar sequences were segmented along the in-plane phase-encoding direction. Echo train lengths (ETLs) of 3 and 5 and signal bandwidths of +/-25, +/-33, and +/-50 KHz were tested along with a conventional (ETL = 1) 3-D MR flow study in six healthy volunteers and in five patients with known arteriosclerotic disease involving the carotid bifurcation as confirmed by conventional angiography. The volunteer data were used to rank the techniques with respect to vessel dimension, vessel/background contrast, and quality by four trained neuroradiologists. For the patient studies, the percentage of stenoses was measured for all MR studies and compared against the conventional angiographic data using the criteria of the North American Symptomatic Carotid Endarterectomy Trial. RESULTS: Using Wilcoxon's test statistic and a significance level of .05, we found that the conventional MR flow examination was better than the segmented techniques and that the segmented techniques with ETL of 3 were superior to their counterparts with ETL of 5. For the ETL of 3 techniques, the high-bandwidth studies were inferior to their lower bandwidth counterparts; however, there was no significant difference between the performance of the medium- and low-bandwidth sequences. The patient data revealed that the segmented techniques consistently overestimated the severity of stenosis; however, in no instance did any of the segmented examinations erroneously indicate the presence of disease. CONCLUSIONS: The reduction in acquisition time and the zero false-positive rate we obtained suggest that segmented 3-D echo-planar MR flow techniques may be used as a screening/locating study for cervical carotid artery disease.

Adult↗

PubMed1994

Three-dimensional time-of-flight MR angiography with variable TE (VARIETE) for fat signal reduction.

Uniform fat saturation over a large region of interest remains a problem in time-of-flight (TOF) magnetic resonance angiography applications. We demonstrate that a variable echo time with an opposed phase value at low spatial slice select frequencies can effectively reduce most of the fat signal in an otherwise standard three-dimensional TOF acquisition. We evaluated this method at 1.5 T using a short TE = 5.3 ms and a long TE = 6.75 ms for different values of the slice encoding gradient (i.e., different kz values). Shorter echo time (TE = 5.3 msec) was used at higher spatial slice select frequencies, but all echoes have the same gradient structures. By keeping the number of slice encoding steps with longer echoes to a minimum, field inhomogeneity effects on flow compensation remained small. A magnetization transfer saturation pulse was used to suppress signal of brain parenchyma. Overall, highly uniform and selective fat signal reduction was obtained while maintaining superior flow compensation in all volunteer studies.

Adipose Tissue↗

PubMed1994

Optimizing three-dimensional time-of-flight MR angiography with variable repetition time.

PURPOSE: To implement and evaluate a variable repetition time (TR) modification of three-dimensional (3D) time-of-flight (TOF) magnetic resonance (MR) angiography. MATERIALS AND METHODS: Variable- and constant-TR 3D TOF MR angiography of the intracranial vasculature was performed in 20 healthy volunteers and 10 patients with known or suspected cerebrovascular disease. Total acquisition time was short and held constant for all studies. Frequency-selective fat saturation (FS) and magnetization transfer saturation (MTS) pulses were applied. The associated maximum-intensity projection (MIP) images were evaluated quantitatively for contrast-to-noise (C/N) and qualitatively for depiction of vessels that exhibit slow flow. RESULTS: Variable-TR MIP images exhibited improved C/N and depiction of small peripheral vessels and venous structures when compared with constant-TR MIP images. These observations were consistent for all studies. CONCLUSIONS: The variable-TR modification improved the depiction of intracranial vessels that exhibit slow flow.

Adult↗

PubMed1994

Sequential three-dimensional time-of-flight MR angiography of the carotid arteries: value of variable excitation and postprocessing in reducing venetian blind artifact.

