Robert J. Herfkens

Nuclear Magnetic Resonance Imaging of Acute Myocardial Infarction in Dogs: Alterations in Magnetic Relaxation Times

Nuclear magnetic resonance (NMR) imaging was used to study 24-hour-old acute myocardial infarctions in 8 dogs. Images and measurements of excised hearts were obtained in a 6.5 ml bore-resistive NMR imager (0.35 Tesla). Spin echo NMR imaging in each instance demonstrated the area of infarction as a region of increased signal intensity compared with that in normal myocardium. The T1 and T2 values of the area of infarction were greater than those of normal myocardium in all dogs. For each dog the T1 value was greater for the infarct region; however, the group mean value for T1 (ms) of the infarct region (728 +/- 94) was not significantly greater than that for the normal region (650 +/- 87). The T2 value (ms) was discriminate for all dogs, and the mean value for the infarct region (48 +/- 2) was significantly different (p less than 0.01) from the value for normal myocardium (42 +/- 1). The percent water content of the infarct (79 +/- 1%) was significantly greater (p less than 0.01) than that of normal regions (76 +/- 1%). The linear relationship between T2 value and percent water content showed a good correlation coefficient (r = 0.90; p less than 0.01). NMR imaging detects acute myocardial infarction as a positive image without contrast media. Increased signal intensity of the infarct is related to increased hydrogen density and increased T2 relaxation time.

Time-resolved three-dimensional phase-contrast MRI.

To demonstrate the feasibility of a four‐dimensional phase contrast (PC) technique that permits spatial and temporal coverage of an entire three‐dimensional volume, to quantitatively validate its accuracy against an established time resolved two‐dimensional PC technique to explore advantages of the approach with regard to the four‐dimensional nature of the data.

CT-determined pulmonary artery diameters in predicting pulmonary hypertension.

This study was to determine if the diameters of pulmonary arteries measured from computed tomographic (CT) scans could be used 1) as indicators of pulmonary artery hypertension and 2) as a reliable base for calculating mean pulmonary artery pressure. The diameters of the main, left, proximal right, distal right, interlobar, and left descending pulmonary arteries were measured from CT scans in 32 patients with cardiopulmonary disease and in 26 age- and sex-matched control subjects. Diameters were measured using a special computer program that could display a CT density profile of the artery and its adjacent tissues. The upper limit of normal diameter for the main pulmonary artery was found to be 28.6 mm (mean + 2 SD). In the patient group, the diameters were correlated with data from cardiac catheterization. In these patients, a diameter of the main pulmonary artery above 28.6 mm readily predicted the presence of pulmonary hypertension. The calculated cross-sectional areas of the main and interlobar pulmonary arteries (normalized for body surface area [BAS]) were found to give the best estimates of mean pulmonary artery pressure (r = 0.89, P less than 0.001 and r = 0.66, P less than 0.001). Multiple regression analysis gave the useful equation: mean pulmonary artery pressure = -10.92 + 0.07646 X area of main pulmonary artery/BSA + 0.08084 X area of the right interlobar pulmonary artery/BSA (r = 0.93, P less than 0.0001). Because CT allows precise, noninvasive measurement of the diameter of pulmonary arteries, it can be of value in detecting pulmonary hypertension and estimating mean pulmonary artery pressure.

Time-resolved 3-dimensional Velocity Mapping in the Thoracic Aorta: Visualization of 3-directional Blood Flow Patterns in Healthy Volunteers and Patients

Objective: An analysis of thoracic aortic blood flow in normal subjects and patients with aortic pathologic findings is presented. Various visualization tools were used to analyze blood flow patterns within a single 3-component velocity volumetric acquisition of the entire thoracic aorta Methods: Time-resolved, 3-dimensional phase-contrast magnetic resonance imaging (3D CINE PC MRI) was employed to obtain complete spatial and temporal coverage of the entire thoracic aorta combined with spatially registered 3-directional pulsatile blood flow velocities. Three-dimensional visualization tools, including time-resolved velocity vector fields reformatted to arbitrary 2-dimensional cut planes, 3D streamlines, and time-resolved 3D particle traces, were applied in a study with 10 normal volunteers. Results from 4 patient examinations with similar scan prescriptions to those of the volunteer scans are presented to illustrate flow features associated with common pathologic findings in the thoracic aorta. Results: Previously reported blood flow patterns in the thoracic aorta, including right-handed helical outflow, late systolic retrograde flow, and accelerated passage through the aortic valve plane, were visualized in all volunteers. The effects of thoracic aortic disease on spatial and temporal blood flow patterns are illustrated in clinical cases, including ascending aortic aneurysms, aortic regurgitation, and aortic dissection. Conclusion: Time-resolved 3D velocity mapping was successfully applied in a study of 10 healthy volunteers and 4 patients with documented aortic pathologic findings and has proven to be a reliable tool for analysis and visualization of normal characteristic as well as pathologic flow features within the entire thoracic aorta.

Comparison of flow patterns in ascending aortic aneurysms and volunteers using four‐dimensional magnetic resonance velocity mapping

To determine the difference in flow patterns between healthy volunteers and ascending aortic aneurysm patients using time‐resolved three‐dimensional (3D) phase contrast magnetic resonance velocity (4D‐flow) profiling.

