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Biomedical subjects

R I Pettigrew

Publications and source records attributed to R I Pettigrew.

At least 37 records · Page 2Linked to original sources
PubMed1996

Two-dimensional coronary MR angiography without breath holding.

PURPOSE: To determine whether breath holding can be eliminated in two-dimensional magnetic resonance (MR) imaging of the coronary arteries by using real-time respiratory gating. MATERIALS AND METHODS: Thirty-one subjects (20 healthy volunteers, 11 patients) underwent MR imaging. In 13 subjects, a respiratory monitoring belt was used, and in 18 subjects, a navigator echo was used. MR imaging was performed with breath holding, respiratory gating, and respiratory gating with two signals acquired. Three reviewers conducted a blinded review of the images, and overall image quality was rated on a scale from 1 (poor) to 5 (excellent). RESULTS: Respiratory gating with two signals acquired provided image quality superior to that with breath-hold imaging (3.7 vs 3.0, respectively; P < .05). Measurements of signal-to-noise ratio (14.5 for respiratory gating with two signals acquired and 11.9 for breath holding) supported the results of the image review. Navigator-echo gating provided better image quality than the monitoring belt (3.7 vs 3.1, respectively; P < .05). CONCLUSION: Breath holding may be eliminated by gating image acquisition to a real-time monitor of respiratory position. Respiratory gating enables improved resolution by means of acquisition of multiple signals, provides aligned sections of coronary arteries, and improves patient tolerance.

Adult↗
PubMed1996

The accuracy of magnetic resonance phase velocity measurements in stenotic flow.

Nuclear magnetic resonance (MR) can be used to measure velocities in fluid flow using the technique of phase velocity mapping. Advantages of MR velocimetry include the simultaneous mapping of the entire flow field through a non-contacting, magnetic window. The phase velocity mapping technique assumes that velocity is constant over the measurement time (typically around 10 ms). For many fluid flows, this assumption is not valid. The current study showed that MR phase velocity measurements of velocity through stenotic flow can be in error by over 100% immediately upstream and downstream of the stenosis throat and by 20% far downstream of the throat in comparison with laser Doppler anemometer measurements taken at the same location. Highly turbulent flow also led to significant errors in velocity measurement. These errors can be attributed to several sources including low signal-to-noise ratio, additional phase shifts due to non-constant velocities, and non-stationary transit-time effects. Velocity measurement errors could be reduced to under 30% at all measurement locations through the use of MR sequences with high signal-to-noise ratios, low echo times, and thick slices.

Arterial Occlusive Diseases↗
PubMed1995

Turbulent fluctuation velocity: the most significant determinant of signal loss in stenotic vessels.

Studies of flow in a 90%-stenosis phantom were conducted to elucidate the parameters and mechanisms responsible for signal loss in MR angiographic images. The studies independently evaluated the effect of velocity, Reynolds number, turbulent fluctuation velocity, and turbulence intensity on the amount of post-stenotic signal loss. Results suggested that the magnitude of the turbulent fluctuation velocity, not merely the presence of turbulence or the intensity of turbulence, was the parameter that determined the extent of the signal loss. The study suggests that future flow phantom studies should be conducted with fluids having physiologic velocities and viscosities to obtain accurate levels of turbulent fluctuation velocities and hence reproduce results of in-vivo signal-loss patterns. The mechanism for signal loss is that the temporal and spatial variations of the turbulent fluctuation velocity cause a range of phases to be present within a voxel. Examination of the theoretical aspects of fluid turbulence suggest that shortening gradient durations and imaging during diastole may help reduce signal loss.

Blood Flow Velocity↗
PubMed1995

Incidence of aneurysm formation after Dacron patch aortoplasty repair for coarctation of the aorta: long-term results and assessment utilizing magnetic resonance angiography with three-dimensional surface rendering.