OBJECTIVE: Multiple overlapping three-dimensional (3D) time-of-flight carotid MR angiography potentially combines many of the desirable features of two-dimensional (2D) and single-volume 3D MR angiographic imaging techniques. Yet the maximum-intensity-projection images from such acquisitions are often degraded by artifact due to nonuniform signal intensity of contiguous imaging volumes and inadequate, yet arduous, postprocessing. The former has been termed venetian blind artifact. To date, the severity of the artifact has been minimized by the use of very thin slabs with a large percentage of overlap. However, the artifact typically is still appreciable, and the required acquisition and postprocessing times are increased. The purpose of this study was to examine the value of technical modifications of both the multislab acquisition and postprocessing procedures to reduce this artifact on images of healthy volunteers. SUBJECTS AND METHODS: Spatially variable RF pulses along the direction of flow were applied as excitation pulses in the multislab time-of-flight MR angiographic acquisitions to compensate for the nonuniform blood signal intensity caused by spin saturation. An automatic postprocessing technique was used to optimally combine the image data in overlapping slices by selecting the higher-intensity pixel of the two on a pixel-by-pixel basis. Ratios of the standard deviation of signal intensity to the mean signal intensity were computed as a function of RF profile and postprocessing method along the long axes of the arteries to measure the uniformity of the signal intensity of the blood. The spatially variable and sinc RF pulse acquisitions, combined with automatic and conventional manual postprocessing, were compared. RESULTS: Compared with the sinc pulse acquisition, the MR angiograms acquired with spatially variable excitation pulses improved the signal uniformity of the arteries with thicker volumes and less overlap, thereby reducing the acquisition time by 25% for similar spatial coverage. When used with the automatic postprocessing technique, the severity of the venetian blind artifact on maximum-intensity-projection images was minimized and the postprocessing time was reduced by roughly a factor of 5. CONCLUSION: The combined use of spatially variable excitation pulses and an automatic postprocessing technique can improve the uniformity of the signal from blood across the slab and allow thicker slabs to be acquired with less overlap. Data acquisition and postprocessing times can be reduced significantly. This work suggests it may be possible to easily produce overlapping 3D MR angiograms that should be superior to conventional 2D and 3D studies.

Adult↗

PubMed1994

Fast spin-echo imaging of the neck: comparison with conventional spin-echo, utility of fat suppression, and evaluation of tissue contrast characteristics.

PURPOSE: To determine whether fast spin-echo sequences could replace conventional spin-echo methods in the evaluation of head and neck neoplasms and associated adenopathy and to evaluate differences in tissue contrast characteristics between conventional spin-echo and fast spin-echo examinations of head and neck disease. METHODS: Twenty-seven patients with squamous cell carcinoma were imaged on a 1.5-T imager with both conventional spin-echo and fast spin-echo sequences with identical section thickness and position. Twenty-one of the 27 fast spin-echo studies were performed with frequency-selective fat suppression. Three radiologists independently evaluated the images using a five-point scale to compare primary lesion margin definition and conspicuity, lymph node margin definition and conspicuity, gross motion artifact, and flow artifact. Quantitative percent contrast and contrast-to-noise ratios were calculated and compared in 7 cases with fat-suppressed fast spin-echo. RESULTS: Fast spin-echo was preferred by all three readers for lesion margin conspicuity and lymph node conspicuity. Gross motion and flow artifact demonstrated trends toward reader preference for fast spin-echo. Quantitative contrast values for fast spin-echo were significantly greater than those for conventional spin-echo. CONCLUSIONS: Fast spin-echo with fat suppression can replace conventional spin-echo at a time savings of more than 50% and improves tissue contrast and the conspicuity and definition of margins for primary lesions and lymph nodes. Fat-suppression heterogeneity remains the major limitation of this technique. Thus, careful attention to fat-suppression failure and unwanted water saturation is essential.

Adipose Tissue↗

PubMed1993

Intracranial MR angiography: application of magnetization transfer contrast and fat saturation to short gradient-echo, velocity-compensated sequences.

To circumvent artifacts related to saturation, turbulence, and data processing in three-dimensional (3D) time-of-flight (TOF) magnetic resonance (MR) angiography, the authors used a high-resolution (256 x 512), short, asymmetric velocity-compensated gradient-echo sequence, magnetization transfer contrast (MTC) between all excitation pulses to suppress brain parenchyma signal, and frequency-offset fat saturation (FS) to suppress fat signal. Signal intensity and blood-background contrast was measured in several regions of interest on conventional 3D TOF MR images, MTC images, and those with MTC and FS in seven volunteers. One patient each with an arteriovenous malformation, an aneurysm, and stenosis in the intracranial vascular system underwent the protocols with an echo time (TE) of 5 msec and then a TE of 8 msec. Use of the 5-msec-TE sequence with MTC and FS led to reduced problems associated with signal void in areas of fast flow in patients. In volunteers and patients, vascular visualization was superior to that of current 3D TOF techniques.

Artifacts↗

PubMed1990

Optimizing blood vessel contrast in fast three-dimensional MRI.

