Three-Dimensional Dosimetry of a Beta-Emitting Radionuclide Using PRESAGE Dosimeters.

Three-dimensional dose distributions from liquid brachytherapy were measured using PRESAGE(®) dosimeters. The dosimeters were exposed to Y-90 for 5.75 days and read by optical tomography. The distributions are consistent with estimates from beta dose kernels.

Interpretation of multigated Fourier functional images.

First-harmonic Fourier analysis is currently used to aid in the interpretation of multigated cardiac studies. Its intrinsic inaccuracies are not generally appreciated. This study investigates the characteristics and magnitudes of the errors of this technique. The study analyzes computer-generated phantoms that isolate the various motions of the ventricles (contraction, translation, and rotation) with the first-harmonic approach. The first-harmonic output is compared with a more accurate fitting scheme using multiple terms of the Fourier expansion. Significant artifacts of the inaccuracy of the first harmonic appear in the phantom studies and are observed in patient examples. We conclude that caution is needed in interpreting first-harmonic phase and amplitude images, and particularly in associating them with parameters like the onset of contraction and the stroke volume.

Characterization of acute myocardial infarction by magnetic resonance imaging.

The T2-weighted spin-echo technique is currently the most frequently used magnetic resonance imaging (MRI) method to visualize acute myocardial infarction. However, image quality is often degraded by ghost artifacts from blood flow, and respiratory and cardiac contractile motion. To enhance the usefulness of this technique for detailed characterization of infarction, a velocity-compensated spin-echo pulse sequence was tested by imaging a flow phantom, 6 normal subjects and 17 patients with acute myocardial infarction. After preliminary studies were performed in 7 patients to determine optimal imaging parameters, a standardized imaging protocol was used in the next 10. The location of myocardial infarction identified by the electrocardiogram and coronary anatomy was correctly identified in 10 of 10 patients. Distribution of the injury within the left ventricle was clearly visualized, and showed that patients often had a mixture of transmural and nontransmural injury. Heterogenous distribution of signal intensity within the infarction suggested the presence of hemorrhage. Papillary muscle involvement was readily apparent. Signal intensity of the infarction (brightest segment) was increased by 89 +/- 31% compared with the mean of the remote segments. The myocardial/skeletal muscle ratio was significantly (p less than 0.001) increased for the infarction segments compared with that for remote myocardium, allowing quantitative analysis of segmental signal intensity. The MRI wall motion study obtained as part of this protocol demonstrated wall thickening in 58% of the infarction segments and in 6 of 10 patients. This finding suggested the presence of reversibly injured myocardium. In conclusion, the results demonstrate the potential of MRI for detailed tissue characterization after acute myocardial infarction.

Magnetic resonance velocity measurements in small arteries. Comparison with Doppler ultrasonic measurements in the aortas of normal rabbits.

RATIONALE AND OBJECTIVES: Magnetic resonance imaging (MRI) can be used to measure motion. This study compares MRI blood flow velocity measurements to Doppler ultrasound velocity measurements in an animal model. MATERIALS AND METHODS: Blood flow in the abdominal aortas of nine normal rabbits was measured using 16-frame, velocity-resolved MRI and Doppler ultrasound. The MRI data were processed into velocity spectra to aid in their interpretation. RESULTS: Maximum velocity measurements made by range-gated Doppler ultrasound were predicted by the maximum velocity values derived from MR velocity spectra with a slope of 0.861, an intercept of -2.78 cm/second, and an R-value of 0.935 in 70 measurements. CONCLUSIONS: Despite the longer time required for the MR measurement, the MR velocity measurement may be useful in the assessment of deep vessels or those obscured by other structures, which are difficult to measure with ultrasound.

Magnetic resonance imaging for assessment of tissue rejection after heterotopic heart transplantation.

