Biomedical subjects

J R MacFall

Publications and source records attributed to J R MacFall.

At least 55 records · Page 3Linked to original sources

PubMed1992

Thick-section, single breath-hold magnetic resonance pulmonary angiography.

RATIONALE AND OBJECTIVES: Approaches to performing magnetic resonance angiography (MRA) of the pulmonary vasculature are described using very fast (repetition time [TR] less than 13 mseconds) radiofrequency (rf)-spoiled, gradient-recalled pulse sequences and the standard quadrature body imaging coil of a commercial 1.5-T MR imaging system. METHODS AND RESULTS: Signal-to-noise (SNR) is improved by signal averaging (Nex greater than or equal to 4) in a two-dimensional, single thick-section approach and by volume acquisition (Nex = 1) in a three-dimensional approach. Blood signal loss is minimized by using short, asymmetric echoes (echo time [TE] less than or equal to 2.7 mseconds). Respiratory motion is eliminated by keeping the scan time short enough (approximately 15 seconds) for image acquisition within a single breath-hold. Cardiac motion artifacts are reduced with section orientations that avoid intersecting the heart and/or use of small flip angle (alpha less than or equal to 25 degrees). CONCLUSIONS: Images of healthy volunteers showed that while single thick sections have superior SNR, the three-dimensional approach appears to produce better visualization of the peripheral vascular segments and offers improved ability to process the images to remove overlapping structures.

Adult↗

PubMed1992

Pulmonary vasculature: single breath-hold MR imaging with phased-array coils.

The authors obtained magnetic resonance images of the pulmonary vasculature with reduced artifacts caused by cardiac and respiratory motion by acquiring a series of moderately thin sections in a single breath-hold with an ultrafast gradient-echo pulse sequence. The series of two-dimensional images was postprocessed with a maximum-intensity projection algorithm. Time-of-flight inflow enhancement increased the signal intensity of arteries and veins while radiofrequency phase spoiling produced limited stationary spin suppression of the chest wall. Moderately thin (8-mm) section thicknesses were used to attain the resolution necessary to visualize smaller pulmonary vascular segments up to the chest wall while the number of acquired sections was minimized. Because the body coil did not provide an adequate signal-to-noise ratio (S/N) for a single excitation and thin-section acquisitions, phased-array coils covering either the right or left lung were used to single breath-holds prevented the misregistration and blurring that occurred in examinations performed with multiple breath-holds.

Humans↗

PubMed1991

The use of gradient flow compensation to separate diffusion and microcirculatory flow in MRI.

This paper describes a new MR imaging technique termed Modified Stejskal Tanner versus Flow Compensation (MST/FC) for the separation of diffusion and microcirculatory flow. The theory behind the sequence is explained, along with a five-component model of microcirculation applicable to any "perfusion" imaging technique. Phantom data is presented showing that (1) diffusion effects can be matched between MST and FC (suggesting the possibility of flow-compensated diffusion imaging), and (2) the technique is a quantitative method of separating diffusion and slow (less than 0.25 mm/s) tortuous flow through a Sephadex column. Furthermore, animal images show the technique to be feasible and quantitative in measuring rat brain microcirculation under normal, vasodilated (hypercarbia), and no-flow (post mortem) conditions.

Acetone↗

PubMed1991

Diffusion/microcirculation MRI in the rat brain.

The CO2 fraction of an anesthetized rat's breathing mixture was changed (from 0 to 10%) to attempt to change the brain microcirculation and observe these changes in diffusion measurements of the neural tissue. Brain apparent diffusion coefficients were measured to be (0.71 +/- 0.01) X 10(-3) mm2/s before sacrifice and (0.39 +/- 0.01) X 10(-3) mm2/s after sacrifice. Multiple diffusion components were observed, consistent with flowing material, but the extra components did not increase with increased CO2. It is proposed that the additional components may be due to extracellular, extravascular water such as CSF.

Animals↗

PubMed1991

Pre- and postmortem diffusion coefficients in rat neural and muscle tissues.

Pulsed gradient diffusion-weighted spin-echo images (7 to 11 gradient strengths) were obtained in a coronal slice through the midbrain for five normal adult white rats before and after sacrifice in a 2-T CSI system with air temperature control. The pulse sequence was cardiac gated and respiratory synchronized in order to minimize motion artifacts (Tr greater than 2 s. Te = 30 ms). Diffusion coefficients reflecting several tissue compartments (D*) in brain and muscle were calculated and referenced to simultaneously imaged tubes of water. In the living animals, brain cortical matter had a value of D* = (0.82 +/- 0.02) x 10(-3) mm2/s. deeper brain regions had a value of D* = (0.73 +/- 0.02) x 10(-3) mm2/s, and the muscle had a value of D* = (1.4 +/- 0.1) x 10(-3) mm2/s. Postmortem the values in brain dropped by approximately 30%, while remaining constant in muscle. Signal intensity in the spin-echo images for muscle tissue rose by 50% over a 1- to 2-h interval after sacrifice while that of brain tissue remained relatively stable.

