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Biomedical subjects

D Chien

Publications and source records attributed to D Chien.

54 records · Page 3Linked to original sources
PubMed1990

0.3-second FLASH MRI of the human heart.

Flow-suppressed FLASH MR images of the human heart have been recorded within a measuring time of 0.3 s using a 2.0-T whole-body research system (Siemens Magnetom) equipped with a conventional 10 mT m-1 gradient system. Subsecond imaging times have been achieved by reducing the repetition time to TR = 4.8 ms and by lowering the spatial resolution to 64 X 128 measured data points. The flip angle of the slice-selective radiofrequency (rf) pulses was adjusted to 10 degrees. Cardiac chambers, ventricular walls, and valves are well delineated in images from a single cardiac cycle using a field of 250 mm and a slice thickness of 8 mm. No motion artifacts were observed as a consequence of the short echo time of TE = 2.8 ms. Distinction between flowing blood and solid structures has been achieved by spatial presaturation of adjacent slices using two slice-selective 90 degrees rf pulses preceding the entire imaging sequence.

Heart↗
PubMed1990

Advances in cardiac applications of subsecond flash MRI.

Flow-suppressed, subsecond FLASH MR images of the normal human heart have been obtained from single cardiac cycles using a 2.0-T whole-body MRI/MRS system (Siemens Magnetom) equipped with conventional 10 mT m-1 gradients. The present results demonstrate further technical improvements as compared to a previous report on the same subject (Magn. Reson. Med. 13:150-157; 1990). Measuring times of 139 msec and 209 msec were achieved by reducing the repetition time to TR = 4.36 msec (TE = 2.8 msec) and the spatial resolution to 32 x 128 or 48 x 128 measured data points, respectively. The flip angle was optimized to 12 degrees. Spatial pre-saturation of 60 mm thick sections adjacent to the imaging plane resulted in a suppression of the blood signal and a clear delineation of the myocardium. Oblique rotation of the imaging slice provides convenient access to the anatomical long axis and short axis views of the heart. EKG-triggered images from separate heartbeats but at different cardiac phases demonstrate that the effective time resolution is considerably less than the actual imaging time.

Electrocardiography↗
PubMed1989

Localized NMR spectroscopy in vivo. Progress and problems.

Metabolites in brain and muscle of normal human volunteers have been studied by localized 1H and 31P NMR spectroscopy in vivo. Localization was achieved by means of stimulated echo (STEAM) sequences for both water-suppressed 1H NMR (TE = 20 ms) and 31P NMR (TE = 3 ms). Volumes-of-interest and measuring times selected for brain spectroscopy were 8 mL and 6.5 min for 1H NMR and 125 mL and 13 min for 31P NMR, respectively. General problems relating to spatial localization, spectral resolution, and quantitation of in vivo NMR data are discussed with respect to the nucleus and organs under investigation. They are correlated to studies of tissue extracts obtained at field strengths of 2.35 T (Bruker Biospec) and 7.0 T (Bruker MSL 300). Human studies were performed at 2.0 T on a whole-body research system (Siemens Magnetom).

Brain↗
PubMedDate not supplied

Strategies to improve contrast in turboFLASH imaging: reordered phase encoding and k-space segmentation.

TurboFLASH (fast low-angle shot) sequences enable the acquisition of an image in a fraction of a second. However, unique to T1-weighted ultrafast imaging, the magnetization variation during image acquisition can produce artifacts along the phase-encoding direction. In this study, the signal behavior and nature of these artifacts were analyzed with various acquisition schemes to improve image contrast. The magnetization variation during image acquisition and its filtering effect on the image were simulated for three different approaches to T1-weighted turboFLASH imaging: standard turboFLASH with (a) monotonically ascending phase-encoding steps, (b) reordered phase encoding, and (c) k-space segmentation. Each of the modified data acquisition schemes has advantages. However, for subsecond imaging, reordered phase encoding produced improved image contrast over that of standard turboFLASH, and segmented k-space imaging gave superior tissue contrast compared with that of both standard and reordered turboFLASH, with imaging time that permits breath-hold studies.

Computer Simulation↗
PubMedDate not supplied

High-speed black blood imaging of vessel stenosis in the presence of pulsatile flow.