OBJECTIVES: Magnetic resonance angiography with three-dimensional surface rendering was performed to determine its value in assessing anatomic detail in patients with suspected aortic aneurysms. BACKGROUND: Dacron patch aortoplasty repair of coarctation of the aorta carries an inherent risk of aneurysm development. Sudden death from aortic rupture prompted discontinuing this operation and evaluating 39 patients (16 girls; mean age 6.3 years, range 10 days to 14.5 years) undergoing repair between January 1976 and October 1987. The aorta ruptured in 10 patients; 6 died at a mean interval of 8.1 years (range 0.75 to 12.4) after repair. All 33 survivors were interviewed and examined. METHODS: Conventional magnetic resonance imaging was performed in 26 patients, magnetic resonance angiography in 18. Angiographic slices were used to reconstruct three-dimensional images. No catheterization or contrast angiography was performed. Surgical intervention was based on clinical findings and magnetic resonance images. RESULTS: Twenty patients (11 girls) developed aneurysms, of which nine were detected in patients studied by magnetic resonance. Ruptures occurred in eight female patients, three of whom were pregnant. Surface renderings accurately defined aortic anatomy or aneurysms in all patients. On follow-up, no aneurysms have been detected in patients with negative magnetic resonance study results. Precise anatomic correlation with operative findings was reported. CONCLUSIONS: Magnetic resonance angiography with three-dimensional surface rendering provides noninvasive, radiation-free and contrast agent-free high resolution images of the thoracic aorta. These images can be reviewed and have three-dimensional form and perspective. These techniques were preferred over invasive angiography by surgeons and clinicians as definitive, risk-free procedures before surgical intervention.

Adolescent↗
PubMed1995

Three-dimensional displays of left ventricular epicardial surface from standard cardiac SPECT perfusion quantification techniques.

UNLABELLED: Two methods for generating left ventricular epicardial surface from SPECT perfusion tomograms are described and validated. Both methods use the locations of the maximal reconstructed count values determined from a perfusion quantification procedure as a basis for generating surfaces. METHODS: The first method fits circular contours, which are perpendicular to the long-axis, to the points obtained from perfusion quantification. The second method applies median and linear filters to the points to remove noise but maintain the basic shape of the surface. Both models are validated against an automatic technique and against the user-traced surfaces of both the perfusion image and an MR image of the same patient. RESULTS: The median-filtered model was found to be closer to the standard surfaces than the circular model in all cases, and 85% of the points on the median-filtered surfaces were within one SPECT pixel length of the hand-traced MR surfaces. CONCLUSION: Accurate, three-dimensional left ventricular epicardial surfaces can be generated quickly and easily from already existing perfusion quantification software. The resulting images may be useful for realistic displays of ventricular size, shape and the three-dimensional distribution of perfusion.

Coronary Circulation↗
PubMed1994

Determining the accuracy of calculating systolic wall thickening using a fast Fourier transform approximation: a simulation study based on canine and patient data.

UNLABELLED: The high court yields of 99mTc-sestamibi make possible the acquisition of multiple gated SPECT studies with relatively high count densities. By reorienting these studies into gated short-axis slices, and extracting the three-dimensional myocardial perfusion distribution, we can study wall thickening using an amplitude and phase analysis methodology that examines the change in counts throughout the cardiac cycle. There have been two main concerns raised about this count-based technique: (1) What effect does the sampling rate have on the calculation of systolic wall thickening? and (2) What effect does count density have on the calculation of systolic wall thickening? METHODS: We designed a simulation study using myocardial wall thickening data obtained from ultrasonic crystals implanted in the myocardium of a normal canine. This data was modified to produce wall thickening curves with various percent systolic wall thickening measurements, sampling rates and count densities. RESULTS: The results show that using at least eight frames per cardiac cycle, systolic wall thickening can be calculated with enough accuracy to separate normal patients from those with cardiac dysfunction, even in areas of hypoperfused myocardium. Also, the results show the importance of calculating and interpreting phase (onset of contraction) information. CONCLUSIONS: This count-based technique continues to show promise as a tool for calculating systolic wall-thickening from multiple gated myocardial perfusion SPECT studies, but needs to be validated in a prospective multi-center trial before being applied in a clinical setting.

Animals↗
PubMed1994

Reproducibility of thallium-201 exercise SPECT studies.