Animals↗

PubMed1991

Muscle activity localization with 31P spectroscopy and calculated T2-weighted 1H images.

Using 31P spectroscopy and magnetic resonance imaging (MRI), the authors studied changes in muscle phosphorous metabolites and T2 with isometric knee extension to evaluate the potential role of T2 images in coil placement for exercise spectroscopy studies. Increased signal intensity was visible in active muscles on T2 images after exercise. Calculated T2-weighted values were elevated immediately after exercise in the quadriceps (P less than .01). T2 increases for individual quadricep muscles varied, with the largest changes in the rectus femoris and the least in the vastus lateralis. 31P spectroscopy studies demonstrated similar findings: percent change in T2 correlated positively with inorganic phosphorus to phosphocreatine ratio (Pi/PCr) (r = 0.89, P less than .01) and negatively with pH (r = -0.88, P less than .01). The correlations between imaging and spectroscopy suggest that T2 images may allow more precise placement of phosphorous coils in exercise studies. The heterogeneity of T2 changes within the quadriceps with exercise suggests that assumptions about muscle activity may be misleading. T2 images may provide muscle activity verification for exercise studies.

Adult↗

PubMed1990

Truncated sinc slice excitation for 31P spectroscopic imaging.

The shortest possible delay (Td) between slice selection and data acquisition is important for producing high quality 31P spectra. In single slice multivoxel spectroscopic imaging, conventional excitation using sinc-shaped rf pulses within typical gradient limitations can have values of Td that lead to significant spectral distortion and loss of signal. Truncated sinc excitation, which ends the excitation close to the center of the main rf lobe has been suggested for MR angiographic applications to produce short values of Td. In this work, the slice profiles, spectral signal-to-noise ratio (SNR) and spectral distortions are compared using the minimum delay achievable on a commercial MRI system for conventional 'sinc' rf excitation and truncated sinc excitation. Slice profiles are calculated using the Bloch equations and measured with a phantom. SNR and spectral distortions are evaluated from whole slice spectra on a human volunteer. On an MRI system with 1 G/cm gradients (0.5 msec risetime), for a 2.5-cm slice at 31P frequencies, conventional excitation can be adjusted to achieve Td = 2.5 msec while truncated sinc excitation yields Td = 1.5 msec. The truncated sinc excitation's shorter value of Td leads to much smaller spectral distortions, but its slice profile has "dispersive tails" which increase as more of the rf is truncated. Slice profile corrected SNR for the beta-ATP peak of 31P on a human volunteer is equivalent for both sequences while, qualitatively, in the PDE region the truncated sinc approach has improved SNR.

Brain↗

PubMed1989

Effects of intravoxel incoherent motions (IVIM) in steady-state free precession (SSFP) imaging: application to molecular diffusion imaging.

A theoretical analysis of the effects of diffusion and perfusion in steady-state free precession (SSFP) imaging sequences sensitized to intravoxel incoherent motions by magnetic field gradients is presented and supported by phantom studies. The capability of such sequences to image diffusion and perfusion quickly was recently demonstrated. The possible residual effects of T1 and T2 in diffusion measurements are evaluated, as are the effects of the sequence design and the acquisition parameters (repetition time, flip angle, gradient pulses). It is shown theoretically and confirmed by experiments on phantoms that diffusion coefficients can be directly measured from SSFP images when large enough diffusion gradient pulses are used.

Diffusion↗

PubMed1989

Contrast and accuracy of relaxation time measurements in acquired and synthesized multislice magnetic resonance images.

The effects of interslice spacing, the number of data points and other factors on the accuracy of relaxation time measurements and contrast have been investigated for both acquired and synthesized multislice MR images using experiments and computer simulations. The cross-excitation between adjacent slices in multislice imaging affects both contrast and derived relaxation times. Such measurements also are affected by the T1 and T2 of the materials imaged, the pulse sequence timing parameters, and the number of data points used to estimate the relaxation times. Errors in T1 and T2 may be severe, particularly for slice spacings less than 0.5 slice thickness and for long T1 and T2 materials. Consequently, the difference in signal intensities between two materials with different relaxation times also varies with slice spacing and between acquired and synthetic images, particularly for strongly T1-weighed images.

