Bulk Flow and Near Wall Hemodynamics of the Rabbit Aortic Arch and Descending Thoracic Aorta: A 4D PC-MRI Derived Computational Fluid Dynamics Study.

Animal models offer a flexible experimental environment for studying atherosclerosis. The mouse is the most commonly used animal, however, the underlying hemodynamics in larger animals such as the rabbit are far closer to that of humans. The aortic arch is a vessel with complex helical flow and highly heterogeneous shear stress patterns which may influence where atherosclerotic lesions form. A better understanding of intraspecies flow variation and the impact of geometry on flow may improve our understanding of where disease forms. In this work, we use magnetic resonance angiography (MRA) and 4D phase contrast magnetic resonance imaging (PC-MRI) to image and measure blood velocity in the rabbit aortic arch. Measured flow rates from the PC-MRI were used as boundary conditions in computational fluid dynamics (CFD) models of the arches. Helical flow, cross flow index (CFI), and time-averaged wall shear stress (TAWSS) were determined from the simulated flow field. Both traditional geometric metrics and shape modes derived from statistical shape analysis were analyzed with respect to flow helicity. High CFI and low TAWSS were found to colocalize in the ascending aorta and to a lesser extent on the inner curvature of the aortic arch. The Reynolds number was linearly associated with an increase in helical flow intensity (R = 0.85, p < 0.05). Both traditional and statistical shape analyses correlated with increased helical flow symmetry. However, a stronger correlation was obtained from the statistical shape analysis demonstrating its potential for discerning the role of shape in hemodynamic studies.

Noninvasive Assessment of Intracranial Pressure Status in Idiopathic Intracranial Hypertension Using Displacement Encoding with Stimulated Echoes (DENSE) MRI: A Prospective Patient Study with Contemporaneous CSF Pressure Correlation.

BACKGROUND AND PURPOSE: Intracranial pressure is estimated invasively by using lumbar puncture with CSF opening pressure measurement. This study evaluated displacement encoding with stimulated echoes (DENSE), an MR imaging technique highly sensitive to brain motion, as a noninvasive means of assessing intracranial pressure status. MATERIALS AND METHODS: Nine patients with suspected elevated intracranial pressure and 9 healthy control subjects were included in this prospective study. Controls underwent DENSE MR imaging through the midsagittal brain. Patients underwent DENSE MR imaging followed immediately by lumbar puncture with opening pressure measurement, CSF removal, closing pressure measurement, and immediate repeat DENSE MR imaging. Phase-reconstructed images were processed producing displacement maps, and pontine displacement was calculated. Patient data were analyzed to determine the effects of measured pressure on pontine displacement. Patient and control data were analyzed to assess the effects of clinical status (pre-lumbar puncture, post-lumbar puncture, or control) on pontine displacement. RESULTS: Patients demonstrated imaging findings suggesting chronically elevated intracranial pressure, whereas healthy control volunteers demonstrated no imaging abnormalities. All patients had elevated opening pressure (median, 36.0 cm water), decreased by the removal of CSF to a median closing pressure of 17.0 cm water. Patients pre-lumbar puncture had significantly smaller pontine displacement than they did post-lumbar puncture after CSF pressure reduction (P = .001) and compared with controls (P = .01). Post-lumbar puncture patients had statistically similar pontine displacements to controls. Measured CSF pressure in patients pre- and post-lumbar puncture correlated significantly with pontine displacement (r = 0.49; P = .04). CONCLUSIONS: This study establishes a relationship between pontine displacement from DENSE MR imaging and measured pressure obtained contemporaneously by lumbar puncture, providing a method to noninvasively assess intracranial pressure status in idiopathic intracranial hypertension.

Imaging time after Gd-DTPA injection is critical in using delayed enhancement to determine infarct size accurately with magnetic resonance imaging.