Stenosis phantoms were created to study the ability of "black blood" methods to image a vessel stenosis in the presence of pulsatile flow. Black blood images were acquired with a modified TurboFLASH (fast low-angle shot) method that eliminates flow signal by applying a set of prepulses before segmented data acquisition. With this high-speed approach, imaging can be completed within 16 seconds. This technique was compared with conventional spin-echo black blood, gradient-echo black blood, and gradient-echo bright blood methods. Loss of flow signal, which extended beyond the site of the stenosis, was seen on the gradient-echo bright blood images. The pattern of signal loss varied with the type of stenosis. Flow voids were achieved with spin-echo black blood imaging; however, substantial ghosting artifacts were seen. With gradient-echo black blood imaging, it was difficult to eliminate all flow signal, particularly for in-plane flow. The modified TurboFLASH method produced high-quality black blood images in a fraction of the time needed for spin-echo imaging. It showed no ghosting artifacts even in the presence of pulsatile flow.

Blood Flow Velocity↗
PubMedDate not supplied

Fast spin-echo studies of contrast and small-lesion definition in a liver-metastasis phantom.

A liver-metastasis model was used to study the ability of fast spin-echo (FSE) imaging to show small lesions (1 pixel in diameter) relative to conventional spin-echo imaging. FSE images of the liver-metastasis phantom were acquired with various phase-encode reordering schemes to manipulate T2 contrast. The imaging time for multisection acquisitions was 27 seconds for FSE imaging and 6 minutes 48 seconds for conventional spin-echo imaging. Computer simulations were performed to determine how the point spread function varies with the different phase-encoding orders in FSE imaging. Contrast-to-noise ratios and signal profiles of the lesions were measured as a function of the effective TE and lesion size. Experimental results and theoretical simulations showed that T2-weighted FSE imaging provides high contrast and good edge definition even for small lesions. The results indicate that FSE imaging may become a powerful method for the early detection of liver metastases.

Computer Simulation↗
PubMedDate not supplied

MR angiography with a cardiac-phase--specific acquisition window.

A method for cardiac-phase-specific magnetic resonance (MR) angiography is presented. An electronics module permits incrementing of phase-encoding gradients and storage of incoming data only during a chosen portion of the cardiac cycle. Suppression of stationary material is maintained by delivering radio-frequency pulses at constant TR throughout the cycle. Imaging of a pulsatile flow phantom demonstrates that acquiring data only during systole substantially increases the signal intensity of flowing material. In addition, phase-encoding ghost artifacts are eliminated from the neighborhood of the vessel. Image acquisition time is minimized by acquiring only the low-frequency phase-encoding lines in the cardiac-phase-specific mode. In healthy volunteers, greatly improved MR angiograms of the lower extremities are obtained. Fat saturation and magnetization transfer further enhance vessel/background contrast. Acquiring data only during systole ensures rapid inflow for all phase-encoding lines, permitting a near-longitudinal section orientation without in-plane saturation. This substantially reduces total acquisition time relative to axial acquisition.

Blood Vessels↗
PubMedDate not supplied

Novel application of breath-hold turbo spin-echo T2 MRI for detection of acute myocardial infarction.

To assess the clinical utility of the breath-hold turbo spin-echo T2-weighted MRI in patients with acute myocardial infarction, the results of MRI were compared with those of electrocardiography, coronary angiography, and thallium-201 single photon emission tomography (SPECT) in 23 patients and 5 healthy volunteers. To compare MRI and thallium-SPECT, the left ventricle was divided into five segments, and the presence of myocardial infarction was determined in each segment. MRI demonstrated an abnormally bright signal in 49 of 140 segments (five segments each from 23 patients and 5 volunteers); thallium-SPECT showed a fixed perfusion defect in 52 segments, for an 85% diagnostic concordance rate. The size of the myocardial infarction measured on MRI corresponded well to that measured on thallium-SPECT (r = .70, P < .01). Breath-hold turbo spin-echo T2 MRI can be used for detection of acute myocardial infarction in conjunction with thallium-SPECT, especially when accurate localization of lesion, increased spatial resolution, and anatomic landmarks are needed.

Coronary Angiography↗
PubMedDate not supplied

Comparison of multishot turbo spin echo and HASTE sequences for T2-weighted MRI of liver lesions.

