Blood flow measurements in the aorta and major arteries with MR velocity mapping

The purpose of this study was to measure antegrade and retrograde flow in the aorta and the major arterial pathways in the body noninvasively with cine magnetic resonance (MR) velocity mapping, to determine the hemodynamic significance of retrograde flow in arteries. Two hundred forty cine velocity maps for blood flow measurements were obtained at 29 sites in the aorta and the major arteries in 31 healthy human subjects of varying age at rest. Synchronous or isolated antegrade and retrograde flow was found in the entire aorta and in arteries supplying muscles. No retrograde flow was found in arteries supplying internal organs, such as the internal carotid or splanchnic arteries. The retrograde flow in the aorta and the extremity arteries contributes substantially to supplying diastolic perfusion of internal organs such as the heart, brain, and kidneys. Antegrade flow tends to be helical in the thoracic aorta.     

Immediate postgadolinium spoiled gradient-echo MRI for evaluating the abdominal aorta in the setting of abdominal MR examination

To assess the reproducibility and image quality of immediate postgadolinium chelate spoiled gradient-echo MRI in demonstrating disease of the abdominal aorta. All patients (27 patients: 21 men, 6 women) with substantial disease of the abdominal aorta, who underwent abdominal MR examinations at 1.5 T between 1991 and 1995, were entered in the study. Patients were referred for evaluation of suspected aortic disease (14 patients) or other abdominal diseases (13 patients). Three experienced investigators manually measured luminal and external aortic wall diameters and rated image quality, definition of inner and outer walls, extent of disease, and presence of other abdominal abnormalities, in an independent fashion. A cardiovascular surgeon then rated all studies to determine whether clinical management could be based on the MR findings alone. There was 98 to 99% agreement in measurements of luminal and external wall diameter between the three investigators. Overall image quality was rated as good in 77.8 to 88.9% of patients. A total of 31 additional nonaortic abdominal abnormalities were detected by all observers. The cardiovascular surgeon rated 25 of 27 studies as adequate to determine clinical management based on MR findings alone. Immediate postgadolinium spoiled gradient-echo MRI is a reproducible technique for the demonstration of abdominal aortic disease and possesses good image quality. Advantages of this technique include simultaneous evaluation of other nonvascular diseases of the abdomen, short examination time, and easy implementation as part of routine abdominal MRI scanning protocol.     

Exercise-related changes in aortic flow measured with spiral echo-planar MR velocity mapping

Spiral echo-planar magnetic resonance (MR) velocity mapping was used to measure exercise-related changes in flow in the descending thoracic aorta in 10 healthy volunteers. Flow was measured at rest and Immediately after dynamic exercise, with a 0.5-T imager with a surface receiving coil and electrocardiographic triggering. Supine exercise was performed with a home-built pedaling apparatus. Spiral velocity mapping was performed in a transverse plane through the descending thoracic aorta with the subject at rest. The subject was then asked to perform maximum exercise, stop, and hold his breath during a four-heartbeat acquisition time. Eight cine frames with a temporal resolution of 50 msec were acquired through systole. Each image was acquired in 40 msec during spiral acquisition of k-space data, starting at the center, 6 msec after the excitation pulse. Reproduclbility of the technique was established by repeating the flow measurement in four consecutive heartbeats. At rest, the heart rate (mean ± standard deviation), mean aortic flow, peak aortic flow, and time to peak flow were 68 beats per minute ± 6, 41 mllliliters per beat ±8, 107 mL/sec ± 20, and 175 msec ± 25, respectively. After exercise, the heart rate and mean and peak aortic flow were significantly increased (P < 0.001), measuring 101 beats per minute ±12, 57 milliliters per beat ± 11, and 158 mL/sec ±29, respectively, while the time to peak flow (115 msec ±32) was significantly reduced (P < 0.001). The four sets of values obtained for the first four consecutive heartbeats measured at rest were similar, as were those obtained for the first four heartbeats after exercise.     