Magnetic resonance angiography has matured to the point where clinically useful images can be acquired in half an hour or less. In this paper, the role of 3D imaging techniques is primarily considered. Specifically, the optimal imaging parameters, sequences, and reconstruction techniques are evaluated for moving spins. A variant of FISP known as ROAST with low flip angles, short repeat times, and a thick slab has been found to yield the best 3D survey scan of the cranial vessels with roughly 1 X 1 X 1-mm3 resolution in each of the processed images (slices). For the faster flowing carotids, a sagittal scout with as short a TE as possible is required to avoid spin dephasing. Localization is accomplished in both cases by acquiring thin slab 3D, thin partition, larger flip angle, longer repeat time FLASH sequences. Different choices of dephase/rephase sequences and directions are also reviewed. These choices are discussed from a practical and theoretical perspective. In particular, improvements in contrast and resolution are evaluated using half-Fourier, 512 acquisition, small fields of view and constrained reconstruction for both rephased gradient echo sequences and dephased thin slice long TR spin-echo sequences. A resolution of 0.5-0.75 mm is recommended to obtain sufficient image quality for consistent clinical interpretation of stenoses and vessel abnormalities.

Blood Flow Velocity↗

PubMed1990

Steady-state free precession imaging in the presence of motion: application for improved visualization of the cerebrospinal fluid.

The authors discuss the appropriate FISP (fast imaging with steady-state precession) sequence structure to maintain constant phase at the radio-frequency pulse in the presence of motion. They present preliminary results of its application to head and spine imaging in an effort to maintain contrast between the cerebrospinal fluid (CSF) and the soft tissue. In the usual application of these FISP-like sequences, the gradient structure is modified to avoid unwanted signal (and contrast) variations due to field inhomogeneities. This change makes the signal sensitive to motion with a resulting decrease in signal intensity for moving tissue. The expected high contrast at large flip angles for tissues with low T1/T2 ratios such as CSF is not obtained. The technique discussed here overcomes the effects of field inhomogeneities and compensates for moving spins so that the transverse steady-state equilibrium and hence high contrast are obtained simultaneously.

Cerebrospinal Fluid↗

PubMed1990

Fast MR imaging: techniques and clinical applications.

Fast MR imaging has matured in the past few years and is now of established value for several aspects of clinical MR imaging. The initial impetus for rapid imaging was to reduce scan times. Today its usefulness includes reducing motion artifacts, improved contrast per unit time, three-dimensional (3-D) imaging, real-time imaging, cine-mode imaging, and flow imaging. The focus of this review is on short-TR steady-state gradient-echo imaging. We discuss the basic sequence design of the mainstream fast techniques. Many important applications exist, including gadopentetate dimeglumine-enhanced MR imaging of the brain and spine, subsecond imaging of real-time applications, myelographic imaging of the spine, cardiac cine-mode imaging; 3-D musculoskeletal (knee) imaging, 3-D pituitary imaging; two-dimensional and 3-D body imaging; 3-D carotid and intravascular imaging, and reformatting 3-D images into arbitrary planes.

Fourier Analysis↗

PubMed1990

Functional cardiovascular evaluation by magnetic resonance imaging.

The pursuit of capabilities for the evaluation of functional aspects of cardiovascular disease by MRI has resulted in the development and implementation of a number of interesting techniques that can be performed on a conventional scanner. Some currently available techniques emphasize the production of anatomically accurate images representing different phases of the cardiac cycle; others demonstrate physical changes within the acquired data that reflect motion, such as blood flow. Magnitude data from spin-echo and gradient-echo sequences can be used to produce dynamic images of the cardiovascular system. Phase data can be used to generate flow-based images reflecting the movement of blood protons. These techniques can be applied in the evaluation of ventricular function, valve function, or functional abnormalities in either congenital cardiovascular disease or great artery disease.

Cardiovascular Diseases↗

PubMed1989

Reduction of T2* dephasing in gradient field-echo imaging.

Fast gradient field-echo imaging is becoming more common in morphologic studies but is not as widespread as might be hoped because of its susceptibility to local field inhomogeneities that lead to spin dephasing (reducing T2 to T2*) and geometric distortion (frequency misregistration). These problems are manifested in both the in-plane and section-select directions. The authors show that reversal of many of these adverse effects is possible through the acquisition of the gradient field echo in a three- and four-dimensional mode for a fixed echo time (TE), since phase-encoding leads to the capture of the echo within the sampling window. This echo centering is shown to be equivalent to the reduction of dephasing across a pixel. Improvements are also obtained by reducing TE to as short a value as possible.

Humans↗

PubMed1989

Three-dimensional (volume) gradient-echo imaging of the carotid bifurcation: preliminary clinical experience.

