Faster flow quantification using sensitivity encoding for velocity-encoded cine magnetic resonance imaging: in vitro and in vivo validation

PURPOSE: To test the agreement between conventional and sensitivity-encoded (SENSE) velocity encoded cine (VEC) MRI in a flow phantom and in subjects with congenital and acquired heart disease. MATERIALS AND METHODS: Flow measurements were performed in a 1.5 T scanner using a segmented k-space VEC MRI sequence and then repeated with a SENSE factor of 2. The flow phantom used a piston pump to generate physiologic arterial waveforms (0.5-4.9 L/min). In the subjects, flow measurements were performed in the ascending aorta (N = 33) and/or the main pulmonary artery (N = 24). RESULTS: Utilization of SENSE reduced the scan time by 50%. In the phantom, measurements without and with SENSE agreed closely with a mean difference of 0.01 +/- 0.08 L/min or 0.12% +/- 3.8% (P = 0.68). In the subjects, measurements without and with SENSE also agreed closely with a mean difference of 0.08 +/- 0.36 L/min or 1.3% +/- 7.2% (P = 0.08). Compared with standard imaging, the use of SENSE reduced the signal-to-noise ratio (SNR) by 28% in the phantom (N = 10) and 27% in vivo (N = 22). CONCLUSION: VEC MRI flow measurements with a SENSE factor of 2 were twice as fast and agreed closely with the conventional technique in vitro and in vivo. VEC MRI with SENSE can be used for rapid and reliable quantification of blood flow.     

Determination of the optimal first‐order gradient moment for flow‐sensitive dephasing magnetization‐prepared 3D noncontrast MR angiography

Flow‐sensitive dephasing (FSD) magnetization preparation has been developed for black‐blood vessel wall MRI and noncontrast MR angiography. The first‐order gradient moment, m₁, is a measure of the flow‐sensitization imparted by an FSD preparative module. Determination of the optimal m₁ for each individual is highly desirable for FSD‐prepared MR angiography. This work developed a 2D m₁‐scouting method that evaluates a range of m₁ values for their effectiveness in blood signal suppression in a single scan. The feasibility of using the 2D method to predict blood signal suppression in 3D FSD‐prepared imaging was validated on a flow phantom and the popliteal arteries of 5 healthy volunteers. Excellent correlation of the blood signal measurements between the 2D scouting and 3D FSD imaging was obtained. Therefore, the optimal m₁ determined from the 2D m₁‐scouting scan may be directly translated to 3D FSD‐prepared imaging. In vivo studies of additional 10 healthy volunteers and 2 patients have demonstrated the proposed method can help significantly improve the signal performance of FSD MR angiography, indicating its potential to enhance diagnostic confidence. Further systematic studies in patients are warranted to evaluate its clinical value. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Comparison of the quantitative first pass myocardial perfusion MRI with and without prospective slice tracking: Comparison between breath‐hold and free‐breathing condition

Physiologic motion of the heart is one of the major problems of myocardial blood flow quantification using first pass perfusion–MRI method. To overcome these problems, a perfusion pulse sequence with prospective slice tracking was developed. Cardiac motion was monitored by a navigator directly positioned at heart's basis to overcome no additional underlying model calculations connecting diaphragm and cardiac motion. Additional prescans were used before the perfusion measurement to detect slice displacements caused by remaining cardiac motion between navigator and the perfusion slice readout. The pulse sequence and subsequent quantification of myocardial blood flow was tested in healthy pigs with and without prospective slice tracking under both free‐breathing and breath‐hold conditions. To avoid influences by residual contrast agent concentration time courses were analyzed. Median myocardial blood flow values and interquartile ranges with prospective slice tracking under free‐breathing and in a breath‐hold were (1.04, interquartile range = 0.58 mL/min/g) and (1.20, interquartile range = 0.59 mL/min/g), respectively. This is in agreement with published positron emission tomography values. In measurements without prospective slice tracking (1.15, interquartile range = 1.58 mL/min/g), the interquartile range is significantly (P < 0.012) larger because of residual cardiac motion. In conclusion, prospective slice tracking reduces motion‐induced variations of myocardial blood flow under both during breath‐hold and under conditions of free‐breathing. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Estimation of perfusion and arterial transit time in myocardium using free‐breathing myocardial arterial spin labeling with navigator‐echo