4D magnetic resonance velocity mapping of blood flow patterns in the aorta in young vs. elderly normal subjects.

Four-dimensional magnetic resonance MR velocity mapping was developed to study normal flow patterns in the thoracic aorta using time-resolved cardiac gated three-directional velocity data. Sixteen normal subjects were studied, one young group (average age 31 years) and one group with elderly people (average age 72 years). Blood flowed in a right-handed helix from the ascending aorta to the aortic arch. A straight flow pattern or a left-handed helix was seen in the descending aorta. Blood flow was never parabolic. Blood flowed forward in early systole, retrograde in mid-to-end systole, and forward again in diastole in all subjects as a basic pattern. Continuous retrograde flow over a long distance was not seen, but blood entered a retrograde flow column at various levels. In young people blood passed from the aortic valve to the mid-descending aorta in less than one heartbeat. In people in their sixties it took two heartbeats and in people older than 78 years, it took three heartbeats. The maximum systolic forward velocities were higher in young subjects than in elderly while the retrograde velocities were lower. J. Magn. Reson. Imaging 1999;10:861-869.     

Severity of aortic regurgitation: interstudy reproducibility of measurements with velocity-encoded cine MR imaging.

The interstudy reproducibility of velocity-encoded cine (VEC) magnetic resonance (MR) imaging for quantification of regurgitant volume (RV) and regurgitant fraction (RF) was studied in 10 patients with chronic aortic regurgitation. Each patient underwent two VEC MR imaging studies. RV and RF were measured on the aortic flow curve by quantifying antegrade and retrograde flow per cardiac cycle. VEC MR imaging measurements for RV and RF correlated closely with volumetric measurements for both studies (r greater than .97). Interstudy reproducibility for VEC MR imaging measurement of RV and RF was high (r greater than .97), and the interstudy variability for VEC MR imaging measurements was low. These results demonstrate a high accuracy of VEC MR imaging for measurement of RV and RF in patients with chronic aortic regurgitation. The level of interstudy reproducibility of VEC MR imaging for quantitative assessment of RV and RF indicates the potential of this technique for follow-up and monitoring of response to therapy.     

Abdominal aorta: characterisation of blood flow and measurement of its regional distribution by cine magnetic resonance phase-shift velocity mapping

Magnetic resonance phase-shift-induced velocity mapping is a powerful technique for measuring in vivo blood velocity and flow non-invasively. Using this method we examined dimensions, distensibility, blood flow and its regional distribution in the abdominal aorta in 10 normal volunteers. Data were acquired at three levels of the descending aorta. Thirty percent reduction in diastolic cross sectional area was observed in the caudal direction between these levels. Total blood flow (ml/min) in the abdominal aorta at the three sites was 4094 ± 1600, 2339 ± 910 and 1602 ± 549 respectively. Flows in the coeliac trunk, superior mesenteric artery and renal arteries were also calculated. The net flow in the abdominal aorta above the coeliac trunk was persistently forward, while there was considerable backflow (13% of total instantaneous flow) below the renal arteries during early diastole. Magnetic resonance imaging is a non-invasive technique for quantitative assessment of blood flow in the abdominal aorta and its main branches. Correspondence to: M. Amanuma     

Inadvertent embolic obstruction of abdominal aorta from left atrial thrombus after percutaneous mitral valvuloplasty

Inadvertent embolic obstruction of the distal abdominal aorta and left renal artery during a percutaneous mitral valvuloplasty procedure in a patient with mitral stenosis is reported. The embolism was from a left atrial thrombus which was detected by magnetic resonance imaging (MRI) but not by transesophageal echocardiography.     

Quantification of flow dynamics in congenital heart disease: applications of velocity-encoded cine MR imaging.

