Does IVIM measure classical perfusion?

MR measurements based on motion encoding gradients provide interesting information about diffusion in tissues and have also been advanced as a way to measure tissue perfusion. This communication shows why IVIM cannot measure perfusion in the classical sense. Attempts to do so result from an unclear understanding of classical perfusion measurements and from confusion between terminal deposition (or uptake) and blood volume flow.     

Simultaneous imaging of myocardial motion and chamber blood flow with SPAMM n' EGGS (Spatial Modulation of Magnetization With Encoded Gradients for Gauging Speed)

PURPOSE: To provide simultaneous measurements of one-dimensional (1-D) myocardial displacement and 1-D chamber blood flow in a single breath-held acquisition using an MR imaging technique, SPAMM n' EGGS (Spatial Modulation of Magnetization With Encoded Gradients for Gauging Speed). MATERIALS AND METHODS: Velocity encoding bipolar gradients sensitive to chamber blood flow were played out before the readout gradient in a 1-1 SPAMM-tagged MR imaging pulse sequence. For any given motion-flow encoded direction, the acquired image sequence was later postprocessed to separate the tag motion and blood flow terms. Experiments were performed on seven normal volunteers, and two pigs with moderate ischemic mitral regurgitation. Left-ventricular motion and trans-valvular flow obtained using the SPAMM n' EGGS pulse sequence was compared against measurements obtained using standard tagging and phase-contrast pulse sequences, respectively. RESULTS: Results in normal volunteers and diseased pigs demonstrate multiphase correlated measurements of myocardial motion and chamber blood flow using SPAMM n' EGGS. A close correspondence in these measurements to conventional tagging and phase-contrast sequences is confirmed. CONCLUSION: We have demonstrated that simultaneous acquisition of myocardial motion and chamber blood flow is possible within a single breath-hold. The data obtained using the SPAMM n' EGGS pulse sequence may be useful in the planning and evaluation of mitral-valve repair procedures.     

MR黑血和白血技术诊断肥厚型心肌病的价值

目的：研究MR黑血和白血技术在诊断肥厚型心肌病中的价值。方法：采用心脏MR黑血和白血技术及多平面成像方式，对15例临床拟诊为肥厚型心肌病的患者进行检查。结果：MR黑白技术提供病变部位、厚度、信号、心腔形态、大小等信号。白血技术能反映心肌的运动，流出道有无狭窄及程度，二尖瓣有无返流等情况。多平面成像更全面了解病变的部位和范围，避免漏诊。MR的基本特征有室壁肥厚、心肌信号不均匀、左心室流出道狭窄、心肌运动不均匀、二尖瓣少量返流和心包积液等。结论：MR黑白和白血技术对肥厚型心肌病的诊断能提供更为准确、全面的影像学信息。     

Magnetic resonance velocity measurements in small arteries. Comparison with Doppler ultrasonic measurements in the aortas of normal rabbits.

RATIONALE AND OBJECTIVES. Magnetic resonance imaging (MRI) can be used to measure motion. This study compares MRI blood flow velocity measurements to Doppler ultrasound velocity measurements in an animal model. MATERIALS AND METHODS. Blood flow in the abdominal aortas of nine normal rabbits was measured using 16-frame, velocity-resolved MRI and Doppler ultrasound. The MRI data were processed into velocity spectra to aid in their interpretation. RESULTS. Maximum velocity measurements made by range-gated Doppler ultrasound were predicted by the maximum velocity values derived from MR velocity spectra with a slope of 0.861, an intercept of -2.78 cm/second, and an R-value of 0.935 in 70 measurements. CONCLUSIONS. Despite the longer time required for the MR measurement, the MR velocity measurement may be useful in the assessment of deep vessels or those obscured by other structures, which are difficult to measure with ultrasound.     

MRI measurement of hepatic magnetic susceptibility-phantom validation and normal subject studies.