BACKGROUND: In patients with acute myocardial infarction (MI), delayed enhancement is seen in MRI 5 to 7 minutes after gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA) injection, and the enhancement occurs in regions that later show recovery of function. However, in a canine model of acute MI, delayed enhancement 20 to 30 minutes after injection only occurs in necrotic regions and not in surrounding, reversibly injured myocardium. The objective of the present study was to determine (1) if the size of the enhanced region varies with time after Gd-DTPA injection and (2) if and when the size of the enhanced region corresponds to the true infarct size. METHODS AND RESULTS: The left coronary artery was occluded in 15 Lewis rats for 30 minutes (n=9) or 2 hours (n=6); this was followed by reperfusion. MRI scans were performed 48+/-2 hours after-MI. Midventricular short-axis images were obtained continuously for 40 minutes after Gd-DTPA injection (0.3 mmol/kg). The sizes of enhanced regions at each time were determined by threshold analysis and compared with triphenyltetrazolium chloride-stained sections of the excised rat heart. In all animals, the enhanced region overestimated infarct size (28+/-5%) immediately after the injection of Gd-DTPA, although it then gradually receded to match the size of the infarct. The time required for enhancement to accurately determine infarct size was significantly different between 2-hour infarcts (16+/-2 minutes) and 30-minute (26+/-4 minutes) infarcts (P<0.05). CONCLUSIONS: In reperfused acute MI, accurate determination of infarct size by delayed enhancement MRI requires imaging at specific times after Gd-DTPA injection, and this time varies with the duration of occlusion.

Improved measurement of pressure gradients in aortic coarctation by magnetic resonance imaging.

OBJECTIVES: This study evaluated whether magnetic resonance imaging (MRI) and magnetic resonance (MR) phase velocity mapping could provide accurate estimates of stenosis severity and pressure gradients in aortic coarctation. BACKGROUND: Clinical management of aortic coarctation requires determination of lesion location and severity and quantification of the pressure gradient across the constricted area. METHODS: Using a series of anatomically accurate models of aortic coarctation, the laboratory portion of this study found that the loss coefficient (K), commonly taken to be 4.0 in the simplified Bernoulli equation delta P = KV2, was a function of stenosis severity. The values of the loss coefficient ranged from 2.8 for a 50% stenosis to 4.9 for a 90% stenosis. Magnetic resonance imaging and MR phase velocity mapping were then used to determine coarctation severity and pressure gradient in 32 patients. RESULTS: Application of the new severity-dependent loss coefficients found that pressure gradients deviated from 1 to 17 mm Hg compared with calculations made with the commonly used value of 4.0. Comparison of MR estimates of pressure gradient with Doppler ultrasound estimates (in 22 of 32 patients) and with catheter pressure measurements (in 6 of 32 patients) supports the conclusion that the severity-based loss coefficient provides improved estimates of pressure gradients. CONCLUSIONS: This study suggests that MRI could be used as a complete diagnostic tool for accurate evaluation of aortic coarctation, by determining stenosis location and severity and by accurately estimating pressure gradients.

Magnetic resonance phase-shift velocity mapping in pediatric patients with pulmonary venous obstruction.

OBJECTIVES: This study evaluated the accuracy, advantages and clinical efficacy of magnetic resonance (MR) phase-shift velocity mapping, in delineating the site and the hemodynamic severity of pulmonary venous (PV) obstruction in patients with congenital heart disease (CHD). BACKGROUND: Magnetic resonance phase-shift velocity mapping of normal pulmonary veins and of obstructed PV pathways have been previously reported in a mainly adult population. METHODS: The study population (33 pts) underwent MR phase-shift velocity mapping of their PV pathways. These results were compared with cardiac catheterization and Doppler echocardiography data. RESULTS: The study population (0.4 to 19.5 years) consisted of a study group (PV pathway obstruction, n = 7) and a control group (no PV obstruction, n = 26). No patients had any left-to-right shunt lesions. The MR imaging displayed precise anatomical detail of the pulmonary veins. Phase velocities in the control group ranged from 20 to 71 cm/s, whereas velocities in the study group ranged from 100 to 250 cm/s (p = 0.002). The MR phase velocities (154 +/- 0.53 cm/s) compared favorably with Doppler echocardiography (147 +/- 0.54 cm/s), (r = 0.76; p = 0.05). The MR velocity mapping was 100% specific and 100% sensitive in detecting PV obstruction, although the absolute gradient measurements among MR phase mapping, echocardiographic Doppler and catheterization did not show statistically significant correlation. CONCLUSIONS: In the absence of any associated left-to-right shunt lesions, PV velocities of 100 cm/s and greater indicated significant obstruction. The MR phase-shift velocity mapping, together with MR spin echocardiography and MR angiography, provides comprehensive anatomic and physiologic data that may obviate the need for further invasive studies.