The purpose of this study was to compare the relative usefulness of multishot turbo spin echo (TSE) and half-Fourier single-shot turbo spin echo (HASTE) for determination of optimal breath-hold fast T2-weighted technique in terms of lesion detection, lesion-to-liver contrast-to-noise ratio (CNR), and image quality. The images of TSE with and without fat suppression (FS) and of HASTE with and without FS were retrospectively reviewed for 49 patients with 128 lesions. Without FS, TSE and HASTE images allowed depiction of focal hepatic masses (112 of 128, sensitivity = 87.5%) at the same rate. TSE with FS depicted more focal lesions (115 of 128, 89.8%) than HASTE with FS (109 of 128, 85.2%), but the difference was not statistically significant (P > .05). The CNR of each lesion on HASTE sequences was greater (P < .01) than that on TSE sequences. The CNR of hemangioma was distinct from that of solid tumors and cystic lesions in all sequences, and the range of CNR in each group of pathologies overlapped less and were well separated in the HASTE sequences. HASTE sequences produced better image quality with fewer artifacts (P < .0001). The results of this study suggest that HASTE sequences allow differentiation between solid tumors, hemangiomas, and cystic lesions in terms of CNR, producing fewer image artifacts, with acceptable sensitivity in lesion detection.

Artifacts↗
PubMedDate not supplied

Fast T2-weighted liver MR imaging: comparison among breath-hold turbo-spin-echo, HASTE, and inversion recovery (IR) HASTE sequences.

BACKGROUND: To evaluate the diagnostic efficacy of fast T2-weighted magnetic resonance (MR) imaging sequences on image quality, hepatic lesion detection, and lesion conspicuity. METHODS: Three breath-hold, fast T2-weighted sequences with turbo-spin-echo (TSE), half-Fourier acquisition single-shot TSE (HASTE), and inversion recovery (IR) HASTE techniques were examined for 43 lesions in 20 consecutive patients. Evaluation was performed qualitatively on image quality and lesion detectability and quantitatively on lesion conspicuity by using lesion/liver signal-intensity and contrast-to-noise ratios. RESULTS: Artifacts were significantly less present on the HASTE sequence (p < 0.01). Both TSE and HASTE sequences detected 39 lesions (91% each); the IR HASTE sequence detected 37 (86%). IR HASTE sequence showed a significantly higher signal-intensity ratio than did the others (p < 0.01). CONCLUSIONS: Breath-hold TSE versus breath-hold HASTE or IR HASTE is still the most robust sequence in lesion detection, image quality, and lesion conspicuity. However, the HASTE sequence offers good lesion detection and image quality, and the IR HASTE has a better signal-intensity ratio.

Adolescent↗
PubMedDate not supplied

Specificity of enzyme immunoassay for hepatitis B core antibody used in screening blood donors.

Hepatitis B core antibody (anti-HBc) is currently tested by a competitive inhibition enzyme immunoassay (EIA), using recombinant DNA-produced core antigen. We have used the anti-HBc assay in routine screening of voluntary blood donors in San Francisco. The detection rate of anti-HBc was 2.08 percent. The specificity of the antibody test was established by an absorption method using purified HBc antigen (HBcAg) produced by recombinant DNA technology and covalently coupled to Sepharose 4B. Bovine serum albumin was used in the preparation of a control conjugate. The absorption test demonstrated that out of 98 anti-HBc-positive specimens, 97 could be specifically neutralized. Only one specimen was indeterminate. The absorption test was particularly useful in confirming the specificity of EIA in eight specimens inconsistently positive for anti-HBc. We conclude that the current EIA for anti-HBc is highly specific and we are of the opinion that it could be used as a rational basis for donor deferral since it gives evidence of active or previous HBV infection.

Blood Donors↗
PubMedDate not supplied

MR diffusion imaging of the human brain.

Magnetic resonance diffusion imaging reflects the water mobility at each point in a tomographic image. We have studied brain water diffusion in 18 normal volunteers to identify the different factors that can influence diffusion measurements in vivo. Our results show that both the measurement accuracy and the measured diffusion coefficient strongly depend on the experimental parameters, in particular, echo and diffusion time, cardiac gating, and diffusion encoding gradient direction.

Body Water↗
PubMedDate not supplied

High resolution cine MRI of vessel distension.

OBJECTIVE: We have evaluated the ability of high resolution cine MRI to quantify vessel distension. MATERIALS AND METHODS: Gradient echo imaging was used to acquire high resolution, cardiac-triggered imaging of the ascending aorta in normal volunteers to measure changes in the cross-sectional area of the lumen over the cardiac cycle. The effects of temporal resolution, spatial resolution, and the radiofrequency flip angle on the measurements were investigated. RESULTS: High spatial resolution improved the ability to detect changes in the luminal area. High temporal resolution allowed better tracking of maximal vessel distension. A small flip angle reduced the saturation of slow flow observed in imaging acquired using a large flip angle. CONCLUSION: Our study has demonstrated that high spatial resolution and high temporal resolution can substantially improve the measurement of vessel distension. We have overcome problems of intravoxel dephasing and spin saturation by using high spatial resolution, a short echo time with flow compensation, and a small flip angle.