Detection of myocardial rejection is difficult in patients with heterotopic heart transplantation because of the complex vascular anatomy present after transplant surgery. To determine whether magnetic resonance imaging might be useful for the assessment of heart rejection, eight patients with heterotopic heart transplantation were serially studied on 27 occasions. One patient had two donor hearts implanted, which allowed the study of 33 donor hearts. Data acquisition was gated to the ECG signal of the donor heart. Heavily T2-weighted (TE = 90 ms) velocity compensated spin-echo images were obtained through the midportion of the donor heart to assess tissue rejection. Donor heart function was qualitatively measured by acquiring multiphasic gradient echo images at the same level. A myocardial/skeletal muscle signal intensity ratio was calculated for the donor heart and compared to results of right ventricular biopsy obtained within 24 hours of imaging. A change in signal intensity ratio of 0.14 or more exceeded normal signal variation. All three episodes of rejection detected by biopsy were detected by magnetic resonance imaging. In three instances a significant change in the signal intensity ratio was associated with clinical evidence of rejection and a negative biopsy. Two instances were treated with a steroid bolus, and the signal returned to baseline. In three other instances a significant change in the magnetic resonance imaging signal occurred without clinical or biopsy evidence of rejection. Cardiac toxoplasmosis was present in one of these cases, and signal intensity returned to baseline after treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrocardiographic gating and monitoring in NMR imaging.

ECG gating and monitoring during NMR imaging may be achieved reliably by applying the principles in this tutorial. In order to use the ECG signal both for triggering and for patient monitoring it must have a prominent R-wave, while at the same time must have little artifact from gradient switches or the Lorentz voltage across the aorta, and not be significantly distorted by gradient switching artifacts. The twin goals of no image artifacts and minimal ECG artifacts may be achieved by the following means: (1) using ECG electrodes with minimal metal, (2) selecting electrodes and cables with no ferrous metals, (3) placing the limb electrodes close together, (4) placing the line between the limb electrodes and the leg electrode parallel to the magnetic flux lines and, if possible, parallel to the transverse component of the gradient flux lines, (5) keeping the area between the limb electrodes and the leg electrode small, (6) placing that area in the center of the imager and (7) twisting or braiding the cables. Following these principles allows artifact-free images and reliable ECG monitoring during ECG-gated NMR imaging examinations.

A technique for flow-enhanced magnetic resonance angiography of the lower extremities.

A two-dimensional, flow-enhanced gradient echo pulse sequence for nuclear magnetic resonance angiography is described. It employs interleaved, presaturated slices to acquire data efficiently on imagers which favor interleaved acquisition over sequential acquisition for multislice imaging. It is useful on any imagers when the effective TR is extended to enhance the sensitivity to slow flow. The technique was applied to the region from aortic bifurcation to the iliac bifurcations of three normal volunteers. The right and left common iliac arteries and veins, the separation of the external and internal iliac arteries, and secondary branches were clearly depicted.

Detection of viable myocardium in segments with fixed defects on thallium-201 scintigraphy: usefulness of magnetic resonance imaging early after acute myocardial infarction.

To determine if magnetic resonance imaging (MRI) can be used to detect tissue viability in segments with persistent 201T1 defects early following acute myocardial infarction, 24 patients underwent MRI and adenosine 201T1 single photon emission computed tomography (SPECT) imaging at approximately 6 days. Infarction was demonstrated on MRI using a velocity-compensated, T2-weighted spin-echo pulse sequence. Wall thickening was assessed using a gradient-echo pulse sequence obtained in the same anatomic position. Viable myocardium was defined by MRI as a segment with increased signal intensity and preserved wall thickening. A fixed defect on the 201T1 SPECT images was defined as the absence of any redistribution 4 hours after the 201T1 infusion. Of 11 patients with redistribution on the 201T1 images in the infarction region, 10 (91%) had preserved wall thickening by MRI. Of 13 patients with fixed defects on the 201T1 images in the infarction region, 6 (46%) had preserved wall thickening by MRI. Of 7 patients with absent thickening, all had one or more segments with absent perfusion on redistribution imaging. Wall thickening tended to occur in patients who received thrombolytic therapy or who underwent revascularization procedures prior to imaging. The results of the present study suggest that spin-echo MRI with motion compensation can be used to identify viable myocardium in patients with fixed defects on 201T1 SPECT following acute myocardial infarction.

