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Biomedical subjects

A Shimakawa

Publications and source records attributed to A Shimakawa.

17 recordsLinked to original sources
PubMed1991

Quantitation of the susceptibility difference between trabecular bone and bone marrow: experimental studies.

In this study we quantify the effects of different relaxation mechanisms on the signal intensity in gradient-echo images of tissue such as bone marrow in the presence of trabecular bone. The susceptibility difference between trabecular bone and soft tissue produces distortions in the magnetic lines of force which induce strong inhomogeneities in the static magnetic field. Diffusion of tissue protons in such magnetic field gradients produce a shortening of the transverse relaxation time T2, while the dephasing of the transverse magnetization due to susceptibility differences produces a shortening of the apparent relaxation time T2* as demonstrated in gradient-echo images. We have used specimens of dried human vertebrae with different bone densities immersed in either saline to simulate tissue water or an emulsion of oil and water to simulate bone marrow to quantify these relaxation mechanisms in vitro. We have measured the MR relaxation times T1, T2, and T2* of protons within the trabecular spaces and correlated their variations with trabecular bone density. We have found that in vitro, at 1.5 T, the relaxation times T1 and T2 do not show significant variations with bone density and there are no significant contributions to the transverse relaxation rate due to the diffusion of tissue water in the magnetic field gradients. However, the relaxation rate, 1/T2*, of saline in the presence of trabecular bone increases at a rate of 0.2 s-1/mg/cc due to the dephasing of the transverse magnetization in the magnetic field inhomogeneities. Similar bone density-related T2* variations were observed for fat protons within the trabeculae where the chemical-shift-induced modulations of signal intensity in an oil-water emulsion have been separated from the susceptibility-induced relaxation effects. In addition, we have verified these effects in vivo and quantified in vivo variations in fat and water relaxation rates of bone marrow in the epiphysis and diaphysis in the appendicular skeleton of normal volunteers and found that both fat and water T2* are shorter in the epiphysis compared to the diaphysis, which correlates well with previous observations.

Bone Marrow↗
PubMed1991

Right and left ventricular stroke volume measurements with velocity-encoded cine MR imaging: in vitro and in vivo validation.

The accuracy of measurements of flow velocity determined by using cine MR phase velocity mapping--velocity-encoded cine (VEC) MR--was assessed by comparing VEC MR data with independent measurements in a flow phantom and in human subjects. Constant flow velocities generated in a phantom (range, 20-408 cm/sec) were determined correctly by VEC MR (r = .997, standard error of the estimate [SEE] = 7.9 cm/sec). Peak systolic velocities in the main pulmonary artery determined by VEC MR correlated well with the measurements obtained by using continuous-wave Doppler echocardiography (r = .91). Stroke volumes measured at the aorta by VEC MR and continuous-wave Doppler imaging also correlated well with each other (r = .80). VEC MR measurements of aortic and pulmonary flow provided left and right ventricular stroke volumes that correlated well with left ventricular stroke volumes determined by short-axis cine MR images (r = .98, SEE = 3.7 ml, and r = .95, SEE = 4.8 ml, respectively). Intra- and interobserver variabilities were small for both left and right ventricular stroke volumes as measured with VEC MR. These results indicate that VEC MR accurately and reproducibly measures aortic and pulmonary flow velocities and volumes in the physiologic range of humans, and can be used to measure right and left ventricular stroke volumes under normal flow conditions.

Adult↗
PubMed1991

Phase contrast cine magnetic resonance imaging.

Phase contrast cine magnetic resonance imaging (MRI) combines the flow-dependent contrast of phase contrast MRI with the ability of cardiac cine imaging to produce images throughout the cardiac cycle. Two pulse sequence types are used for sensitivity to flow in one direction, whereas four are needed for sensitivity in all directions. Several alternatives for synchronization of the data to the cardiac cycle exist. Retrospectively interpolated methods can image the entire cardiac cycle efficiently. Rapid interleaving of the various sequence types ensures immunity to motion misregistration. The technique produces images in which contrast is related to flow velocity as well as magnitude images such as those of conventional cine MRI. The data can be interpreted qualitatively to demonstrate the presence, magnitude, and direction of flow, and quantitatively to provide estimates of flow velocity, volume flow rate, and displaced volumes. Phase contrast cine MRI is helpful in the diagnosis of aortic dissections, in the study of flow distributions in large vessels such as pulmonary arteries, as well as in smaller vessels such as carotid and basilar arteries, and in the evaluation of complex anatomical variants. Future developments are expected to reduce imaging time and expand the quantitative applications.

Cardiovascular System↗
PubMed1988

Valvular regurgitation: dynamic MR imaging.

Cine magnetic resonance (MR) imaging is a new technique that combines short repetition times, limited flip angles, gradient refocused echoes, and cardiac gating. This technique has a temporal resolution of up to 32 time frames per cardiac cycle and accentuates signal from flowing blood. Cine MR images of 56 valves in 27 patients were evaluated and compared with either Doppler echocardiograms or cardiac catheterization images. An area of decreased signal that correlated spatially and temporally with regurgitant blood flow was seen in all instances in which valvular incompetence was demonstrated on either Doppler echocardiograms or cardiac catheterization images (20 valves). This abnormality was seen in nine of 36 cases without valvular incompetence. Cine MR imaging may be sensitive to turbulence and thus sensitive to valvular regurgitation.