Velocity-encoded cine (VEC) magnetic resonance (MR) imaging is a valuable technique for quantitative assessment of flow dynamics in congenital heart disease (CHD). VEC MR imaging has a variety of clinical applications, including the measurement of collateral flow and pressure gradients in coarctation of the aorta, differentiation of blood flow in the left and right pulmonary arteries, quantification of shunts, and evaluation of valvular regurgitation and stenosis. After surgical repair of CHD, VEC MR imaging can be used to monitor conduit blood flow, stenosis, and flow dynamics. There are some pitfalls that can occur in VEC MR imaging. These include potential underestimation of velocity and flow, aliasing, inadequate depiction of very small vessels, and possible errors in pressure gradient measurements. Nevertheless, VEC MR imaging is a valuable tool for preoperative planning and postoperative monitoring in patients with CHD.     

Delivery and assessment of endovascular stents to repair aortic coarctation using MR and X-ray imaging

PURPOSE: To investigate the utility of MR and X-ray imaging for characterizing aortic coarctation and flow, and guiding the endovascular catheter to place a stent to repair the coarctation. MATERIALS AND METHODS: The descending aorta in eight dogs was looped with elastic band and tightened distal to the subclavian artery. Balanced fast field echo (bFFE) and velocity-encoded cine (VEC) MRI sequences were used for device tracking and measuring aortic flow. A T1-weighted fast-field echo sequence (T1-FFE) was used to visualize the coarctation and roadmap the aorta. Nitinol stents were guided by a nitinol guidewire and placed under MR guidance. RESULTS: Aortic coarctation was visible on MR and X-ray imaging. The procedure success rate was 88%. VEC MRI measured the changes in aortic flow (baseline = 1.3 +/- 0.2, coarctation = 0.2 +/- 0.02, and stent placement = 0.8 +/- 0.1 liters/minute). A significant reduction in iliac blood pressure was measured after coarctation, but it was reversed by stent placement. The stent lumen was visible on X-ray fluoroscopy, but not on MRI. CONCLUSION: Stent deployment to repair aortic coarctation is feasible under MR guidance. The combined use of MR and X-ray imaging is effective for anatomic and functional evaluation of aortic coarctation dilation, which may be crucial for optimal therapy.     

Analysis of MR phase-contrast measurements of pulsatile velocity waveforms

Errors in the measurement of the mean velocity of pulsatile velocity waveforms with ungated phasecontrast techniques were studied theoretically and experimentally. Waveforms consisting of a constant and two sinusoidal components were analyzed. Variations in magnitude and phase of the vascular magnetic resonance (MR) signal resulted in errors, the severity of which increased when either factor increased. Magnitude variations always resulted in overestimation. The general shape of the waveform greatly influenced the error, with certain waveforms producing greater inherent error than others. Experimental measurements were performed, validating the predicted sensitivity of these errors to changes in imaging parameters, including TR and flow-encoding sensitivity. Errors generally became more severe with increased flow-encoding sensitivity. The theoretical and experimental results suggest that accurate mean velocity measurements in many vessels of the body-with acquisition times of less than 15 seconds-should be attainable with ungated imaging techniques and with careful selection of relevant imaging parameters.     

In vitro validation of flow measurements in an aortic nitinol stent graft by velocity-encoded MRI

Purpose

To validate flow measurements within an aortic nickel–titanium (nitinol) stent graft using velocity-encoded cine magnetic resonance imaging (VEC MRI) and to assess intraobserver agreement of repeated flow measurements.

Materials and methods

An elastic tube phantom mimicking the descending aorta was developed with the possibility to insert an aortic nitinol stent graft. Different flow patterns (constant, sinusoidal and pulsatile aortic flow) were applied by a gear pump. A two-dimensional phase-contrast sequence was used to acquire VEC perpendicular cross-sections at six equidistant levels along the phantom. Each acquisition was performed twice with and without stent graft, and each dataset was analysed twice by the same reader. The percental difference of the measured flow volume to the gold standard (pump setting) was defined as the parameter for accuracy. Furthermore, the intraobserver agreement was assessed.