Changes in intervertebral disc cross-sectional area with bed rest and space flight.

STUDY DESIGN: We measured the cross-sectional area of the intervertebral discs of normal volunteers after an overnight rest; before, during, and after 5 or 17 weeks of bed rest; and before and after 8 days of weightlessness. OBJECTIVES: This study sought to determine the degree of expansion of the lumbar discs resulting from bed rest and space flight. SUMMARY OF BACKGROUND DATA: Weightlessness and bed rest, an analog for weightlessness, reduce the mechanical loading on the musculoskeletal system. When unloaded, intervertebral discs will expand, increasing the nutritional diffusion distance and altering the mechanical properties of the spine. METHODS: Magnetic resonance imaging was used to measure the cross-sectional area and transverse relaxation time (T2) of the intervertebral discs. RESULTS: Overnight or longer bed rest causes expansion of the disc area, which reaches an equilibrium value of about 22% (range 10-40%) above baseline within 4 days. Increases in disc area were associated with modest increases in disc T2. During bed rest, disc height increased approximately 1 mm, about one-half of previous estimates based on body height measurements. After 5 weeks of bed rest, disc area returned to baseline within a few days of ambulation, whereas after 17 weeks, disc area remained above baseline 6 weeks after reambulation. After 8 days of weightlessness, T2, disc area, and lumbar length were not significantly different from baseline values 24 hours after landing. CONCLUSIONS: Significant adaptive changes in the intervertebral discs can be expected during weightlessness. These changes, which are rapidly reversible after short-duration flights, may be an important factor during and after long-duration missions.

Interactive design of motion-compensated gradient waveforms with a personal computer spreadsheet program.

A personal computer spreadsheet program was used to compute the amplitudes of the gradient pulses in motion-compensated gradient waveforms. The resulting designs for velocity-compensated, gradient-echo, frequency-encoding gradients and velocity-compensated section-select waveforms required little or no modification when implemented on two clinical magnetic resonance imagers.

Automatic adjustment of contrast and brightness of magnetic resonance images.

Two statistics of the magnetic resonance (MR) image, the median and the standard deviation of the values of the significant pixels, can be used along with the type of image to adjust the contrast and brightness of the image (ie, to "window" it) automatically and robustly. The essential parts of this approach to automatic windowing are (1) avoidance of irrelevant pixels, (2) identification of the type of MR image from information stored in the image header, and (3) use of algorithms for the maximum and minimum values that reflect the preference of the intended viewer using a specific monitor and ambient lighting conditions for the different types of images. An evaluation in thirteen clinical studies yielded 91.5% (2312/2526) images requiring no further adjustment and the remaining 8.5% (214/2526) being improved by further adjustment.

Nuclear magnetic resonance velocity spectra of pulsatile flow in a rigid tube.

Velocity spectra can be derived from velocity-encoded nuclear magnetic resonance (NMR) images. Velocity spectra are histograms showing the amounts of fluid flowing at different velocities in the sensitive volume of the measurement. Velocity spectra may prove to be useful in characterizing the flow of blood in small vessels, for example, in detecting the presence of stenoses and in evaluating their severity. NMR velocity spectra acquired in vivo are sufficiently complicated that a model system was designed and tested to investigate the velocity spectra of pulsatile flow. This study measured the NMR velocity spectra of pulsatile flow in a rigid tube and compared them to velocity spectra derived from Doppler ultrasound measurements and to velocity spectra inferred from a theoretical model driven by the measured pressure difference function. The experimental results from each technique agree.

Monitoring of acutely ill patients during nuclear magnetic resonance imaging: use of a time-varying filter electrocardiographic gating device to reduce gradient artifacts.

A time-varying filter electrocardiographic gating device designed to reduce NMR-induced gradient artifacts during NMR imaging of the acutely ill cardiac patient is described. When used in conjunction with multiple electrocardiographic display monitors, accurate assessment of the electrocardiogram for morphologic changes and arrhythmias during all phases of the NMR examination is possible.