A magnetic resonance (MR) imaging method with the potential for assessing hepatic iron overload from measurements of hepatic magnetic susceptibility in vivo is described. Using the blood in the portal and hepatic veins as an internal reference, this technique uses the orientation dependence of signal phase to measure the susceptibility of the liver parenchyma. Computer simulations were done to investigate the requirements on spatial resolution and contrast ratio between the vessels and the background liver tissue for data acquisition. Validation studies were conducted using tube-embedded gel phantoms doped with iron-dextran from 0 to 10 mg Fe/mL to mimic healthy and iron-overloaded livers. The phantom measurements were conducted without motion and flow, under respiration-like oscillatory motion, and with flow. Studies on six normal human subjects demonstrated excellent reproducibility and precision. All images were collected at 1.5 T using a 3D T(1)-weighted turbo field echo sequence for inflow MR angiographies with full flow compensation and capable of cardiac synchronization, navigator gating, and motion correction.     

Prospective motion correction for single‐voxel 1H MR spectroscopy

Head motion during ¹H MR spectroscopy acquisitions may compromise the quality and reliability of in vivo metabolite measurements. Therefore, a three‐plane image‐based motion‐tracking module was integrated into a single‐voxel ¹H MR spectroscopy (point‐resolved spectroscopy) sequence. A series of three orthogonal spiral navigator images was acquired immediately prior to the MR spectroscopy water suppression module in order to estimate head motion. By applying the appropriate rotations and translations, the MR spectroscopy voxel position can be updated such that it remains stationary with respect to the brain. Frequency and phase corrections were applied during postprocessing to reduce line width and restore coherent averaging. Spectra acquired during intentional head motion in 11 volunteers demonstrate reduced lipid contamination and increased spectral reproducibility when motion correction is applied. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

MR thermometry

Minimally invasive thermal therapy as local treatment of benign and malignant diseases has received increasing interest in recent years. Safety and efficacy of the treatment require accurate temperature measurement throughout the thermal procedure. Noninvasive temperature monitoring is feasible with magnetic resonance (MR) imaging based on temperature-sensitive MR parameters such as the proton resonance frequency (PRF), the diffusion coefficient (D), T1 and T2 relaxation times, magnetization transfer, the proton density, as well as temperature-sensitive contrast agents. In this article the principles of temperature measurements with these methods are reviewed and their usefulness for monitoring in vivo procedures is discussed. Whereas most measurements give a temperature change relative to a baseline condition, temperature-sensitive contrast agents and spectroscopic imaging can provide absolute temperature measurements. The excellent linearity and temperature dependence of the PRF and its near independence of tissue type have made PRF-based phase mapping methods the preferred choice for many in vivo applications. Accelerated MRI imaging techniques for real-time monitoring with the PRF method are discussed. Special attention is paid to acquisition and reconstruction methods for reducing temperature measurement artifacts introduced by tissue motion, which is often unavoidable during in vivo applications.     

Bright pleural effusion and ascites on gradient-echo MR images: a potential source of confusion in vascular MR studies

Motion of fluids other than blood can cause flow-related signal enhancement on MR images, including MR angiograms. In order to study this problem, the appearance of ascites (20 patients) and pleural effusions (five patients) was assessed on MR images made during suspended respiration with flow-compensated gradient-echo sequences as well as T1- and T2-weighted sequences. Signal intensities of vessels, fluid collections, and muscle were measured and vessel/muscle and vessel/fluid contrast were calculated. Fluid motion was measured with a bolus tracking technique that tags a selected volume of fluid with an RF presaturation. Fluid collections had a bright signal in four of five patients with pleural effusion and in 15 of 20 patients with ascites. The average contrast ratio between bright components of the fluid collections and vessels was only 0.03 +/- 0.09. Bright fluid collections were seen on MR angiograms and could obscure blood vessels. Bolus tracking measurements of ascites revealed multidirectional flow, suggesting that its bright signal is related to motion that continues during suspended respiration. Fluid collections appeared dark on T1-weighted images in all patients, indicating that a short T1 relaxation time was not a cause of the high signal intensity. The results indicate that, despite breath-holding, ascites and pleural effusions can show bright signal intensity on gradient-echo images. Awareness of this phenomenon will avoid confusion between moving fluid collections and flowing blood and identify a source of image degradation on both gradient-echo and T2-weighted spin-echo MR acquisitions.     

