Variability of consecutive in vivo MR flow measurements in the main portal vein.

OBJECTIVE: The variability of consecutive cine phase-contrast MR flow measurements could significantly affect their use for clinical decisions, especially during provocative testing. The purposes of this study were to determine the normal variability of flow and consecutive flow measurements in the main portal vein on MR images and to determine how intraobserver variability, interobserver variability, and MR imager variability affect these measurements. SUBJECTS AND METHODS: MR flow measurements were acquired four consecutive times at the same location in the main portal vein of 12 subjects and three consecutive times at the same location in a nonpulsatile vessel model. All acquisitions were completed within 10 min. All main portal vein MR data sets were evaluated manually in a blinded review by two independent observers during three separate sessions spaced a mean of 4.5 weeks apart. Flow model data sets were evaluated during a single session by one observer. Variabilities were subsequently calculated by a components-of-variance analysis and by the coefficient of variation (SD/mean x 100). RESULTS: Of the total variance, 90% was due to flow variability among subjects (intersubject), 6% to flow variability within one subject (intrasubject), 2% to intraobserver variability, and 2% to interobserver variability. The coefficient of variation of consecutive MR portal vein flow measurements within a single subject was 11% +/- 5% (range, 3-23%). Intra- and interobserver variabilities were 5% +/- 2% (range, 1-11%) and 4% +/- 4% (range, 0-17%), respectively. MR imager variability was 1% +/- 1% (range, 0-2%). CONCLUSION: The mean variability of consecutive cine phase-contrast MR flow measurements in the main portal vein is 11% +/- 5% and could affect research and clinical protocols that employ this technique.

Abdominal phase-contrast MR angiography: breath-hold versus non-breath-hold techniques.

Breath-hold magnetic resonance (MR) imaging is now replacing many non-breath-hold pulse sequences in the upper abdomen because of faster imaging times and improved image quality. The authors compared non-breath-hold cine phase-contrast (PC) and breath-hold 2D phase-contrast (2DPC) magnetic resonance (MR) angiograms of the main portal vein (MPV) and superior mesenteric artery (SMA) in 12 volunteers. All angiograms were graded in overall image quality, vessel conspicuity, and signal-to-noise ratios (SNR). In the MPV MR angiograms, the breath-hold 2DPC sequence produced better images than the non-breath-hold cine PC sequence as graded by overall image quality (P = .016) and SNR (P = .004). Conversely, in the SMA MR angiograms, the non-breath-hold cine PC sequence produced better images than the breath-hold sequence in terms of overall image quality (P = .008) and SNR (P = .008). By reducing the most significant cause of image artifacts, (ie, using a breath-hold 2DPC sequence to decrease respiratory misregistration of the MPV, and using a cardiac-gated cine PC sequence to minimize pulsatile artifacts of the SMA), one can clearly optimize the quality of MR angiography.

Upper abdominal phase-contrast MR angiography: comparison of cine and non-cine techniques.

PURPOSE: To evaluate image quality, vessel conspicuity, and signal-to-noise ratio (S/N) of four two-dimensional magnetic resonance (MR) phase-contrast techniques: cine phase-contrast with respiratory compensation (CPC-R), cine phase-contrast without respiratory compensation (CPC), phase-contrast with two signals acquired (PC-2), and phase-contrast with eight signals acquired (PC-8). MATERIALS AND METHODS: MR angiographic images of the portal vein were obtained in 14 patients. RESULTS: Overall image quality was rated excellent in 10 of 14 patients (CPC-R), in one of 14 patients (CPC), in none of 14 patients (PC-2), and in one of 14 patients (PC-8). Views of 10 upper abdominal vessels were evaluated with each technique. The mean (+/- standard deviation) number of well-imaged vessels was 6 +/- 2 (CPC-R), 3 +/- 2 (CPC), 2 +/- 2 (PC-2), and 3 +/- 2 (PC-8). CPC-R images were significantly better in overall image quality and number of vessels seen (P < .01). Mean S/Ns were 10.8 +/- 3.4 (CPC-R), 6.8 +/- 2.7 (CPC), 4.5 +/- 1.7 (PC-2), and 6.4 +/- 2.1 (PC-8). S/N with CPC-R was significantly greater than with CPC, PC-2, or PC-8. CONCLUSION: CPC-R provided consistently higher quality angiographic images than were obtained with the other sequences studied.