Results

Mean accuracy of flow volume measurements was −0.45 ± 1.63% without stent graft and −0.18 ± 1.45% with stent graft. Slightly lower accuracy was obtained for aortic flow both without (−2.31%) and with (−1.29%) stent graft. Accuracy was neither influenced by the measurement position nor by repeated acquisitions. There was significant intraobserver agreement with an intraclass correlation coefficient of 0.87 (without stent graft, p < 0.001) and 0.80 (with stent graft, p < 0.001). The coefficient of variance was 0.25% without stent graft and 0.28% with stent graft.

Conclusion

This study demonstrated high accuracy and excellent intraobserver agreement of flow measurements within an aortic nitinol stent graft using VEC MRI. VEC MRI may give new insights into the haemodynamic consequences of endovascular aortic repair.     

Combined MR imaging and CFD simulation of flow in the human descending aorta

A combined MR and computational fluid dynamics (CFD) study is made of flow in the upper descending thoracic aorta. The aim was to investigate further the potential of CFD simulations linked to in vivo MRI scans. The three-dimensional (3D) geometrical images of the aorta and the 3D time-resolved velocity images at the entry to the domain studied were used as boundary conditions for the CFD simulations of the flow. Despite some measurement uncertainties, comparisons between simulated and measured flow structures at the exit from the domain demonstrated encouraging levels of agreement. Moreover, the CFD simulation allowed the flow structure throughout the domain to be examined in more detail, in particular the flow separation region in the distal aortic arch and its influence on the downstream flow during late systole. Additional information such as relative pressure and wall shear stress, which could not be measured via MRI, were also extracted from the simulation. The results have encouraged further applications of the methods described. J. Magn. Reson. Imaging 2001;13:699-713.     

Measurement of collateral blood flow in a porcine model of aortic coarctation by velocity-encoded cine MRI

The purpose of this study was to investigate the time course of development of collateral blood flow in an animal model of aortic coarctation. A juxtaductal aortic stenosis (model coarctation) was surgically created in five juvenile pigs. MRI was performed preoperatively, 1 to 2 days postoperatively, and 2 to 10 weeks postoperatively. Aortic blood flow was measured by velocity-encoded cine MR (VENC-MR). The percent change in aortic blood flow (ΔBF) from proximal to distal descending thoracic aorta was calculated, and a multiple-comparison paired t test used to assess changes in ΔBF over time. Invasive flow measurements were obtained in one animal before sacrifice using an ultrasonic probe. ΔBF preoperatively was ?2 ± 8% (mean ± SE). ΔBF increased to 32 ± 7% (mean ± SE, P = .022) 2 days postoperatively and 55 ± 19% (P = .032) 2 to 8 weeks postoperatively. Invasive measurements were in qualitative agreement with the VENC-MR data. VENC-MR is an accurate noninvasive method of measuring collateral blood flow in aortic coarctation. Recruitment and development of collateral flow pathways occur rapidly in an animal model.     

Coronary sinus flow measurement by means of velocity-encoded cine MR imaging: validation by using flow probes in dogs

PURPOSE: To validate coronary sinus flow measurements for quantification of global left ventricular (LV) perfusion by means of velocity-encoded cine (VEC) magnetic resonance (MR) imaging and flow probes. MATERIALS AND METHODS: Measurements of coronary sinus flow were performed in seven dogs by using VEC MR imaging at baseline, single coronary arterial stenosis, dipyridamole stress, and reactive hyperemia. These measurements were compared with flow probe measurements of coronary blood flow (CBF) in the left anterior descending coronary (LAD) and circumflex (CFX) arteries (CBF(LAD+CFX)) and coronary sinus. LV blood perfusion was calculated in milliliters per minute per gram from coronary sinus flow, and LV mass was obtained by using VEC and cine MR imaging. LV mass was validated at autopsy. RESULTS: CBF(LAD+CFX) and coronary sinus flow at VEC MR imaging showed close correlation (r = 0.98, P: <.001). The difference between CBF(LAD+CFX) and MR coronary sinus flow was 3.1 mL/min +/- 8.5 (SD). LV mass at cine MR imaging was not significantly different from that at autopsy (73.2 g +/- 12.8 vs 69. 4 g +/- 12.8). At baseline, myocardial perfusion was 0.40 mL/min/g +/- 0.09 at VEC MR imaging, and CBF(LAD+CFX) was 0.44 mL/min/g +/- 0. 08 (not significant). Reactive hyperemia resulted in 2.7- and 2. 3-fold increases in coronary sinus flow at VEC MR imaging and flow probe CBF(LAD+CFX), respectively. CONCLUSION: VEC MR imaging has the potential to measure coronary sinus flow during different physiologic conditions and can serve as a noninvasive modality to quantify global LV perfusion in patients.     