Computer Simulation↗

PubMed1988

Integrated MR imaging and spectroscopy with chemical shift imaging of P-31 at 1.5 T: initial clinical experience.

A section-selective three-dimensional phosphorus-31 chemical shift imaging (CSI) experiment was evaluated as the spatial localization method for spectroscopy in an integrated clinical magnetic resonance (MR) imaging and spectroscopy examination. The results of a CSI experiment can be displayed as either spectra related to specific voxels or "metabolite maps," in which the relative concentration of a given metabolite is displayed as an overlay of the MR image. This method was applied to the study of a soft-tissue mass and to a meningioma. The total imaging time in each case was 17 minutes with a voxel size of 27 cm3 in the extremity and 64 cm3 in the brain. The total time to set up this part of the study was about 10 minutes. No additional shimming was necessary when the center of the field of view selected for the CSI experiment was located at or near isocenter. The promising results obtained with this approach make the CSI method an attractive choice of spatial localization method.

Adult↗

PubMed1987

Methodology for the measurement and analysis of relaxation times in proton imaging.

Measurements of proton T1 and T2 were performed on GdCl3 solutions (20 less than T2 less than 500 msec, 90 less than T1 less than 1000 msec) on large-bore NMR imaging systems operating at 1.0T and 1.5T. CPMG multi-echo (ME), multiple saturation recovery (MSR) and modified fast inversion recovery (MFIR) pulse sequences as well as a sequence that combines and interleaves T1 and T2 weighted data acquisition (which we call "multiple saturation-recovery multiple-echo" (MSRME) were used. The relaxation data are compared to those obtained on a small bore NMR spectrometer operated at 1.5T. T1 and T2 values for the solutions were found to be the same within 10% for the two fields. Reproducibility of measurements of T1, T2 and the unnormalized spin density of the solutions was better than 5%. Systematic errors, amenable to correction through calibration, are noted in the imager T1 and T2 values. T1 and T2 values for some typical neural tissues at 1.5T and body tissue at 1.0T for human volunteers were obtained and are tabulated.

Gadolinium↗

PubMed1986

Reproducibility of relaxation and spin-density parameters in phantoms and the human brain measured by MR imaging at 1.5 T.

The reproducibility of T1, T2, and proton density, measured in phantoms and the human brain was evaluated by proton imaging techniques. The sequence used to derive T1 and density values was a multiple-saturation recovery which consists of four pairs of 90 degrees pulses, followed by a 180 degrees phase reversal pulse, generating four T1-weighted images. T2 was derived from a multiple-echo sequence, generating four T2-weighted images. The data were analyzed by fitting the pixel intensities to the respective equations by means of nonlinear multiparameter least-squares analysis. Short-term reproducibility between four consecutive scans was evaluated to be 1-4% depending on location with a phantom covering the entire span of physiologic T1 and T2 values. A second phantom containing a series of identical samples served to study the dependence of the apparent T1 and T2 on position, both radially and axially, with respect to magnet isocenter. Reproducibility across the field of view was found to be better than 7% (T1 and T2). This phantom was further used to evaluate effects of long-term reproducibility, which at each location varied from 5-14% (T1) and 2-10% (T2). Finally, interinstrument reproducibility, tested by means of the same protocol on three different instruments, all operating at the same magnetic field and using largely identical hardware for each location, was found to be 1-14% (T1) and 2-10% (T2). The positional dependence of the apparent relaxation times appears to be systematic and may be due to variations in the effective field, caused by magnet and rf inhomogeneity. Finally, brain tissue relaxation and spin-density data were determined using the same protocol in 37 scans performed on 27 normal volunteers. The tissues analyzed were putamen, thalamus, caudate nucleus, centrum semiovale, internal capsule, and corpus callosum. Excellent accordance was further obtained between left and right hemispheres.

Brain↗

PubMed1986

T2 estimates in healthy and diseased brain tissue: a comparison using various MR pulse sequences.