Arterial spin labeling (ASL) provides noninvasive measurement of tissue blood flow, but sensitivity to motion has limited its application to imaging of myocardial blood flow. Although different cardiac phases can be synchronized using electrocardiography triggering, breath holding is generally required to minimize effects of respiratory motion during ASL scanning, which may be challenging in clinical populations. Here a free‐breathing myocardial ASL technique with the potential for reliable clinical application is presented, by combining ASL with a navigator‐gated, electrocardiography‐triggered TrueFISP readout sequence. Dynamic myocardial perfusion signals were measured at multiple delay times that allowed simultaneous fitting of myocardial blood flow and arterial transit time. With the assist of a nonrigid motion correction program, the estimated mean myocardial blood flow was 1.00 ± 0.55 mL/g/min with a mean transit time of ∼400 msec. The intraclass correlation coefficient of repeated scans was 0.89 with a mean within subject coefficient of variation of 22%. Perfusion response during mild to moderate stress was further measured. The capability for noninvasive, free‐breathing assessment of myocardial blood flow using ASL may offer an alternative approach to first‐pass perfusion MRI for clinical evaluation of patients with coronary artery disease. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Test-retest reliability of arterial spin labeling with common labeling strategies

Purpose

To compare the test–retest reproducibility of three variants of arterial spin labeling (ASL): pseudo‐continuous (pCASL), pulsed (PASL) and continuous (CASL).

Materials and Methods

Twelve healthy subjects were scanned on a 3.0T scanner with PASL, CASL, and pCASL. Scans were repeated within‐session, after 1 hour, and after 1 week to assess reproducibility at different scan intervals.

Results

Comparison of within‐subject coefficients of variation (wsCV) demonstrated high within‐session reproducibility (ie, low wsCV) for CASL‐based methods (gray matter [GM] wsCV for pCASL: 3.5% ± 0.02%, CASL: 4.1% ± 0.07%) compared to PASL (wsCV: 7.5% ± 0.06%), due to the higher signal‐to‐noise ratio (SNR) associated with continuous labeling, evident in the 20% gain in temporal SNR and 58% gain in raw SNR for pCASL relative to PASL. At the 1‐week scan interval, comparable reproducibility between PASL (GM wsCV 9.2% ± 0.12%) and pCASL (GM wsCV 8.5% ± 0.14%) was observed, indicating the dominance of physiological fluctuations.

Conclusion

Although all three approaches are capable of measuring cerebral blood flow within a few minutes of scanning, the high precision and SNR of pCASL, with its insensitivity to vessel geometry, make it an appealing method for future ASL application studies. J. Magn. Reson. Imaging 2011;33:940–949. © 2011 Wiley‐Liss, Inc.     

Helium‐3 MR q‐space imaging with radial acquisition and iterative highly constrained back‐projection

An undersampled diffusion‐weighted stack‐of‐stars acquisition is combined with iterative highly constrained back‐projection to perform hyperpolarized helium‐3 MR q‐space imaging with combined regional correction of radiofrequency‐ and T₁‐related signal loss in a single breath‐held scan. The technique is tested in computer simulations and phantom experiments and demonstrated in a healthy human volunteer with whole‐lung coverage in a 13‐sec breath‐hold. Measures of lung microstructure at three different lung volumes are evaluated using inhaled gas volumes of 500 mL, 1000 mL, and 1500 mL to demonstrate feasibility. Phantom results demonstrate that the proposed technique is in agreement with theoretical values, as well as with a fully sampled two‐dimensional Cartesian acquisition. Results from the volunteer study demonstrate that the root mean squared diffusion distance increased significantly from the 500‐mL volume to the 1000‐mL volume. This technique represents the first demonstration of a spatially resolved hyperpolarized helium‐3 q‐space imaging technique and shows promise for microstructural evaluation of lung disease in three dimensions. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

Estimation of intersubject variability of cerebral blood flow measurements using MRI and positron emission tomography

Purpose:

To investigate the within and between subject variability of quantitative cerebral blood flow (CBF) measurements in normal subjects using various MRI techniques and positron emission tomography (PET).