Measurement of the blood flow velocity in the pulmonary arteries using the magnetic resonance technique]

MR blood velocity measurements were performed by the RACE technique in a plane perpendicular to the flow of the pulmonary arteries. MR findings were correlated with those of perfusion scintigraphy, Doppler US and right heart catheter (thermodilution). The ratio of MR blood flow measurements of right and left pulmonary arteries correlated well with the results of perfusion scintigraphy (RPA to LPA) and Doppler. Poor correlation was found when comparing MR blood flow measurements with right heart catheter since absolute flow measurements can be superimposed by neighboring blood vessels in complex anatomic situations.     

Regional pulmonary blood flow: Comparison of dynamic contrast‐enhanced MR perfusion and phase‐contrast MR

Regional pulmonary blood flow can be assessed using both dynamic contrast‐enhanced (DCE) MR and phase‐contrast (PC) MR. These methods provide somewhat complementary information: DCE MR can assess flow over the entire lung while PC MR can detect rapid changes in flow to a targeted region. Although both methods are considered accurate, one may be more feasible than the other depending on pathology, patient condition, and availability of an intravenous route. The objective of this study was to establish a consensus between the two methods by comparing paired DCE MR and PC MR measurements of relative blood flow in Yorkshire piglets (N = 5, age = 7 days, weight = 3.3 ± 0.6 kg) under various physiological states including regional lung collapse. A strong correlation (R² = 0.71, P < 0.01) was observed between the methods. In conclusion, DCE MR and PC MR provide a consistent measure of changes in regional pulmonary blood flow. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Interleaved magnetic resonance and ultrasound by electronic synchronization

Increasing attention has been directed toward using magnetic resonance imaging (MRI) to assess blood flow velocity. Complete acceptance of this application requires validation of MRI-derived flow measurements against an accepted flow measurement technique such as Doppler ultrasound in an in vivo situation. To provide an accurate correlation in the presence of rapid changes in blood flow, the MR acquisition should be made nearly simultaneously with the ultrasonic measurements. Unfortunately, standard ultrasound equipment generates radio frequency signal which interferes with MRI. Near-simultaneous acquisition of MR data and ultrasonic blood flow data should be possible if the two measurements are properly synchronized. In the technique presented, ultrasound is made to peacefully coexist with MRI by gating the ultrasound so that it is disabled during the time of MR data acquisition. Phantom and animal experiments confirm the use of this procedure. Although we did not specifically test new fast-scan MR techniques, our technique is completely general and should work equally well with spin-echo as well as newer fast scanning MRI techniques.     

First-pass renal perfusion imaging using MS-325, an albumin-targeted MRI contrast agent.

RATIONALE AND OBJECTIVES: MR angiography is proving to be a useful clinical study for the diagnosis of vascular disorders of renal arteries. However, its utility in terms of stenosis characterization is still limited. Renal perfusion could provide supplemental information that could allow for a comprehensive evaluation of renal artery stenosis by MR imaging. METHODS: MS-325 is a small-molecule blood pool agent that reversibly binds with serum albumin and hence leads to higher relaxivity and longer residence times in the blood. In this study, the authors evaluated the use of MS-325 to perform first-pass perfusion imaging and contrast-enhanced MR angiography in the characterization of renal artery stenosis in an animal model. RESULTS: Quantitative perfusion estimates were obtained in the renal cortex (258 +/- 19.8 mL/min/100 g) and are comparable to microsphere measurements (198 +/- 12.2 mL/min/100 g), given the practical constraints. Based on these measurements, perfusion showed minimal changes even when the diameter reductions reached 75%. CONCLUSIONS: MS-325 could provide quantitative perfusion estimates that when combined with MR angiography may lead to comprehensive evaluation of renal artery stenosis.     