MR flow quantification using race: Clinical application to the carotid arteries

Real time MR flow quantification was performed with real time acquisition and evaluation of motion (RACE) in a rigid phantom under steady flow conditions and in the common carotid arteries of 43 subjects aged 24–78 years. Hemodynamic information included the intraluminal velocity distribution during the complete cardiac cycle, the distensibility of the arterial wall, and age-dependent changes of the flow curves. Systolic peak velocities of 51 ± 33 cm/s and time-averaged volume flow rates of 4.3 ± 2.0 ml/s were measured in healthy subjects. Flow rates below 3.0 ml/s and the observation of abnormal flow patterns indicated stenoses greater than 70% in the region of the bifurcation (sensitivity: 83.3%; specificity: 93.7%; accuracy: 71.4%). Improvements may be achieved from a combination with MR angiography, providing both functional and morphologic vascular information noninvasively within one observer-independent examination. MR imaging, therefore, has a strong potential for the diagnosis of critical stenoses in symptomatic patients.     

Pulsatile flow artifacts in two-dimensional time-of-flight MR angiography: Initial studies in elastic models of human carotid arteries

Initial experimental and numerical analysis of artifacts due to pulsatile flow in two-dimensional time-of-flight (2D-TOF) magnetic resonance (MR) angiography are presented. The experimental studies used elastic models of the carotid artery bifurcation cast from fresh cadavers and accurately reproducing the twisting and tapering of the human blood vessels, allowing direct comparison of images with and without flow. Prominent image artifacts, including periodic ghosts and signal loss, were produced by pulsatile flow even though flow-compensated gradient waveforms were used. The dependence of artifacts due to partial saturation on pulse sequence parameters (TR and flip angle) was investigated theoretically for a simple pulsatile velocity profile and compared with experimental results from a model of a normal carotid artery. Signal reduction was observed proximal and distal to the stenosis in a model with a 70% internal carotid artery (ICA) stenosis and a model with 90% stenoses in both the ICA and the external carotid artery. Although this study deals exclusively with 2D-TOF imaging, the methods can also be applied to evaluate other MR angiography techniques.     

Biophysical properties of the normal-sized aorta in patients with Marfan syndrome: evaluation with MR flow mapping

PURPOSE: To investigate the feasibility of magnetic resonance (MR) flow mapping in the assessment of aortic biophysical properties in patients with Marfan syndrome and to detect differences in biophysical properties in the normal-sized aorta distal to the aortic root between these patients and matched control subjects. MATERIALS AND METHODS: Seventy-eight patients with Marfan syndrome with aortic root dilatation and 23 matched control subjects underwent MR flow mapping in four locations in the normal-sized aorta (1, ascending aorta; 2, thoracic descending aorta; 3, descending aorta at the level of the diaphragm; and 4, abdominal descending aorta). Distensibility at each location and flow wave velocity between locations were calculated. RESULTS: Compared with the control subjects, patients with Marfan syndrome had decreased aortic distensibility at three of the four locations (levels 1, 2, and 4; P <.05) and increased flow wave velocity between all locations (P <.05) in the aorta. In patients with Marfan syndrome, flow wave velocity was also significantly increased along the entire aortic tract beyond the aortic root (from level 1 to level 4). CONCLUSION: MR imaging reveals abnormal biophysical properties of the normal-sized aorta in patients with Marfan syndrome. Monitoring of these properties is relevant for evaluating disease progression and treatment options.     