Materials and Methods:

Repeated CBF measurements were performed in 17 healthy, young subjects using three different MRI techniques: arterial spin labeling (ASL), dynamic contrast enhanced T1 weighted perfusion MRI (DCE) and phase contrast mapping (PCM). All MRI measurements were performed within the same session. In 10 of the subjects repeated CBF measurements by ¹⁵O labeled water PET had recently been performed. A mixed linear model was used to estimate between subject (CV_betw) and within subject (CV_with) coefficients of variation.

Results:

Mean global CBF, CV_betw and CV_with using each of the four methods were for PCM 65.2 mL/100 g/min, 17.4% and 7.4%, for ASL 37.1 mL/100 g/min, 16.2% and 4.8%, for DCE 43.0 mL/100 g/min, 20.0%, 15.1% and for PET 41.9 mL/100 g/min, 16.5% and 11.9%, respectively. Only for DCE and PCM a significant positive correlation between measurements was demonstrated.

Conclusion:

These findings confirm large between subject variability in CBF measurements, but suggest also that in healthy subjects a subject‐method interaction is a possible source of between subject variability and of method differences. J. Magn. Reson. Imaging 2012;35:1290–1299. © 2012 Wiley Periodicals, Inc.     

Precision of the CASL-perfusion MRI technique for the measurement of cerebral blood flow in whole brain and vascular territories

PURPOSE: To analyze the precision of cerebral blood flow (CBF) measurements made with continuous arterial spin labeling(CASL) perfusion magnetic resonance imaging (MRI) over experimentally relevant intervals. MATERIALS AND METHODS: CASL perfusion MRI measurements of CBF on a 1.5-T GE Signa magnet were repeated in young healthy male and female subjects at one hour and one week. Precision of the measurement was evaluated at both time intervals. RESULTS: CASL perfusion MRI measurements of CBF yielded within-subject coefficients of variation (wsCV) of 5.8% for global and 13% for individual vascular regions when measurements were repeated within one hour. Differences in these values represent the error in post-processing. Global and regional CBF measurements over one week yielded wsCVs of 13% and 14%, respectively. At one week, error secondary to physiologic variability affected global and regional measurements to the same degree and masked the software post-processing error seen at one hour. The magnitude of the difference in repeated measures correlated with the magnitude of the measurement. CONCLUSION: CASL perfusion MRI CBF measurements are accurate and precise. Variability over longer periods of time appears attributable to physiologic factors. Repeatability of the CASL measurement is sensitive to the magnitude of the measurement. This should be taken into account when studies requiring repeated measures involve subjects with significant variability in CBF.     

Renal blood flow in neonates: quantification with color flow and pulsed Doppler US.

Color flow and pulsed Doppler ultrasound measurements of renal artery blood flow were compared with cardiac output in 22 preterm and 19 full-term healthy neonates. Renal arteries were insonated 3-5 mm from the abdominal aorta at an angle of less than 15 degrees. Vessel diameter was estimated from color flow diameter. Total renal blood flow increased significantly (P less than .001) with advancing birth weight and gestational age, because of increased vessel diameter, but flow velocity did not. The mean (+/- 1 standard deviation) value for both right and left renal artery blood flow was 21 mL/min/kg +/- 5, and the mean proportion of cardiac output to the kidneys was 16.1% +/- 3.7. Seven preterm infants with a symptomatic patent ductus arteriosus had increased cardiac output values (444 mL/min/kg +/- 45) and reduced right (15 mL/min/kg +/- 7) and left (16 mL/min/kg +/- 7) renal artery blood flow, with 6.6% +/- 2.0 of cardiac output directed to the kidneys. These values normalized after closure of the ductus. This study provides normative data for renal artery blood flow in neonates during the first week of life.     

Renal blood flow measurements with use of phase-contrast magnetic resonance imaging: normal values and reproducibility