UNLABELLED: A detailed analysis of intrapatient reproducibility of exercise SPECT thallium studies is presented. METHODS: Twenty patients in stable condition were re-examined with exercise-redistribution SPECT201 Tl within 3-9 days without intervening procedures. At peak stress, 3.5 mCi 201Tl were given intravenously 1 min prior to exercise termination. SPECT imaging started at 5 and 180 min. Acquisition and processing protocols were the same for all studies. Coronary angiography was performed on 19 patients and showed coronary artery disease (CAD) in 18, and no CAD in one; one patient did not have coronary arteriography. RESULTS: For 16 of 20 patients, exercise levels and ECG were comparable for both studies. Ten patients reproduced ST-segment depression; two reproduced angina; one had left bundle branch block (LBBB) on both studies after 1 min of exercise. The remaining seven patients had no ECG changes or symptoms during exercise. Four of 20 (20%) thallium scans differed: three in degree of redistribution and one (5%) in presence of a second stress defect. In three of four patients whose thallium studies showed some nonreproducibility, there were differences in exercise. Thallium results were identical in 15 of 16 patients whose ECG/exercise tests were reproducible (94%). Interobserver agreement was 95%. CONCLUSION: There was excellent reproducibility of 201Tl SPECT scintigraphy in patients who reproduced exercise test performance and symptoms.

Coronary Angiography↗
PubMed1990

Evaluation of magnetic resonance velocimetry for steady flow.

Whole body magnetic resonance (MR) imaging has recently become an important diagnostic tool for cardiovascular diseases. The technique of magnetic resonance phase velocity encoding allows the quantitative measurement of velocity for an arbitrary component direction. A study was initiated to determine the ability and accuracy of MR velocimetry to measure a wide range of flow conditions including flow separation, three-dimensional secondary flow, high velocity gradients, and turbulence. A steady flow system pumped water doped with manganese chloride through a variety of test sections. Images were produced using gradient echo sequences on test sections including a straight tube, a curved tube, a smoothly converging-diverging nozzle, and an orifice. Magnetic resonance measurements of laminar and turbulent flows were depicted as cross-sectional velocity profiles. MR velocity measurements revealed such flow behavior as spatially varying velocity, recirculation and secondary flows over a wide range of conditions. Comparisons made with published experimental laser Doppler anemometry measurements and theoretical calculations for similar flow conditions revealed excellent accuracy and precision levels. The successful measurement of velocity profiles for a variety of flow conditions and geometries indicate that magnetic resonance imaging is an accurate, non-contacting velocimeter.

Aorta, Thoracic↗
PubMed1990

Reversibility bull's-eye: a new polar bull's-eye map to quantify reversibility of stress-induced SPECT thallium-201 myocardial perfusion defects.

Myocardial ischemia is currently interpreted from SPECT thallium-201 (201Tl) tomograms by the subjective visual finding of stress-induced perfusion defects which "normalize" or "reverse" by 4 hr. Thus, we have developed a computer method to quantify and display the three-dimensional distribution of reversible segments. Circumferential profiles generated from the short axis slices are normalized to the reference area in the stress study. The stress is subtracted from the normalized delayed data, and then displayed as a polar bull's-eye plot so that positive values show areas that have "reversed" or "improved." Patient profiles are compared to means and standard deviations of reversibility for all pixels determined from the Emory normal male files. Criteria for reversibility were developed from studies of 42 male patients found to have 48 defects, as determined by the consensus of five blinded expert observers. There was computer agreement with the experts on 25 of 31 relatively fixed and 14 of 17 reversible defects. Our preliminary results indicate that this new method promises to aid observers to more consistently identify and quantify the reversibility of SPECT 201Tl myocardial perfusion defects.

Adult↗
PubMed1989

Dynamic cardiac MR imaging. Techniques and applications.

Current state-of-the-art dynamic or cine cardiac MR imaging is a time-effective study that offers a comprehensive evaluation of cardiac structure and function at rest. The major limitation at present is its inability to screen for coronary artery disease and evaluate ventricular function under stress. The future promises routine quantification of aortic, pulmonic, and other vascular flow rates, ultrafast (seconds) to real-time imaging, and paramagnetic or magnetic susceptibility perfusion markers. With the combination of real-time imaging and magnetic perfusion agents, imaging to detect coronary artery disease should also be possible. The use of pharmacologic stress or possibly physical stress with real-time imaging may also make this a reality in the not-too-distant future. MR coronary angiography remains a theoretical possibility but requires significant technologic advances, and is thus more remote. Existing and developing technology, however, seems to indicate that in the near-to-intermediate term, dynamic MR imaging will be an effective tool for evaluating almost all major types of cardiovascular disease.