MR measurements of mesenteric venous flow: prospective evaluation in healthy volunteers and patients with suspected chronic mesenteric ischemia.

PURPOSE: To quantify portal vein (PV) and superior mesenteric vein (SMV) flow before and after a standardized meal in healthy volunteers and to prospectively evaluate patients with a clinical suspicion of chronic mesenteric ischemia on the basis of magnetic resonance (MR) measurement of flow in the mesenteric venous system in volunteers. MATERIALS AND METHODS: Cine phase-contrast flow measurements were acquired in 10 asymptomatic volunteers and in 10 patients. RESULTS: In volunteers, the difference between the fasting and post-prandial flows in the SMV and PV was significant (P < .001), with a peak flow augmentation of 245% +/- 74 and 70% +/- 29, respectively. Postprandial augmentation of peak flow in the SMV was significantly less in patients with mesenteric ischemia compared with volunteers (64% +/- 28; P = .02). SMV flow augmentation in patients without mesenteric ischemia did not differ significantly from that in volunteers (206% +/- 36; P = .31). CONCLUSION: Measurement of postprandial flow augmentation in the SMV with MR imaging shows promise as a noninvasive screening test for chronic mesenteric ischemia.

Cine phase-contrast MR flow measurements: improved precision using an automated method of vessel detection.

OBJECTIVE: The purpose of this study was to construct a method of vessel edge detection that correctly identifies vessel pixels and to compare the interuser variability of cine phase contrast MR volumetric flow rates obtained with the conventional manual method and an automated method. MATERIALS AND METHODS: The automated method was developed based on a magnitude image threshold value and compared with the manual method in a flow phantom (three users) and in velocity images of the portal vein (five users). The threshold value determined from the magnitude image was applied to a region of interest surrounding the vessel of interest on the magnitude image to construct a vessel edge detection mask m(x,y). The velocity images were then multiplied by the mask m(x,y) and volumetric flow rates determined using the identified vessel pixels. RESULTS: In the flow phantom, flow measurements with the magnitude threshold method had significantly less interuser variability compared with the manual method (p < 0.01) and were within 10% (mean 6.0%) of the actual flow versus 35% (mean 18.6%) with the manual method. Regarding flow measurements in the portal vein of six volunteers, the magnitude threshold method was significantly more precise (p < 0.01) than the manual method with a mean standard deviation between the five users of 40.4 +/- 12.9 ml/min (range 22-60 ml/min) and 110.4 +/- 32.7 ml/min (range 70-155 ml/min), respectively. CONCLUSION: The magnitude threshold method of vessel edge detection developed in this study yields flow measurements that are accurate in the model system and have significantly less interuser variability than the manual method. This method shows promise for improving the precision of cine phase contrast flow measurements.

Correlation of arterial and venous blood flow in the mesenteric system based on MR findings. 1993 ARRS Executive Council Award.

OBJECTIVE: The purpose of this study was to determine if a consistent relationship exists between (1) flow in the portal vein and superior mesenteric vein and (2) flow in the mesenteric arteries. SUBJECTS AND METHODS: Cine phase-contrast methods were used to determine volumetric flow in the celiac axis, superior mesenteric artery, portal vein, and superior mesenteric vein in 11 healthy volunteers. RESULTS: The mean (+/- SD) volumetric flow rates in the celiac axis, superior mesenteric artery, superior mesenteric vein, and portal vein were 9.9 +/- 3.2, 6.0 +/- 1.8, 5.7 +/- 2.0, and 13.7 +/- 1.8 ml/min per kilogram, respectively. A strong correlation was found between measured mesenteric arterial blood flow, defined as the sum of flow in the celiac axis and the superior mesenteric artery, and portal venous flow (r = .97, p = .00002). Good agreement was found between flow in the superior mesenteric artery and that in the superior mesenteric vein (r = .98; p = .00001; bias = 20 ml/min). CONCLUSION: There is a consistent relationship between MR measurements of flow in the portal or superior mesenteric vein and the measured flow in the arteries supplying those veins. Flow in the superior mesenteric vein is an accurate predictor of flow in the superior mesenteric artery.