Aortic Valve Insufficiency↗
PubMed1987

Rapid calculation of T1 using variable flip angle gradient refocused imaging.

We present a method for rapid measurement of T1 relaxation times using gradient refocused images at limited flip angles and short repetition times. This "variable nutation" techniques was investigated using a T1 phantom. There was a high correlation between measurements obtained with the variable nutation and partial saturation techniques. The ability of this method to create calculated T1 images is also demonstrated. We conclude that the variable nutation method may allow measurement of T1 relaxation times with a significant reduction in acquisition time compared to partial saturation techniques.

Humans↗
PubMed1987

MR vascular imaging with a fast gradient refocusing pulse sequence and reformatted images from transaxial sections.

The authors present a method for obtaining magnetic resonance (MR) images of intra- and extracranial vessels from thin contiguous transaxial sections. A section-selective gradient refocusing pulse sequence with a short repetition time caused flow-related enhancement from spins that flowed perpendicular to the transaxial sections. The signal was further enhanced by means of flow compensation gradients to rephase any phase shifts resulting from moving spins in the presence of the imaging gradients. Coronal and sagittal sections, reformatted from multiple transaxial sections, are shown to have excellent vessel contrast without the use of contrast material. These images were obtained in 12 minutes of acquisition time from as many as 60 sections of 3-mm thickness. Such a technique shows significant promise for MR angiography.

Cerebral Arteries↗
PubMed1987

Two-second MR images: comparison with spin-echo images in 29 patients.

MR images can be obtained with a 2-sec scan time when an extremely short repetition rate (22 msec), limited flip angle (30 degrees), and gradient refocused echoes are used. Comparison of 415 such images obtained in 29 patients with routine T1-weighted (TR 500, TE 25) and T2-weighted (TR 2000, TE 80) images showed that images free of respiratory artifacts could be obtained in all patients. Although abdominal organs were well seen with 2-sec scan time, overall evaluation of these organs was better on routine T1-weighted images. Vascular structures, however, were seen as well or better on the 2-sec images in 60% of cases. The images were extremely sensitive to field nonhomogeneity, and metallic artifact was exaggerated in five patients with surgical clips. Two-sec MR images provide a rapid method of localizing abdominal organs for further evaluation. The sensitivity to blood flow may assist in the assessment of vascular patency.

Adrenal Glands↗
PubMed1987

Cine MR determination of left ventricular ejection fraction.

Cine MR imaging provides tomographic images of the heart with both high spatial and high temporal resolution. As many as 32 images per cardiac cycle can be acquired with up to four separate anatomic slices and a total imaging time of 128 cardiac cycles. End-diastolic and end-systolic volumes were determined in 11 patients, and ejection fractions were calculated. The results correlated linearly with those from cardiac catheterization (correlation coefficient of .88). We conclude that cine MR imaging can be used to obtain quantitative information about the heart and has the potential to become a valuable noninvasive means of cardiac evaluation.

Cardiac Catheterization↗
PubMed1986

A time-of-flight method of measuring flow velocity by magnetic resonance imaging.

A new time-of-flight method for direct imaging of flow velocities by magnetic resonance is presented. The technique uses selective exciting and refocusing RF pulses to selectively affect planes oriented in orthogonal directions in space, with the region of excitation perpendicular to the flow and the refocusing region parallel to and including the flow. The positions of the sources of the resulting spin echoes are imaged, showing a displacement equal to the product of the velocity and the echo time. These images clearly show the profile of the velocity distribution, both in laminar and nonlaminar flows.

Blood Flow Velocity↗
PubMed1986

Effect of intersection spacing on MR image contrast and study time.

The effects of section separation on image contrast and calculated T1 relaxation times were investigated in healthy volunteers and a phantom using an early commercial version magnetic resonance imaging system. The effects are explained qualitatively on the basis of side lobes of excitation occurring outside the selected section resulting in reduction of the time permitted for T1 relaxation. The options for dealing with imperfect section selection, including separation of the sections (i.e., leaving gaps) and nonsequential excitation, are illustrated and the trade-offs involved in each explained.

Brain↗
PubMed1986

Time-of-flight MR flow imaging: selective saturation recovery with gradient refocusing.

A novel magnetic resonance flow-imaging technique is presented and its suitability evaluated for both qualitative and quantitative imaging of flow. The method is derived from a selective saturation-recovery scheme consisting of a tagging and detection pulse followed by a bipolar read gradient. The detrimental phase effects causing signal loss at fast flow are shown to be greatly reduced because of the absence of a 180 degrees pulse and its associated section-selection gradient. The second loss mechanism intrinsic to 180 degrees spin echoes, the washout of excited spins between excitation and detection pulse, likewise is not present with the discussed technique. Assuming a parabolic flow profile, the authors calculated the signal evolution curve and found it to be in agreement with the experimental washout curve. The technique is shown to provide high-intensity signals for arteries such as carotid and vertebral arteries. Arteries and veins can be differentiated by judiciously choosing interpulse intervals or by alternating selective and nonselective tagging pulses.