Animals↗
PubMed1987

Use of standard gradients with compound oblique angulation for optimal quantitative MR flow imaging in oblique vessels.

The earliest described phase-modulation techniques for flow quantification by MR imaging require a phase image obtained by modifying one of the imaging gradients and a reference phase image obtained without the modified gradient. However, by using the same gradients that are used for routine two-dimensional Fourier transform imaging, both anatomic and velocity-encoded images can be obtained in one scan. Although convenient, this technique is sensitive to flow both within and perpendicular to the imaging plane. Consequently, significant errors occur in the measurement of flow in vessels oblique to the image plane. To determine the relative accuracy and practicality of quantitatively measuring flow in oblique vessels, we used standard sequence gradients with routine orthogonal plane imaging and direct compound oblique plane imaging. Phantom studies of flow in a vessel aligned along the z axis showed a significant linear correlation (r = .999; p less than .05) between the spin phase and spin velocity. However, studies of flow at relatively low physiologic rates (12-17 cm/sec) in vessels angled 0-30 degrees off axis showed that obliquities of as little as 10 degrees result in significant quantification errors. This is due to a larger phase shift per unit velocity along the frequency-encoding direction vs along the slice-select direction and to a mixture of velocities within a voxel that is oblique to the flow direction. In most instances, resolution of these errors can be achieved satisfactorily only by electronic plane rotation with compound oblique angulation so that the image plane and vessel are perpendicular. When so used, this technique potentially might provide important adjunctive quantitative flow data in oblique vessels during routine clinical imaging.

Magnetic Resonance Spectroscopy↗
PubMed1987

Quantitative phase-flow MR imaging in dogs by using standard sequences: comparison with in vivo flow-meter measurements.

For evaluation of the feasibility and clinical potential of using the phase data from standard MR imaging sequences to measure blood flow, 11 vessels with diameters of 4 to 7 mm were imaged in seven dogs. The flow in either the superior mesenteric vein or the inferior vena cava was measured first at laparotomy (in ml/min) with electromagnetic flow meters. Immediately thereafter, these vessels were imaged by MR in 25-mm thick sections by using a standard spin echo (SE) 750/30 sequence with a Philips 0.5-T imager. Previous phase-flow calibration of the imager and sequence allowed calculation of the blood flow rates from the phase images that were used to measure the vessels' cross-sectional areas and blood phase values. Comparison of the measurements obtained with each technique showed a significant correlation (r = .977, p less than .05) between MR-imaging values and flow-meter measurements when the blood velocity was less than approximately 40 cm/sec, the known upper limit of the flow dynamic range for the MR hardware and sequence used. There was no correlation for blood velocities greater than 40 cm/sec. However, the range of blood flow velocities in dogs and man extends to more than 100 cm/sec. Thus, these results suggest that this technique might yield valuable adjunctive flow data in routine clinical imaging provided that improvements in hardware and software permit a larger dynamic range.

Animals↗
PubMed1986

Fast-field-echo MR imaging with Gd-DTPA: physiologic evaluation of the kidney and liver.

To determine if magnetic resonance imaging with Gd-DTPA could be used to assess renal and hepatic perfusion and possibly function, a fast-field-echo technique was used to perform sequential imaging of the kidney and liver of five subjects. Sixteen 3-second images of the same section were obtained at 13-second intervals immediately after Gd-DTPA administration, and again at 30 or 50 minutes after injection. From these images, curves of renal and hepatic signal versus time were generated. In each case the renal signal intensity peaked within 2 minutes and decreased to 60% or less by 3.4 minutes, and 35% or less by 50 minutes. Hepatic curves peaked within 2 minutes and approached initial levels by 30 minutes. These results suggest that transit and clearance of the contrast agent can be imaged by this technique.

Humans↗