Blood Vessels↗
PubMedDate not supplied

Signal strength in subsecond FLASH magnetic resonance imaging: the dynamic approach to steady state.

Subsecond fast low-angle shot (FLASH) magnetic resonance imaging (MRI) allows single shot studies of the human heart within measuring times of about 100-300 ms depending on the data matrix. In contrast to conventional FLASH MRI subsecond applications acquire data during the approach to steady state. A detailed analysis of the saturation behavior of the signal is given for the ideal case of a rectangular slice profile. In a second step, realistic slice profiles assuming Gaussian-shaped excitation pulses were taken into account by means of a numerical solution of the Bloch equations. It turns out that the signal strength and the resulting image intensity is considerably higher than may be expected from steady-state considerations. Correspondingly optimized flip angles depend on the number of phase-encoding steps. Assuming long T1 relaxation times as, for example, encountered in muscle and brain tissue and repetition times of 5 ms or less, optimum flip angles are 12 degrees-16 degrees. The use of even higher flip angles (greater than or equal to 20 degrees) causes heavily distorted slice profiles and a dynamic increase of the effective slice thickness. Flip angles of the order of the Ernst angle (6 degrees) correspond to steady-state conditions and lead to considerable signal losses. The theoretical results are confirmed by subsecond FLASH MRI studies of the human heart using a 2.0 T whole-body system (Siemens Magnetom).

Biophysical Phenomena↗
PubMedDate not supplied

Application of spatial modulation of magnetization to cervical spinal stenosis for evaluation of the hydrodynamic changes occurring in cerebrospinal fluid.

OBJECTIVE: To evaluate the hydrodynamic changes occurring in cerebrospinal fluid (CSF) flow in cervical spinal stenosis using the spatial modulation of magnetization (SPAMM) technique. MATERIALS AND METHODS: Using the SPAMM technique, 44 patients with cervical spinal stenosis and ten healthy volunteers were investigated. The degree of cervical spinal stenosis was rated as low-, intermediate-, or high-grade. Lowgrade stenosis was defined as involving no effacement of the subarachnoid space, intermediate-grade as involving effacement of this space, and high-grade as involving effacement of this space, together with compressive myelopathy. The patterns of SPAMM stripes and CSF velocity were evaluated and compared between each type of spinal stenosis and normal spine. RESULTS: Low-grade stenosis (n = 23) revealed displacement or discontinuity of stripes, while intermediate- (n = 10) and high-grade (n = 11) showed a continuous straight band at the stenotic segment. Among low-grade cases, 12 showed wave separation during the systolic phase. Peak systolic CSF velocity at C4-5 level in these cases was lower than in volunteers (p <.05), but jet-like CSF propulsion was maintained. Among intermediate-grade cases, peak systolic velocity at C1-2 level was lower than in the volunteer group, but the difference was not significant (p >.05). In high-grade stenosis, both diastolic and systolic velocities were significantly lower (p <.05). CONCLUSION: Various hydrodynamic changes occurring in CSF flow in cervical spinal stenosis were demonstrated by the SPAMM technique, and this may be a useful method for evaluating CSF hydrodynamic change in cervical spinal stenosis.

Cervical Vertebrae↗
PubMedDate not supplied

MR diffusion imaging of cerebral infarction in humans.

PURPOSE: MR diffusion imaging was performed to investigate changes in water diffusion in patients with cerebral infarction. METHODS: Diffusion maps of the apparent diffusion coefficient (ADC) were created to show local water mobility in the brain tissue in 15 patients. These ADC maps were compared with conventional T2-weighted images. RESULTS: Distinct subregions with different water diffusions were detected, even when the infarcted area appeared homogeneous on a T2-weighted image. The results also show that stroke lesions of the same age can have very different water diffusions. A trend towards an increasing diffusion coefficient in a lesion during the first several days following an acute event was observed in a group of patients imaged at multiple timepoints. CONCLUSION: The measurement of diffusion coefficients in vivo now offers an opportunity for greater understanding of the biophysical changes that occur during the evolution of infarction in humans.

Adult↗