PURPOSE: To assess the validity and the direct, short-term, and long-term reproducibility of renal blood flow (RBF) measurements with phase-contrast (PC) magnetic resonance (MR) imaging. MATERIALS AND METHODS: In 20 healthy volunteers, RBF measurements were repeated with and without repositioning. Internal validity was assessed by comparing the total RBF with the difference in aortic flow above and below the renal arteries. In 19 healthy volunteers, RBF measurements were performed at two different occasions. In 40 healthy volunteers, RBF measurements were performed to assess normal values as a function of age. Analyses were performed according to Bland and Altman. RESULTS: The technical success rate ranged from 78% to 85%. Total RBF and the difference in aortic flow rates showed good agreement (Pearson correlation coefficient, 0.72; P = .002). Directly repeated measurements had a mean difference of 54 mL/min in total RBF with a coefficient of variation (CV) of 17%. For repeated measurements with repositioning, the mean difference in total RBF was 74 mL/min (CV, 23%). Repeated measurements on different occasions showed a CV of 20%. The mean total RBF of the 40 healthy volunteers was 838 mL/min +/- 244 (SD). CONCLUSIONS: RBF measurement with PC MR has a success rate greater than 75%. The demonstrated internal reliability of this method and fair reproducibility of the flow parameters is crucial for further studies of the renal artery with MR imaging.     

Cine flow measurements using phase contrast with undersampled projections: in vitro validation and preliminary results in vivo

PURPOSE: To assess the accuracy of flow measurements in vitro and in vivo during scan times shorter than a breath-hold using a 2D cine phase contrast (PC) undersampled radial acquisition method, which may be useful for measuring flow, especially in vessels subject to motion during respiration. MATERIALS AND METHODS: For in vitro assessment, a flow phantom was imaged at various flow rates and undersampling factors. For in vivo assessment, five normal subjects were imaged and the flow rate in the aorta was compared with the sum of the flow rates in the iliac arteries. RESULTS: For results in vitro, the accuracy of flow measurements was maintained with scan times as low as 13-17 seconds. For results in vivo, scans acquired in less than 25 seconds provided flow measurements in the aorta that corresponded well to the sum of flow measurements in the iliac arteries. CONCLUSION: The undersampled radial acquisition cine PC technique provided accurate flow measurements in a flow phantom and in healthy human volunteers in scan times shorter than a typical breath-hold.     

Magnetic resonance velocity imaging using a fast spiral phase contrast sequence

Time-resolved velocity imaging using the magnetic resonance phase contrast technique can provide clinically important quantitative flow measurements in vivo but suffers from long scan times when based on conventional spin-warp sequences. This can be particularly problematic when imaging regions of the abdomen and thorax because of respiratory motion. We present a rapid phase contrast sequence based on an interleaved spiral k-space data acquisition that permits time-resolved, three-direction velocity imaging within a breath-hold. Results of steady and pulsatile flow phantom experiments are presented, which indicate excellent agreement between our technique and through plane flow measurements made with an in-line ultrasound probe. Also shown are results of normal volunteer studies of the carotids, renal arteries, and heart.     

Hepatic vascular flow measurements by phase contrast MRI and doppler echography: A comparative and reproducibility study

Purpose:

To directly compare and study the variability of parameters related to hepatic blood flow measurements using 3 T phase‐contrast magnetic resonance imaging (PC‐MRI) and Doppler ultrasound (US).

Materials and Methods:

Nine healthy subjects were studied. Blood velocities and flow rate measurements were performed in the portal vein and the proper hepatic artery. MR studies were performed using a 3 T imager. Gradient‐echo fast phase contrast sequences were used with both cardiac and respiratory gating. MR and Doppler flow parameters were extracted and compared. Two methods of calculation were used for Doppler flow rate analysis.

Results:

Compared to Doppler US, PC‐MRI largely underestimated hepatic flow data with lower variability and higher reproducibility. This reproducibility was more pronounced in the portal vein than in the proper hepatic artery associated with poorer velocity correlations. Total hepatic flow values were 1239 ± 223 mL/min and 1595 ± 521 mL/min for PC‐MRI and Doppler US, respectively.

Conclusion:

Free‐breathing PC‐MRI can provide reliable noninvasive measurement of hepatic flow parameters compared to Doppler US. The MR technique could help to improve Doppler flow calculations, thereby allowing standardization of protocols, particularly for applications in disease. J. Magn. Reson. Imaging 2010;31:579–588. © 2010 Wiley‐Liss, Inc.     