Fourteen patients and five healthy individuals underwent magnetic resonance (MR) imaging to determine an effective multiple spin echo pulse sequence for estimating T2. Lesions examined included infarction, glioma, multiple sclerosis, and acute hematoma. A pulse repetition time (TR) of 1,500 msec and echo delays (TEs) of 25, 50, 75, and 100 msec were used. Computed T2 images were derived from all four echoes, the first two echoes, and the first and fourth echoes. T2 values were obtained from specific brain locales using region-of-interest analysis. Use of either the first two echoes or the first and fourth in the T2 fit provided T2 estimates which closely correlated with that of the four-echo analysis. The noise level in T2 maps constructed from the 25- and 100-msec echoes was modestly (typically 10%) higher than that from four echoes; noise level from the 25- and 50-msec echoes was markedly higher, typically 60%. This behavior is remarkably consistent with that predicted from theory. All 19 subjects displayed consistent relative T2 values for specific brain structures; in 13, the absolute T2 values fell within a limited range. Despite the high sensitivity of T2 images, their specificity in the detection of most brain disease appears limited except in acute intracerebral hematoma, which exhibited a decreased T2 relaxation time using high-field-strength MR imaging.

Adolescent↗

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↗

PubMed1985

Quantification of contrast in clinical MR brain imaging at high magnetic field.

The relative contrast between two tissues in a magnetic resonance (MR) image is shown to be quantifiable for any combination of pulse timing parameters, provided the intrinsic parameters are known. Based on multiple inversion-recovery and spin echo images, a region-of-interest T1, T2 and density analysis was conducted at 1.4T in selected patients with diagnosed neuropathology for various brain tissues. The resulting tissue parameters subsequently served to calculate the contrast-to-noise (C/N) ratio for typical tissue interfaces as a function of the operator-variable pulse timing parameters and the data were compared with the images. Although such calculations may be useful as a protocol selection aid, it is obvious that an optimized pulse protocol can only be established for a single tissue interface. The data also reveal that a T2-discriminating pulse sequence like Carr-Purcell-Meiboom-Gill with long repetition time, generally advocated as clinically most effective, may not always be ideal.

Brain Diseases↗

PubMed1985

Synthesized MR images: comparison with acquired images.

Synthesized and directly acquired spin-echo images were compared in order to assess the validity of magnetic resonance (MR) image synthesis as a method enabling retrospective formation of images by interactive manipulation of scan parameters. Synthetic images subjectively compared favorably in both accuracy and precision with acquired images when formed for the same values of echo (TE) and repetition times (TR) and for interpolated and extrapolated values of both TE and TR. Plots of synthetic and acquired signals within the same pixel sectors quantitatively showed comparable values for several regions of interest in the brain. Percent error and noise-normalized differences between acquired and synthetic images were tested as a quantitative measure of accuracy. Percent error was consistently less than 5% for brain parenchyma, and synthetic signals were accurate to within four times the noise level at acquisition. The apparent signal-to-noise ratio of synthetic images was comparable, superior, or inferior to similar acquired images, depending on the values of TE and TR. Total acquisition time required for synthetic formation of images for arbitrary values of TE and TR was equivalent to that of a single direct acquisition with a TR of 2,500 msec.

Brain↗

PubMed1984

The dependence of nuclear magnetic resonance (NMR) image contrast on intrinsic and pulse sequence timing parameters.

In Nuclear Magnetic Resonance (NMR) the image pixel value is governed by at least three major intrinsic parameters: the spin density N (H), the spin-lattice relaxation time T1, and the spin-spin relaxation time T2. The extent to which the signal is weighted toward one or several parameters is related to the history of the spin system preceding detection. On the simplifying, though not generally warranted assumption that the spin density does not vary significantly in soft tissues, relative tissue contrast can be predicted quantitatively provided the relaxation times are known. Signal intensities and contrast were computed on the basis of the Bloch equations and experimentally determined relaxation times as a function of pulse timing parameters and the data compared with those in images recorded at 0.5T field strength. Significant deviations from the equal density hypothesis were found for gray and white substance. Notably partial saturation but also spin echo and inversion-recovery images are not in full accordance with predictions made on the basis of relaxation times alone.

Brain↗

PubMed1984

Mechanisms of contrast in NMR imaging.

Nuclear magnetic resonance pixel intensity and contrast-to-noise has been computed and presented in graphical form for various tissues in the normal central nervous system, on the assumption that the signal intensity is proportional to the macroscopic transverse spin magnetization at the time of detection. T1, T2, and spin density values were experimentally determined using chi-square minimization techniques. Additionally, spin density was derived from partial saturation scans obtained with a long repetition time compared with the spin-lattice relaxation time. Pulse sequences discussed comprise partial saturation, saturation recovery, spin echo, and Carr- Purcell - Meiboom -Gill ( CPMG ). The complicated dependence of signal and contrast-to-noise on the pulse timing parameters and the specific pulse sequence makes it appear desirable to display image intensity so that the dependence on the extrinsic (operator-selectable parameter) is eliminated. Whereas T2 images can be derived from CPMG scans without excessive time penalty, this is not the case for T1 and spin density.

Central Nervous System↗