Evaluation of portal venous hypertension with cine phase-contrast MR flow measurements: high association of hyperdynamic portal flow with variceal hemorrhage.

Thirty-two patients with portal venous hypertension and endoscopically proved esophageal varices who were being evaluated for possible liver transplantation were studied with cine phase-contrast magnetic resonance (MR) imaging. Flow measurements in the main portal vein were obtained and associated with the presence of variceal hemorrhage within 2 years before the MR examination. Low (hypodynamic) flow was present in 22 patients, while high (hyperdynamic) flow was present in 10 patients. The presence of variceal hemorrhage was significantly associated with a high portal venous flow rate (P = .001), high variceal grade (P = .030), and Child class A or B (P = .003); however, only portal venous flow (P = .006) and variceal grade (P = .044) were found to be associated with variceal hemorrhage in a multiple logistic regression analysis. The portal venous flow rate was significantly higher among patients with Child class A or B disease compared with those with class C disease (median, 24.6 vs 8.0 mL/min.kg; P = .004).

Volumetric flow rates in the portal venous system: measurement with cine phase-contrast MR imaging.

OBJECTIVE: The purposes of this study were to (1) validate the accuracy of cine phase-contrast MR flow measurements within the portal vein, (2) develop a suitable protocol for using this method to measure volumetric flow rate in the portal venous system, and (3) use this protocol, with Doppler sonography as a reference, to measure portal venous flow in healthy volunteers and in patients with protal venous hypertension. SUBJECTS AND METHODS: Flow determinations were obtained in a model of fluid movement approximating blood-flow conditions in the portal venous system. A suitable protocol was based on consideration of the theoretical effects of (1) spatial resolution, (2) obliquity of the imaging plane to the direction of flow, and (3) signal-to-noise ratio of the signed quantitative velocity images (in three volunteers) on the accuracy and precision of flow measurements. This protocol was used to obtain cine phase-contrast MR images of the portal venous system in five volunteers and six patients. RESULTS: Values obtained with a flow phantom showed good accuracy of cine phase-contrast-measured vs actual volumetric flow rate (r = .995; p = .0001; MR rate = [0.94 x actual rate] + 65.6 ml/min; standard error of the y estimate = 67.3 ml/min). Velocity encoding and section thickness substantially influenced the signal-to-noise ratio of the velocity images, whereas flip angle and matrix size had only minimal effect. In volunteers and patients, portal volumetric flow rates determined by using MR images and Doppler sonography showed good correlation (r = .94; p = .0003). CONCLUSION: Our results indicate that cine phase-contrast MR imaging is a practical noninvasive method for measuring volumetric flow rates in the portal venous system.

Phase-contrast cine MR angiography in chronic liver disease.

Thirty-one patients with portal venous hypertension underwent phase-contrast cine magnetic resonance (MR) angiography to validate the technique for assessment of main portal vein patency and blood flow direction. Correlative findings from Doppler ultrasonography, conventional angiography, and surgery were acquired in 30, 17, and 16 patients, respectively. Findings from MR angiography correlated well with those of the other methods for determining portal vein patency and flow direction. The main portal vein was patent in 29 patients and was thrombosed in two patients at MR angiography. With the use of a 40-mm slab projection acquisition that required 5 minutes, the splenic and superior mesenteric veins, as well as extensive varices (when present), were seen in the majority of cases. Phase-contrast cine MR angiography is an accurate method of rapidly assessing morphologic characteristics, patency, and flow direction of the portal venous system and thus is a useful method for evaluating the condition of patients with chronic liver disease.