Reproducibility of total cerebral blood flow measurements using phase contrast magnetic resonance imaging

PURPOSE: To evaluate reproducibility of total cerebral blood flow (CBF) measurements with phase contrast magnetic resonance imaging (pcMRI). MATERIALS AND METHODS: We repeated total CBF measurements in 15 healthy volunteers with and without cardiac triggering, and with and without repositioning. In eight volunteers measurements were performed at two different occasions. In addition, measurement of flow in a phantom was performed to validate MR measurements. RESULTS: A difference of 40.4 ml/minute was found between CBF measurements performed with and without triggering (P < 0.05). For repeated triggered measurements, the coefficient of variation (CV) was 7.1%, and for nontriggered measurements 10.3%. For repeated measurements with repositioning, the CV was 7.1% with and 11.2% without triggering. Repeated measurements at different occasions showed a CV of 8.8%. Comparing measured with real flow in the phantom, the triggered differed 4.9% and the nontriggered 8.3%. CONCLUSION: The findings of this study demonstrate that pcMRI is a reliable method to measure total CBF in terms of both accuracy and reproducibility.     

Implementation of multi-echo-based correlated spectroscopic imaging and pilot findings in human brain and calf muscle

Purpose:

To implement a spatially encoded correlated spectroscopic imaging (COSI) sequence on 3 Tesla (T) MRI/MR spectroscopy scanners incorporating four echoes to collect four phase‐encoded acquisitions per repetition time (TR), and to evaluate the performance and reliability of this four‐dimensional (4D) multi‐echo COSI (ME‐COSI) sequence in brain and calf muscle.

Materials and Methods:

Typical scan parameters for the 4D datasets were as follows: repetition time = 1500 ms, 2000 Hz bandwidth, 8 × 8 spatial encoding, one average, 64 Δt₁ increments and the scan duration was 25 min. The performance and test–retest reliability of ME‐COSI were evaluated with phantoms and in the occipitoparietal brain tissues and calf of six healthy volunteers (mean age = 32 years old).

Results:

Regional differences in concentrations of lipids, creatine (Cr), choline (Ch), and carnosine (Car) were observed between spectra from voxels located in tibial marrow, tibialis anterior, and soleus muscle. Diagonal and cross‐peak resonances were identified from several brain metabolites including N‐acetyl aspartate (NAA), Ch, Cr, lactate (Lac), aspartate (Asp), glutathione (GSH), and glutamine\glutamate (Glx). Coefficients of variation (CV) in metabolite ratios across repeated measurements were <15% for diagonal and <25% for cross‐peaks observed in vivo.

Conclusion:

The ME‐COSI sequence reliably acquired spatially resolved 2D Correlated Spectroscopy (COSY) spectra demonstrating the feasibility of differentiating spatial variation of metabolites in different tissues. Multi‐echo acquisition shortens scan duration to clinically feasible times. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.     

Carotid and vertebral artery blood flow in left- and right-handed healthy subjects measured with MR velocity mapping

The goal of the study was to establish normal carotid artery flow rates in left-handed and right-handed individuals as a standard against which patients with carotid artery disease could be compared. Antegrade and retrograde flow were measured in the ascending aorta, in the right and left common, internal, and external carotid arteries, and in the vertebral arteries of 12 healthy subjects. Five subjects were right-handed, five left-handed, and two ambidextrous. Measured flow rates were as follows: common carotid arteries, 360–557 mL/min (mean [± standard deviation], 465 mL/min ± 52); internal carotid arteries, 132–367 mL/min (mean, 265 mL/min ± 60); external carotid arteries, 113–309 mL/min (mean, 186 mL/min ± 51); vertebral arteries from 133–308 mL/min (mean, 244 mL/min ± 43); and cerebral circulation, 546–931 mL/min (mean, 774 mL/min ± 134). All right-handed subjects had higher flow rates in the left internal carotid artery than in the right, and all left-handed subjects had higher flow rates in the right internal carotid artery (P =.007). There were no significant differences in left and right common carotid artery flow rates between left- and right-handed subjects. The standard deviation of a single measurement was 5%. The flow rates were similar to those obtained previously with other techniques and could be used as a normal standard.     

Cardiac flow measurement by ultrafast CT: validation of continuous and pulsatile flow.