This study was designed to test the accuracy of magnetic resonance (MR) imaging with a FLASH (fast low-angle shot) 40 degrees volume pulse sequence by comparing it with intraarterial digital subtraction angiography (DSA) in patients with suspected carotid artery stenoses. Fifteen patients referred for evaluation of anterior circulation in cerebrovascular disease composed the pilot group. Twelve patients underwent correlative intraarterial DSA examinations. The FLASH volume sequence, with an echo time of 7.7 seconds, produced high-signal-intensity vascular images for 28 of 30 bifurcations. Of the 24 carotid bifurcations studied with DSA, 22 were depicted with MR angiography. Among the depicted bifurcations, 21 showed good correlation with the DSA images. These included four of four normal bifurcations, three of three with mild stenosis, four of four with moderate stenosis, eight of nine with severe stenosis, and two of two with occlusions. With respect to ulceration, three of four MR angiographic studies showed good correlation with DSA images. This preliminary experience indicates that the method is reproducible and capable of delineating carotid lesions in patients and that it can be performed in conjunction with conventional spin-echo imaging of the brain with only a small increase in patient examination time.

Adult↗

PubMedDate not supplied

Optimization of three-dimensional T1-weighted gradient-echo imaging of the cervical spine.

Various parameters of the three-dimensional (3D) T1-weighted magnetization-prepared rapid acquisition gradient-echo (MP-RAGE) sequence were evaluated to improve spatial resolution while maintaining T1 contrast and a short examination time in imaging of the cervical spine in volunteers. The most dramatic improvements in image resolution occurred by decreasing section thickness to 1.2 mm and increasing the in-plane matrix to 192 x 256, with a 230-mm field of view. The increase in imaging time due to the increased matrix was offset by the elimination of the preparation pulse and wait time, without dramatic changes in contrast-to-noise ratio or overall image quality. Optimum parameters included elimination of the preparation pulse and wait time, 12 degrees flip angle, 192 x 256 matrix, 1.2-mm section thickness, nonselective excitation (coronal acquisition), RF spoiling, and standard k-space ordering, for an examination time of 5 minutes 21 seconds.

Cervical Vertebrae↗

PubMedDate not supplied

Three-dimensional time-of-flight MR angiography with a specialized gradient head coil.

A gradient head coil has been developed, incorporating two independent gradients within the conventional body coil of the magnetic resonance (MR) system, with reduced rise times (200 microseconds) and maximum amplitudes of 37 and 18 mT/m in the z and y directions, respectively. This gradient coil was systematically evaluated by testing two-dimensional (2D) and three-dimensional (3D) time-of-flight (TOF) MR angiography sequences applied to a pulsatile flow phantom simulating a carotid stenosis and the intracranial vasculature. When standard 2D and 3D TOF MR angiography techniques were used to image the carotid stenosis model, dramatic signal loss in the stenotic segment and a large flow void distal to the stenosis were seen. The shorter (3.8 msec) absolute echo times (TEs) achievable with the gradient coil in 3D sequences substantially reduced the phase dispersion and associated signal loss in the region of stenosis. Shorter TEs alone (3.2 msec) did not minimize signal loss, and first-order flow compensation in the read and section-select directions provided further improvements (despite slightly longer TEs). Reduction of TEs in 2D sequences yielded relatively poor results regardless of the refocusing scheme or TE. This study confirms the predicted benefits of a dedicated coil with improved gradient capabilities for 3D MR angiography. The study suggests the limitations of 2D TOF MR angiography in the evaluation of severe stenoses.

Carotid Arteries↗

PubMedDate not supplied

Pulse sequence strategies for vascular contrast in time-of-flight carotid MR angiography.

A systematic evaluation in healthy volunteers of the relative efficacy of various techniques for background suppression to improve two-dimensional (2D) and three-dimensional (3D) time-of-flight magnetic resonance angiography of the cervical carotid arteries was performed. Conventional 2D and 3D FISP (fast imaging with steady-state precession) sequences with flow compensation were compared with modifications of these sequences, including a tracking saturation pulse (2D), prolonged absolute TEs for fat suppression based on T2* decay (2D and 3D), frequency-selective saturation of fat (2D and 3D), in-plane spatial saturation (2D), and magnetization transfer contrast (2D and 3D). The tracking saturation pulse and slight overlap of the excitation sections provided uniform background suppression without impairing depiction of the morphology of the cervical carotid arteries. Frequency-selective fat saturation was the most effective background suppression scheme among the 2D and 3D techniques but was occasionally compromised by local field inhomogeneities. Magnetization transfer contrast provided little suppression of stationary tissues in the neck because of the intrinsic limitations of the coil. In-plane spatial saturation yielded the highest background suppression but reduced apparent arterial diameters and could not be implemented in a 3D version. The T2* decay method not only reduced the apparent size of the vessels but also their signal intensity.

Adult↗