Proton resonance frequency chemical shift thermometry: experimental design and validation toward high-resolution noninvasive temperature monitoring and in vivo experience in a nonhuman primate model of acute ischemic stroke.

BACKGROUND AND PURPOSE: Applications for noninvasive biologic temperature monitoring are widespread in biomedicine and of particular interest in the context of brain temperature regulation, where traditionally costly and invasive monitoring schemes limit their applicability in many settings. Brain thermal regulation, therefore, remains controversial, motivating the development of noninvasive approaches such as temperature-sensitive nuclear MR phenomena. The purpose of this work was to compare the utility of competing approaches to MR thermometry by using proton resonance frequency chemical shift. We tested 3 methodologies, hypothesizing the feasibility of a fast and accurate approach to chemical shift thermometry, in a phantom study at 3T. MATERIALS AND METHODS: A conventional, paired approach (difference [DIFF]-1), an accelerated single-scan approach (DIFF-2), and a new, further accelerated strategy (DIFF-3) were tested. Phantom temperatures were modulated during real-time fiber optic temperature monitoring, with MR thermometry derived simultaneously from temperature-sensitive changes in the water proton chemical shift (∼0.01 ppm/°C). MR thermometry was subsequently performed in a series of in vivo nonhuman primate experiments under physiologic and ischemic conditions, testing its reproducibility and overall performance. RESULTS: Chemical shift thermometry demonstrated excellent agreement with phantom temperatures for all 3 approaches (DIFF-1: linear regression R(2) = 0.994; P < .001; acquisition time = 4 minutes 40 seconds; DIFF-2: R(2) = 0.996; P < .001; acquisition time = 4 minutes; DIFF-3: R(2) = 0.998; P < .001; acquisition time = 40 seconds). CONCLUSIONS: These findings confirm the comparability in performance of 3 competing approaches to MR thermometry and present in vivo applications under physiologic and ischemic conditions in a primate stroke model.

Diagnosis of cardiac sarcoidosis aided by MRI.

We describe herein a case of dilated cardiomyopathy. The diagnosis of myocardial sarcoid was suggested by abnormal findings on an MRI of the chest. This was subsequently confirmed by a histology of a subcutaneous nodule. MRI is well suited in imaging myocardial scarring associated with sarcoidosis. Its use should be considered where sarcoidosis is suspected. Early detection and intervention with steroids may improve function.

Cine-MRI-aided endomyocardectomy in idiopathic hypereosinophilic syndrome.

The idiopathic hypereosinophilic syndrome is a leukoproliferative disorder marked by a predilection to damage specific organs, including the heart. This report describes a patient with extensive endocardial fibrosis accompanying this syndrome. Right ventricular endomyocardectomy with preservation of the tricuspid valve was performed. The procedure was aided by cine-magnetic resonance imaging for preoperative assessment and follow-up of surgical results.

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.

Quantification of the aortic regurgitant volume with magnetic resonance phase velocity mapping: a clinical investigation of the importance of imaging slice location.

BACKGROUND AND AIMS OF THE STUDY: Current techniques for assessment of aortic regurgitation (AR) are mainly qualitative. Magnetic resonance phase velocity mapping (PVM) provides accurate measurements of arterial blood blow. In AR, the aortic regurgitant volume (ARV) can be quantified with a single imaging slice measurement in the ascending aorta. The aim was to use PVM to: (i) quantify the regurgitant volume in patients with AR using an in vitro validated technique; and (ii) confirm in vivo our previous in vitro findings of the importance of measurement location. METHODS: Four healthy volunteers and 19 patients with AR, varying from mild to severe, were examined in a 1.5 Tesla MRI scanner. In 13 patients, the slice was placed: (i) between the aortic valve and the coronary ostia; (ii) at the sinotubular junction (SJ); and (iii) 2 cm above the SJ. In six patients, one measurement was taken as close as technically possible to the aortic valve. PVM measurements of the ARV were compared with angiographic/echocardiographic AR grading. RESULTS: No ARV was measured in healthy subjects. In patients, PVM results correlated well with angiographic/echocardiographic data. Repeatability of the PVM results was excellent and interobserver variability very small. The measured ARV decreased as the slice distance from the aortic valve increased, due to aortic compliance, in agreement to previous in vitro results. Close to the valve, acceleration did not affect the accuracy of velocity measurements. CONCLUSIONS: PVM has great potential to measure AR in a purely quantitative manner. Measurement location is important and results suggest that the closer the measurement to the valve the more accurate the ARV quantification.