Motion correction in MR thermometry of abdominal organs: A comparison of the referenceless vs. the multibaseline approach

Reliable temperature and thermal‐dose measurements using proton resonance frequency shift‐based magnetic resonance (MR) thermometry for MR‐guided ablation of abdominal organs require a robust correction of artefacts induced by the target displacement through an inhomogeneous and time‐variant magnetic field. Two correction approaches emerged recently as promising candidates to allow continuous real‐time MR‐thermometry under free‐breathing conditions: The multibaseline correction method, which relies on a pre‐recorded correction table allowing to correct for periodic phase changes, and the referenceless method, which depends on a background phase estimation in the target area based on the assumption of a smooth spatial variation of the phase across the organ. This study combines both methods with real‐time in‐plane motion correction to permit both temperature and thermal‐dose calculations on the fly. Subsequently, the practical aspects of both methods are compared in two application scenarios, a radio frequency‐ablation and a high‐intensity focused ultrasound ablation. A hybrid approach is presented that exploits the strong points of both methods, allowing accurate and precise proton resonance frequency‐thermometry measurements during periodical displacement, even in the presence of spontaneous motion and strong susceptibility variations in the target area. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Quantification of myocardial perfusion using free‐breathing MRI and prospective slice tracking

Quantification of myocardial perfusion using first‐pass magnetic resonance imaging (MRI) is hampered by respiratory motion of the heart. Prospective slice tracking (PST) potentially overcomes this problem, and may provide an attractive alternative or supplement to current breath‐hold techniques. This study demonstrates the feasibility of patient‐adapted 3D PST on a 3.0 Tesla MR system. Eight patients underwent free‐breathing studies of myocardial perfusion, simultaneously collecting data with and without PST. On average, PST reduced residual in‐plane motion by a factor of 2, compared to the noncorrected images, resulting in a fourfold improvement of perfusion measurements. In addition, a comparison of perfusion measurements performed with and without PST showed that through‐plane motion can contaminate measurements of myocardial perfusion. However, the quality of the navigator echoes on this field strength constituted a major source of error and needs further improvement to increase the accuracy and robustness of the method. Magn Reson Med, 2009. © 2008 Wiley‐Liss, Inc.     

Renal disease: value of functional magnetic resonance imaging with flow and perfusion measurements

PURPOSE: To differentiate healthy kidneys from diseased kidneys, we propose a combined magnetic resonance (MR) examination that includes measurements of renal arterial blood flow and parenchymal perfusion. MATERIALS AND METHODS: A total of 130 kidneys (patients/healthy volunteers: 83/47) were examined using renal artery MR flow measurements and renal parenchymal perfusion measurements, as well as contrast-enhanced MR angiography. Cine phase-contrast-flow measurements were performed using an ECG-gated fast low angle shot pulse sequence; perfusion was measured with an arterial spin labeling flow-sensitive alternating inversion recovery technique. Contrast-enhanced MR angiography was performed with a fast 3D gradient echo sequence in a single breath hold. For evaluation, kidneys were divided into groups based on nephrologic diagnosis of the patient. Recursive partitioning and Wilcoxon rank-sum tests were used to separate the different groups. RESULTS: Significant differences in mean renal artery flow and parenchymal perfusion were found in kidneys with renal artery stenosis as well as parenchymal disease as compared with healthy kidneys. Using a classification tree derived from the recursive partitioning, a specificity of 99% and sensitivity of 69% with a positive/negative predictive value of 97%/84% was achieved for the separation of healthy kidneys from kidneys with vascular, parenchymal or combined disease. The overall accuracy was 88%. CONCLUSION: The combination of cine PC flow measurements and MR perfusion measurements offers a comprehensive assessment of both renovascular and renoparenchymal disease and provide a noninvasive approach to differentiate between these kidneys and normal kidneys.     