Right and left ventricular stroke volume measurements with velocity-encoded cine MR imaging: in vitro and in vivo validation

The accuracy of measurements of flow velocity determined by using cine MR phase velocity mapping--velocity-encoded cine (VEC) MR--was assessed by comparing VEC MR data with independent measurements in a flow phantom and in human subjects. Constant flow velocities generated in a phantom (range, 20-408 cm/sec) were determined correctly by VEC MR (r = .997, standard error of the estimate [SEE] = 7.9 cm/sec). Peak systolic velocities in the main pulmonary artery determined by VEC MR correlated well with the measurements obtained by using continuous-wave Doppler echocardiography (r = .91). Stroke volumes measured at the aorta by VEC MR and continuous-wave Doppler imaging also correlated well with each other (r = .80). VEC MR measurements of aortic and pulmonary flow provided left and right ventricular stroke volumes that correlated well with left ventricular stroke volumes determined by short-axis cine MR images (r = .98, SEE = 3.7 ml, and r = .95, SEE = 4.8 ml, respectively). Intra- and interobserver variabilities were small for both left and right ventricular stroke volumes as measured with VEC MR. These results indicate that VEC MR accurately and reproducibly measures aortic and pulmonary flow velocities and volumes in the physiologic range of humans, and can be used to measure right and left ventricular stroke volumes under normal flow conditions.     

MR measurement of blood flow in the true and false channel in chronic aortic dissection

Velocity encoded (VEC) cine MR imaging is a new noninvasive technique for the quantification of blood flow velocity in the cardiovascular system. Six patients with type B aortic dissection underwent VEC cine MR imaging at 1.5 T. This technique provides cine MR magnitude and VEC phase images at approximately 16 equally spaced intervals during an average cardiac cycle. A region of interest encompassing a vascular structure, i.e., false channel, provides a spatially averaged velocity for the time interval at which the image was acquired. Interpretation of velocity values from the 16 intervals during the cardiac cycle provides a temporally average velocity. Velocity mapping across the aortic lumen in these six cases showed average spatial and temporal velocity of 13.4 +/- 1.49 cm/s in the true channel and 3.1 +/- 0.84 cm/s in the false channel (p less than 0.05). The peak systolic velocity (temporal peak) was 43.6 +/- 7.20 cm/s in the true channel and 14.3 +/- 2.30 cm/s in the false channel (p less than 0.05). The flow volume per cardiac cycle was not significantly different between the ture (23.1 +/- 5.04 ml/cycle) and false channel (27.1 +/- 10.14 ml/cycle). There was substantial retrograde flow in the false channel of two patients. The intraobserver and interobserver variability was less than 10% (r = 0.98 to 0.99) for the measurement of flow parameters in both the true and the false channel. We conclude that VEC cine MR imaging demonstrates substantial differences in the hemodynamic pattern in the true and false channel in aortic dissection.     

Characterization of atherosclerosis with a 1.5-T imaging system

It is shown that a conventional 1.5-T magnetic resonance (MR) imaging system can help characterize some of the key components of atherosclerotic plaque ex vivo. Fresh human aorta with atheromata was suspended in solutions of agarose and manganese chloride and heated to body temperature. The specimens were imaged with modified Dixon and projection-reconstruction imaging sequences. The specimens were then examined histologically to obtain direct correlation between images, spectra, and histologic characteristics. The results show that vessel wall and plaque components can be identified by means of their MR characteristics and correlated with their histologic appearance. The authors were able to identify normal vessel wall components, such as adventitial lipids and smooth muscle. They were also able to identify and localize plaque components such as fibrous tissue, calcification, lipids, and possible areas of hemorrhage and hemosiderin deposition.