To gauge the accuracy of ultrafast CT in measuring cardiac output and myocardial perfusion in humans, measurements of continuous and pulsatile flow were made in a large asymmetrical phantom. The variation in the relationship between Hounsfield number and contrast concentration was assessed in a human thorax phantom. Radiopaque contrast medium was injected during perfusion of the phantom at a range of flow rates between 1.5 and 8 L/min. The phantom was scanned in two modes (50 and 100 ms) during continuous and pulsatile flow and with the phantom surrounded by air and by water. Flow in the tubes was calculated using indicator dilution theory, and flow in the tissue-equivalent chamber was calculated by applying first-pass distribution principles. The standard deviation of the difference between calculated and measured flow varied from 0.2 to 0.6 L/min, giving 95% limits of agreement from 0.4 to 1.2 L/min. The constant (K) relating Hounsfield unit number to iodine concentration varied widely both in different locations within the phantom and under different scan conditions (17.2-27.6 HU/mg I). Within a human thorax phantom, K varied from 14.15 to 23.18 HU/mg I and was dependent on location within the thorax phantom, the scan mode, and the cross-sectional diameter of the phantom. These data suggest that though the ultrafast CT scanner can measure continuous and pulsatile flow accurately in tubes, precise measurements of cardiac output in humans will require K to be assessed for each subject. Measurements of flow in tissue should be possible.     

Rapid measurement of time-averaged blood flow using ungated spiral phase-contrast.

A novel ungated spiral phase-contrast (USPC) imaging method was developed for rapid measurement of time-averaged blood-flow rates in the presence of pulsatility. The spatial point-spread function was analyzed to provide an intuitive understanding of how spiral trajectories, which sample the k-space origin at every excitation, can mitigate the effects of pulsatility. Pulsatile flow phantom experiments were performed to validate the accuracy and repeatability of the USPC method. The measurement of flow in the renal and femoral arteries of normal volunteers were also performed. The phantom results (error < or = +9%, SD(phantom) < or = 2%, time-averaged pulsatile-flow rates = 3-15 ml/s) and in vivo results (SD(renal) < or = 8%, SD(femoral) < or = 14%) demonstrate the potential of the USPC method for rapidly and repeatedly measuring accurate time-averaged blood flow even in relatively small arteries and in the presence of strong pulsatility.     

Localized blood flow imaging using quantitative flow‐enhanced signal intensity

Flow‐enhanced signal intensity (FENSI) was previously introduced as a novel functional imaging method for measuring changes in localized blood flow in response to a stimulus. However, FENSI was limited to a qualitative functional MRI tool, due to magnetization transfer effects and different tagging plane profiles between tag and control images. In this work, a revised FENSI acquisition is proposed to enable quantitative imaging, which is capable of providing absolute localized blood flow maps free from magnetization transfer and slice profile errors. The feasibility and accuracy of measuring microvascular (arteriole, capillary, and venule) blood flow by using quantitative FENSI was validated by our phantom studies. Additionally, localized cerebral blood flow, 366 ± 45 μL/min/cm² in gray matter and 153 ± 23 μL/min/cm² in white matter, was measured in healthy subjects during resting state, whereas a flow change of 73 ± 13% was detected during a visual task. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Estimation of renal extraction fraction based on postcontrast venous and arterial differential T1 values: an error analysis.

An error analysis for quantifying single kidney extraction fraction (EF) via differential T1 measurements in the renal vein (RV) and renal artery (RA) is presented. Sources of error include blood flow effects, the effect of a short repetition time (TR), and the impact of uncertainties in the T1 estimates on the final EF calculations. Blood flow effects were investigated via simulation. For a range of blood velocities in the renal vein that may be found in kidney disease, incomplete refreshment of blood between readouts results in significant errors in T1 estimation. For a .5-cm slice, 110-ms sampling interval, and T1 of 600 ms, T1 estimation to within 5% of true T1 requires an average through-plane velocity of 6.75 cm/s for parabolic flow, and 3.5 cm/s for plug flow. Improvement can be achieved by accurately estimating the fraction of blood that has not refreshed between readouts (f(old)), while the quality of the T1 estimate varies with the accuracy of f(old) estimation. Shortening of the TR was investigated using phantom and in vivo studies. T1 was estimated to within 3% of the true value on phantoms, and within 5% of the true value for flowing blood for TR = 2T1. The estimated EF is shown to be very sensitive to the difference between T(1RA) and T(1RV). To achieve 10% or 20% uncertainty in the EF estimate, T1 in the renal vein and renal artery must be estimated to within approximately 1% or 2%. Because of limitations on measurement accuracy and precision, this method appears to be impractical at this time.