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.

Effects of acceleration on the accuracy of MR phase velocity measurements.

Acceleration in blood flow can affect the accuracy of phase velocity measurements. Convective acceleration is due to changes in flow geometry and is independent of the time-varying acceleration caused by flow pulsatility. To analyze the effects of convective acceleration on flow velocity measurements, phase velocity measurements were obtained in steady laminar flow in the convergent segment of a 90%, hourglass-shaped stenosis phantom at a Reynolds number of 1,500. Measurements at the stenosis indicated that convective acceleration caused the measured values of average cross-sectional velocity to deviate as much as 37% from the theoretical values. The magnitude of the error could be accounted for by including the convective acceleration term in the phase shift equation. Convective acceleration effects should not be ignored in flow velocity measurements through stenoses, even when time-dependent acceleration due to flow pulsatility can be neglected.

Computational simulation of turbulent signal loss in 2D time-of-flight magnetic resonance angiograms.

Time-of-flight magnetic resonance (MR) angiography is currently limited in the evaluation of arterial stenoses by flow-induced signal loss. This signal loss has been attributed to phase dispersion and to phase misregistration. We have developed a fluid mechanics model of 2D time-of-flight MR angiograms to study the amount of signal loss caused by random turbulence. The simulations were created by stochastic analysis of particle pathlines determined by computational fluid dynamics for turbulent flow. The images obtained by the model compare well to actual MR images of flow in stenoses. By selectively removing the random turbulent motion in the simulation, it can be seen that random phase dispersion is the dominant mechanism of signal loss. Phase misregistration and mean flow phase dispersion act as secondary effects. The MR simulation model recreates accurately the variation of signal loss over a range of echo times. The model can be used further to explore and design new pulse sequences. For example, the current study showed that high slew rate gradient waveforms can significantly reduce poststenotic signal loss. In conclusion, computational modeling of MR angiography can be a useful approach for the analysis of MRA signal loss and the design of improved pulse sequences.

Magnetic resonance coronary angiography using navigator echo gated real-time slice following.

Navigator echo gating allows for the elimination of breath-holding in MR imaging by providing a real-time monitor of respiratory position to gate image acquisition. In this study we examined the advantages and utility of real-time, navigator echo gated slice following technique in 2D magnetic resonance coronary angiography of patients with coronary artery disease. Thirteen patients with coronary artery disease were examined. MR images of the right coronary artery (RCA) were obtained parallel to the atrioventricular groove to image long sections of the RCA in a small number of slices. In-plane resolution was 0.7 x 0.9 mm and 2-6 signals were averaged to support this high spatial resolution. Targeted maximum intensity projection (MIP) images were generated from the slices to present the RCA in a single image. All patients had x-ray angiograms available for comparison with the MR images. Using the navigator echo gated real-time slice following technique, MRI successfully obtained images in 11 of 13 cases. The technique failed in two patients with irregular breathing patterns. The average length of the RCA seen in the 11 successful MR exams was 61 mm and the average length seen in the x-ray angiograms was 80 mm. Eight patients were determined to be without disease in the RCA by x-ray angiography, and all eight were correctly identified as normal on the MRI exam. In the three patients who had a successful MRI exam and were determined to have disease in the RCA by x-ray angiography, MRI identified the lesion in two cases. In the third case MRI indicated a discrete lesion and x-ray angiography indicated diffuse disease without a focal lesion. Navigator echo gating improves patient tolerance, provides aligned sections of coronaries over multiple slices, and allows for improved resolution through signal averaging. This preliminary patient study suggests that navigator echo gated magnetic resonance coronary angiography may play a role in evaluating coronary artery disease.

Quantitative prediction of improvement in cardiac function after revascularization with MR imaging and modeling: initial results.