Blood Flow Velocity↗
PubMedDate not supplied

Encoding strategies for three-direction phase-contrast MR imaging of flow.

Three encoding strategies for the measurement of flow velocities in arbitrary directions with phase-contrast magnetic resonance imaging are presented; their noise and dynamic range performance are compared by means of theoretical analysis and computer simulation. A six-point measurement strategy is shown to be quite inefficient in terms of velocity variance per unit time. A simple four-point method exhibits equal dynamic range; its noise depends on flow direction but on average is equal to that of the six-point method. An alternate, balanced four-point method has noise that is direction independent and has, depending on implementation, possibly lower noise levels. Either four-point method is more efficient and is preferred over the six-point approach.

Blood Flow Velocity↗
PubMedDate not supplied

Minimizing TE in moment-nulled or flow-encoded two- and three-dimensional gradient-echo imaging.

A method for minimizing field-echo delay in moment-nulled gradient-echo imaging is presented. Even though ramps are accounted for, the analysis yields simple closed-form solutions. The method is then generalized to the section-select waveform for three-dimensional volume imaging and to flow encoding for phase-contrast imaging. Three strategies for first-moment selection in phase-contrast imaging are discussed, including a new strategy that always yields the minimum echo delay. Trapezoidal and triangular gradient lobe shapes are analyzed.

Algorithms↗
PubMedDate not supplied

MR imaging of venous and arterial flow by a selective saturation-recovery spin echo (SSRSE) method.

Flow velocity imaging studies have been conducted by means of a selective saturation-recovery spin echo technique, and the dependence of signal amplitude on interpulse interval, echo delay, slice-selection gradient, and flow velocity was evaluated experimentally. The simple theory predicting a steady increase of signal intensity with increasing interpulse interval until this latter equals the transit time could be verified in phantoms and was shown to permit measurement of blood flow velocity in venous structures such as the femoral vein. The flow phantom experiments further showed that the final intensity, attained when inversion time (TI) = transit time, decreases with increasing flow velocity, an effect that cannot be explained by influx of spins between the 90 degree detection pulse and the 180 degree refocusing pulse. This signal reduction is due to slice-selection gradient-induced phase shifts across the pixel, caused by the intralumenal velocity gradient, leading to destructive interference of the spin isochromats. The velocity distribution can be mapped by plotting signal intensity as a function of interpulse interval for pixels in different radial positions. To highlight arterial flow, gating is required with the acquisition delay selected such that the interpulse period TI falls in a time zone of slow flow within the cardiac cycle. By subtracting images recorded with different acquisition delays, flow images showing arterial enhancement only can be obtained, as illustrated for the femoral artery in the thigh.

Blood Flow Velocity↗
PubMedDate not supplied

Dural sinus occlusion: evaluation with phase-sensitive gradient-echo MR imaging.

The purpose of this study was to evaluate the usefulness of limited-flip-angle, phase-sensitive velocity imaging with gradient-recalled-echo (VIGRE) MR when combined with spin-echo MR in the diagnosis of dural sinus thrombosis. The VIGRE sequence consists of a rapid single-slice acquisition, 50/15/2 (TR/TE/excitations), and 30 degrees flip angle. At each slice position, a total of four images were reconstructed; these consisted of one magnitude image and three images sensitive to proton motion in each orthogonal direction. The flow direction and flow velocity (cm/sec) were obtained from each of the phase images, and results were correlated with data obtained from a phantom experiment. In normal controls, dural sinus velocities ranged from a mean of 9.9 to 14.4 cm/sec for the transverse and superior sagittal sinuses, respectively. Three patients with proved dural sinus occlusion were studied with spin-echo images at 1.5 T. Three-dimensional time-of-flight MR angiography was also performed in one patient. The presence of dural sinus occlusion was determined by the lack of flow void on the spin-echo images, the absence of phase shift on the VIGRE study, and the presence of retrograde flow on the phase image in the sinus proximal to the occluded segment. Time-of-flight angiography overestimated the extent of the thrombosis caused by spin saturation. Follow-up VIGRE studies detected the formation of collateral flow in one patient and recanalization with the establishment of normal antegrade sinus flow in the other. We conclude that phase-sensitive MR imaging is helpful in establishing the diagnosis and extent of dural sinus occlusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMedDate not supplied

MR imaging of the optic nerve and sheath: correcting the chemical shift misregistration effect.

The interface between soft-tissue structures and adipose tissue may be obscured by the chemical shift misregistration effect on MR images. In the orbit, this effect occurs at the edges at the optic nerve, even on high-resolution local coil images. In both phantom and clinical studies with a 1.5 T local coil imaging system, it was found that the chemical shift misregistration effect can be minimized by positioning the patient so that the optic nerve is parallel to the frequency encoding gradient. Alternatively, the effect can be corrected by using a computer program to combine "lipid" and "water" proton images. The sensitivity of MR for optic nerve lesions should be improved by these technical modifications.

Child↗