Comparison of gradient echo with spin echo magnetic resonance imaging and echocardiography in the evaluation of major aortopulmonary collateral arteries.

This study compared gradient echo magnetic resonance imaging, spin echo magnetic resonance imaging, echocardiography, and echocardiography with x-ray cineangiography in the evaluation of major aortopulmonary collateral arteries. Twelve patients (ages 9 months to 35 years, mean 11 +/- 11 years) with known or suspected major aortopulmonary collateral arteries were studied. The aortic insertion and proximal course of 29 major aortopulmonary collateral arteries demonstrated by x-ray contrast angiography were shown in all 29 cases by gradient echo magnetic resonance imaging but in only 23 of the 29 cases by spin echo magnetic resonance imaging. Color Doppler-echocardiography detected aortopulmonary collateral arteries in four patients but did not define the proximal course or distal anatomy. Gradient echo images of distal aortopulmonary collateral anatomy were qualitatively superior to spin echo images. The contrast-to-noise ratio between the vessel lumen and adjacent lung was greater for gradient echo (6.06 +/- 2.91) than for spin echo (1.45 +/- 1.13)(p < 0.05). Gradient echo magnetic resonance imaging is a useful method for identification and characterization of aortopulmonary collateral arteries in patients of all ages and is superior to spin echo magnetic resonance imaging and echocardiography.

Nuclear magnetic resonance velocity spectra of steady flow.

Nuclear magnetic resonance (NMR) velocity spectra are a compact way to represent the flow information in a velocity-resolved image set. Fully developed steady flow in long tubes gives NMR velocity spectra with average velocities which correlate well with the values derived from the flow rate. The ratio of average velocity to peak velocity correlates well with the Reynolds number. Tubes with compressed cross sections have velocity spectra similar to those of circular tubes. Tubes with irregular walls have velocity spectra in the entrance region that are markedly different from those from smooth-walled tubes.

Significance of the point of expansion in interpretation of gradient moments and motion sensitivity.

The relationship between magnetic field gradient waveform moments and the motion sensitivity of magnetic resonance imaging was explored analytically and by computer simulation. The analysis and simulations revealed several key points. In general, waveform time moments define sensitivity to the time derivatives of position of moving material only at a single time point: the time about which the moments are computed. A Taylor series description of instantaneous position is expanded about this same time point to compute the phase acquired due to specific derivatives of position. A moment is proportional to phase sensitivity to a particular derivative of position throughout the waveform only when sensitivity to all lower-order derivatives is zero. Under restricted conditions of waveform symmetry and motion characteristics, the phase due to motion may be expressed in terms of the average value of a derivative of position over the duration of the waveform. The choice of the moment center, or point of expansion, adds a degree of freedom that may be used advantageously in the design of motion-compensating and motion phase-encoding gradient waveforms. These results facilitate a more complete understanding of the effects of motion through a magnetic field gradient.

Motional phase artifacts in Fourier transform MRI.

Most magnetic resonance imaging (MRI) techniques are subject to a "motional blurring" arising from the acquisition of data in the presence of a frequency-encoding gradient. The Fourier transform of the signal from a spin moving along a magnetic field gradient obeys an equation analogous to the free space Schrödinger equation. Computer simulations of the Bloch equations illustrate the implications of this motional blurring in MRI.

MR imaging of susceptibility-induced magnetic field inhomogeneities.

A 90 degrees-tau 1-90 degrees-tau 2-image acquisition pulse sequence allows spatial mapping of resonant frequency. This sort of sequence has previously been used for magnet shimming, and its use in chemical-shift imaging has been proposed. The authors used this sequence in magnetic resonance imaging of a phantom to demonstrate the magnetic field gradients arising from susceptibility differences within the phantom and allow those gradients to be measured. Gradients may arise near interfaces between substances that cannot support the same magnetic flux density. The pulse sequence was found to work well in lower-field-strength instruments.