PURPOSE: To evaluate a model that can be used quantitatively to predict changes in postrevascularization left ventricular function based on classification of myocardial tissue as hibernating, scarred, or normal with cine magnetic resonance (MR) imaging. MATERIALS AND METHODS: Eleven patients with chronic left ventricular dysfunction were studied before and after revascularization with cine MR imaging. Regional myocardial contractility and wall thickness were used in the model to predict postrevascularization ejection fraction (EF). The actual EF from the postrevascularization MR images was compared with the EF from the prerevascularization images predicted with the model by using regression analysis and Bland-Altman analysis. RESULTS: Correlation between the actual EF after revascularization and the EF predicted by using the model yielded an R value of 0.98, with a standard error of 1.3 EF percentage points. Predicting changes in function in a myocardial segment was less successful because only 55% of segments classified as hibernating actually improved resting function after revascularization. In nonimproved segments, 78% were either adjacent to infarcted segments or had nontransmural wall thinning. CONCLUSION: A simple mathematical model combined with functional information provided by MR imaging was used to predict improvements in global EF resulting from revascularization.

Use of navigator-echo-gated MRI to diagnose a coronary shunt involving an anomalous origin of the right coronary artery from the pulmonary artery.

Origin of the right coronary artery from the main pulmonary artery is an anomaly that can cause formation of a left-to-right coronary shunt, leading to myocardial ischemia and early onset of congestive heart failure. We describe a case in which magnetic resonance imaging was able to show the anomalous origin of the right coronary artery, and magnetic resonance phase velocity mapping was able to demonstrate the presence of a left-to-right shunt through the coronary artery by showing retrograde flow in the right coronary artery.

Determination of wall shear stress in the aorta with the use of MR phase velocity mapping.

MR phase velocity mapping was used to calculate wall shear stress (WSS) in the suprarenal and infrarenal abdominal aorta, two sites with very different proclivities for development of a atherosclerosis. For the eight subjects studied, the average value of the mean (time averaged over the cardiac cycle) WSS in the suprarenal aorta was 10.4 dynes/cm2 at the posterior wall and 8.6 at the anterior wall. In the infrarenal aorta, WSS values were 4.7 at the posterior wall and 6.1 at the anterior wall. Peak WSS over the cardiac cycle was 48 and 54 at the anterior and posterior walls of the suprarenal aorta, respectively, and 33 and 30 at the anterior and posterior walls of the infrarenal aorta, respectively. Wide variation was found in both mean and peak WSS values among subjects. However, for 28 of 32 locations examined, mean and peak WSS were higher in the suprarenal aorta than in the infrarenal aorta. Because atherosclerosis is more likely to form in the infrarenal aorta than in the suprarenal aorta, this study supports the hypothesis that low WSS is a localizing factor for atherosclerosis, and high WSS may act as a deterrent against formation of atherosclerosis.

Comparison of phantom and computer-simulated MR images of flow in a convergent geometry: implications for improved two-dimensional MR angiography.

The signal loss that occurs in regions of disturbed flow significantly decreases the clinical usefulness of MR angiography in the imaging of diseased arteries. This signal loss is most often attributed to turbulent flow; but on a typical MR angiogram, the signal is lost in the nonturbulent upstream region of the stenosis as well as in the turbulent downstream region. In the current study we used a flow phantom with a forward-facing step geometry to model the upstream region. The flow upstream of the step was convergent, which created high levels of convective acceleration. This region of the flow field contributes to signal loss at the constriction, leading to overestimation of the area of stenosis reduction. A computer program was designed to simulate the image artifacts that would be caused by this geometry in two-dimensional time-of-flight MR angiography. Simulated images were compared with actual phantom images and the flow artifacts were highly correlated. The computer simulation was then used to test the effects of different orders of motion compensation and of fewer pixels per diameter, as would be present in MR angiograms of small arteries. The results indicated that the computational simulation of flow artifacts upstream of the stenosis provides an important tool in the design of optimal imaging sequences for the reduction of signal loss.

MRI techniques for cardiovascular imaging.

Over the last several years, cardiovascular MRI has benefited from a number of technical advances which have improved routine clinical imaging techniques. As a result, MRI is now well positioned to realize its longstanding promise of becoming the comprehensive cardiac imaging test of choice in many clinical settings. This may be achieved using a combination of basic advanced techniques. In this overview, the basic cardiac MRI techniques which are clinically useful are reviewed, and the recent technical advances which are clinically promising are described. These advances include routine black blood and cine bright blood techniques that are high speed (<10s per black blood image or cine slice), multislice whole heart perfusion imaging methods, and recently emerging real-time imaging methodologies. J Magn. Reson. Imaging 1999;10:590-601.