The aortopathy of bicuspid aortic valve disease has distinctive patterns and usually involves the transverse aortic arch

OBJECTIVES Bicuspid aortic valves are associated with a poorly characterized connective tissue disorder that predisposes to aortic catastrophes. Because no criterion exists dictating the appropriate extent of aortic resection in aneurysmal disease of the bicuspid aortic valve, we studied the patterns of aortic dilation in this population. METHODS Sixty-four patients with bicuspid aortic valves who underwent computed tomographic or magnetic resonance angiography and echocardiography were retrospectively identified between January 2002 and March 2006. Orthonormal 2-dimensional or 3-dimensional aortic diameters were measured at 10 levels. Agglomerative hierarchic clustering with centered correlation distance measurements and complete linkage analysis was used to detect distinct patterns of aortic dilatation. RESULTS Mean aortic diameter was 28.1 +/- 0.7 mm at the annulus and 21.7 +/- 0.4 mm at the diaphragmatic hiatus. The aorta was largest in the tubular ascending aorta (45.9 +/- 1.0 mm). Compared with the descending aorta, the transverse aortic arch was also dilated (P < .01). Cluster analysis showed 4 patterns of aortic dilatation: cluster I, aortic root alone (n = 8, 13%); cluster II, tubular ascending aorta alone (n = 9, 14%); cluster III, tubular portion and transverse arch (n = 18, 28%); and, cluster IV, aortic root and tubular portion with tapering across the transverse arch (n = 29, 45%). CONCLUSION Distinct patterns of aortic dilatation in patients with bicuspid aortic valves call for an individualized degree of aortic replacement to minimize late aortic complications and reoperation. Patients in clusters III and IV should have transverse arch replacement (plus concomitant root replacement in cluster IV). Patients in cluster I should undergo complete aortic root replacement, whereas in patients in cluster II supracommissural ascending aortic grafting is adequate.

In Vivo Quantification of Blood Flow and Wall Shear Stress in the Human Abdominal Aorta During Lower Limb Exercise

AbstractMagnetic resonance (MR) imaging techniques and a custom MR-compatible exercise bicycle were used to measure, in vivo, the effects of exercise on hemodynamic conditions in the abdominal aorta of eleven young, healthy subjects. Heart rate increased from 73±6.2 beats/min at rest to 110±8.8 beats/min during exercise (p < 0.0001). The total blood flow through the abdominal aorta increased from 2.9±0.6 L/min at rest to 7.2±1.4 L/min during exercise (p < 0.0005) while blood flow to the digestive and renal circulations decreased from 2.1±0.5 L/min at rest to 1.6±0.7 L/min during exercise (p < 0.01). Infrarenal blood flow increased from 0.9±0.4 L/min at rest to 5.6±1.1 L/min during exercise (p < 0.0005). Wall shear stress increased in the supraceliac aorta from 3.5±0.8 dyn/cm2 at rest to 6.2±0.5 dyn/cm2 during exercise (p < 0.0005) and increased in the infrarenal aorta from 1.3±0.8 dyn/cm2 at rest to 5.2±1.3 dyn/cm2 during exercise (p < 0.0005).

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-Mei-boom-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. Index Terms: Nuclear magnetic resonance, contrast mechanisms—Noise—Image display—Nuclear magnetic resonance.

In Vivo MR Angiographic Quantification of Axial and Twisting Deformations of the Superficial Femoral Artery Resulting from Maximum Hip and Knee Flexion

PURPOSE The goal of this study was to quantify in vivo deformations of the superficial femoral artery (SFA) during maximum knee and hip flexion with use of magnetic resonance (MR) angiography to improve description of the complex, dynamic SFA environment. MATERIALS AND METHODS Contrast medium-enhanced MR angiography was performed on the leg vasculature of eight healthy adults in the supine and fetal positions. The SFA was defined as the centerline path of the iliofemoral segment from the profunda femoris to the descending genicular artery. Deformations that resulted from flexion from the supine position to the fetal position were quantified with the SFA path and its branches. RESULTS Fourteen SFAs shortened from the supine position to fetal position, whereas two lengthened. Six of eight left SFAs twisted counterclockwise, and seven of eight right SFAs twisted clockwise. Straightness percentages for supine and fetal SFAs were 99.1%+/-0.4% and 98.7%+/-0.6%, respectively. From the supine position to the fetal position, the SFA shortened 13%+/-11% (P<.001) and twisted 60 degrees+/-34 degrees (P<.001). SFA arc length and percent shortening were strongly correlated (r>.8) between left and right limbs; however, no significant correlation existed for SFA twist angle. CONCLUSIONS Complex and varying vascular and muscular anatomy among study participants made SFA lengths and deformations from the supine position to the fetal position unpredictable a priori; however, there were strong symmetries between left and right SFAs in terms of arc length, length change, and direction of twist. The data show that, from the supine position to the fetal position, the SFA tended to shorten and twist substantially, suggesting these as possible fracture mechanisms and also providing important parameters for stent design.

Evaluation of myocardial motion tracking with cine-phase contrast magnetic resonance imaging.

RATIONALE AND OBJECTIVES.The accuracy of myocardial motion measurements, computed from cine-phase contrast (cine-PC) magnetic resonance (MR) velocity data, was compared with directly visualized motion of ME signal voids caused by implanted tantalum markers in anesthetized dogs. METHODS.Magnetic resonance imaging (MRI) data were electrocardiogram-gated and divided into 16 phases per cardiac cycle. Myocardial trajectories as a function of time in the cardiac cycle were measured using both methods for four to seven markers in each of eight animals. RESULTS.The peak observed in-plane excursion was 4.0 ±2.1 mm. The average deviation between displacements derived from velocity data versus displacements visualized directly was 1.1 ± 0.7 mm (27.5% of the peak displacement). The difference was less if three separate MR scans were used to measure each velocity component in the cine-PC method. This improvement is probably caused by improved temporal resolution. CONCLUSIONS.Cine-PC MRI offers a noninvasive method for accurate quantification of myocardial motion.