MR imaging of microcirculation in rat brain: correlation with carbon dioxide-induced changes in blood flow

Considerable interest has been shown in developing a magnetic resonance (MR) imaging technique with quantitative capability in the evaluation of tissue microcirculation ("perfusion"). In the present study, the flow-dephased/flow-compensated (FD/FC) technique is evaluated for measuring rat cerebral blood flow (CBF) under nearly optimal laboratory conditions. Imaging was performed on a 2.0-T system equipped with shielded gradient coils. Rat CBF was varied by manipulating arterial carbon dioxide pressure (PaCO2). In parallel experiments, optimized MR imaging studies (seven rats) were compared with laser Doppler flowmetry (LDF) studies (nine rats). LDF values showed a high degree of correlation between CBF and PaCO2, agreeing with results in the literature. MR imaging values, while correlating with PaCO2, showed considerable scatter. The most likely explanation is unavoidable rat motion during the requisite long imaging times. Because of this motion sensitivity, the FD/FC technique cannot provide a quantitative measure of CBF. It can, however, provide a qualitative picture.     

Cardiac ejection fraction: phantom study comparing cine MR imaging, radionuclide blood pool imaging, and ventriculography.

The accuracy and reproducibility of cardiac ejection fraction (EF) measurements based on cine magnetic resonance (MR) imaging, radionuclide multigated acquisition (MUGA) blood pool imaging, and angiographic ventriculography were evaluated by comparing them with a volumetrically determined standard. A biventricular, compliant, fluid-filled heart phantom was developed to mimic normal cardiac anatomy and physiology. Ventricular EFs were measured with cine MR imaging by summation of nine contiguous 10-mm-thick sections in short and long axis, with single-plane ventriculography, and with MUGA. Three measurements were performed with each modality for each of three EFs. Ventriculography was least accurate, with average relative errors ranging from 7.9% for the largest EF to 60.1% for the smallest. Cine MR was most accurate, with average relative errors ranging from 4.4% to 8.5%. MUGA EF measurements showed good correlation, with average relative errors ranging from 7.1% to 22.4%. Comparison of the error variances for the three modalities with the F test revealed that MR and MUGA EF measurements were significantly more accurate than those based on ventriculography (P less than .01). No significant difference was demonstrated between the accuracy of short- and long-axis cine MR acquisitions.     

Abdomen: diffusion-weighted MR imaging with pulse-triggered single-shot sequences

Magnetic resonance (MR) diffusion measurements of the abdomen were performed in 12 healthy volunteers by using a diffusion-weighted single-shot sequence both without and with pulse triggering for different trigger delays. Pulse triggering to the diastolic heart phase led to reduced motion artifacts on the diffusion-weighted MR images and to significantly improved accuracy and reproducibility of measurements of the apparent diffusion coefficients, or ADCs, of abdominal organs.     

MR imaging, flow and motion.

The present work is intended as a nonmathematical review of the role of flow and motion in nuclear magnetic resonance (MR) imaging. A historical review of MR flow measurement techniques is given, followed by a short overview of flow models in vitro and in vivo. The theory behind the influence of motion on the modulus and phase MR signal information is discussed and effects such as washin/washout, flow-induced signal void, phase offset, and phase dispersion are defined. A simple approach to the concept of MR angiography is given, and methods for quantitative flow measurements such as the phase mapping technique, are surveyed. Aspects of the measurement of diffusion and microcirculation are given, and finally, an overview of the role of MR flow imaging in present and future clinical application is given.     

Arterial and venous blood flow: noninvasive quantitation with MR imaging.

Quantitative measurements of arterial and venous blood flow were obtained with phase-contrast cine magnetic resonance (MR) imaging and compared with such measurements obtained by means of implanted ultrasound (US) blood flow probes in anesthetized dogs. The US flowmeter was enabled during a portion of each MR imaging sequence to allow virtually simultaneous data acquisition with the two techniques. MR imaging data were gated by means of electrocardiography and divided into 16 phases per cardiac cycle. The rates of portal venous blood flow measured with MR imaging and averaged across the cardiac cycle (710 mL/min +/- 230 [standard deviation]) correlated well with those measured with the flowmeter and averaged in like fashion (751 mL/min +/- 238) (r = .995, slope = 1.053). The correspondence in arterial blood flow was almost as good. No statistically significant difference existed between the paired measurements of blood flow obtained with MR imaging and the implanted probe. It is concluded that, as a noninvasive means of accurate quantification of blood flow, phase-contrast MR imaging may be especially useful in deep blood vessels in humans.