Prebolus quantitative MR heart perfusion imaging.

The purpose of this study was to present the prebolus technique for quantitative multislice myocardial perfusion imaging. In quantitative MR perfusion studies the maximum contrast agent dose is limited by the requirement to determine the arterial input function (AIF). The prebolus technique consists of two consecutive contrast agent administrations. The AIF is determined from a first low-dose bolus, while a second, high-dose bolus allows the measurement of the myocardium with improved signal increase. The results of the prebolus technique using a multislice saturation recovery trueFISP sequence in healthy volunteers are presented. In comparison to a standard dose of 3 ml Gd-DTPA, perfusion values are maintained while the signal increase in the concentration time courses was considerably improved, accompanied by reduced standard deviations of the obtained perfusion values (0.72 +/- 0.13 ml/g/min for 1 ml/8 ml and 0.67 +/- 0.10 ml/g/min for 1 ml/12 ml Gd-DTPA, respectively).     

Single- or dual-bolus approach for the assessment of myocardial perfusion reserve in quantitative MR perfusion imaging.

A dual-bolus protocol can overcome limitations due to T1-induced MR signal attenuation and hence enables more accurate quantification of myocardial blood flow (MBF) by contrast enhanced MR perfusion imaging. The study explores potential benefits of the dual-bolus technique for the assessment of myocardial perfusion reserve (MPR) over a standard single-bolus protocol. Nineteen patients without obstructive coronary artery disease as assessed by cardiac catheterization underwent a stress-rest MR perfusion study using a dual-bolus protocol. Gd-DTPA dosages of 0.005 and 0.05 mmol/kg of bodyweight were delivered as pre- and main-bolus. For comparison arterial input curves where extracted from left ventricular cavity passage including both, pre-bolus and main-bolus data. Global and segmental MPR were determined from semiquantitative and from full quantitative measures of MBF. As a result good agreement between dual- and single-bolus technique was found with relative differences of maximally 10% in global MPR estimates. For the dual bolus approach a significant relative decrease of 30% (P<0.001) was found for the coefficient of segmental MPR variation, which may allow a more reliable detection of hypoperfused segments in clinical studies.     

Mouse myocardial first‐pass perfusion MR imaging

A first‐pass myocardial perfusion sequence for mouse cardiac MRI is presented. A segmented ECG‐triggered acquisition combined with parallel imaging acceleration was used to capture the first pass of a Gd‐DTPA bolus through the mouse heart with a temporal resolution of 300–400 msec. The method was applied in healthy mice (N = 5) and in mice with permanent occlusion of the left coronary artery (N = 6). Baseline semiquantitative perfusion values of healthy myocardium showed excellent reproducibility. Infarct regions revealed a significant decrease in the semiquantitative myocardial perfusion values (0.05 ± 0.02) compared to remote myocardium (0.20 ± 0.04). Myocardial areas of decreased perfusion correlated well to infarct areas identified on the delayed‐enhancement scans. This protocol is a valuable addition to the mouse cardiac MRI toolbox for preclinical studies of ischemic heart disease. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Comparison of different contrast agents and doses for quantitative MR myocardial perfusion imaging

PURPOSE: To investigate three different contrast agents at different injection volumes for repetitive quantitative multislice myocardial perfusion imaging using the prebolus technique. MATERIALS AND METHODS: Two consecutive prebolus perfusion measurements were performed on a 1.5 T scanner using identical injection volumes for the first and second examination to test the reproducibility for possible rest and stress examination in normal volunteers. Either 1-8 mL, 1-12 mL Gd-DTPA, 1-4 mL, 1-6 mL, 1-9 mL Gd-BOPTA, or 1-4 mL, 1-6 mL gadobutrol were applied. RESULTS: In cases where injection volumes were sufficiently small, there was no indication of significant differences in quantitative perfusion values with respect to the different contrast agents. Increasing the bolus volume improved the contrast-to-noise ratio (CNR) but led to saturation effects and underestimation of the true perfusion. The highest CNR was measured for gadobutrol (6 mL, p < 0.0005 compared to 8 mL Gd-DTPA). The smallest difference of perfusion values between the first and the second prebolus examination was found for Gd-BOPTA (p < or = 0.006 compared Gd-DTPA). CONCLUSION: Prebolus examinations for quantitative myocardial perfusion imaging are possible with all three contrast agents for sufficient small injection volumes. Gd-BOPTA was found to be advantageous for a combined quantitative rest and stress examination.     

Direct comparison of myocardial perfusion cardiovascular magnetic resonance sequences with parallel acquisition

PURPOSE: To directly compare the three main myocardial perfusion cardiovascular magnetic resonance (CMR) sequences incorporating parallel acquisition methods. MATERIALS AND METHODS: In 15 subjects (12 men, 57 +/- 15.7 years) referred for diagnostic coronary angiography, we acquired first-pass perfusion images (0.1 mmol/kg gadolinium-DTPA) at rest and during adenosine (140 microg/kg/min) on three separate occasions using three sequences incorporating parallel acquisition methods and approximately equivalent spatiotemporal resolution: hybrid echo planar imaging (hEPI), steady-state free precession (SSFP), and gradient echo imaging (GRE). We calculated the contrast-to-noise ratio (CNR) of each scan and blinded observers scored the presence and severity of artifacts (1, worst to 4, best), diagnostic confidence (0, low to 2, high), transmurality, area, and epicardial vessel territory of perfusion defects. RESULTS: CNR was greatest with SSFP and least with hEPI (13.15 vs 7.85 P < 0.001). The most artifacts were recorded with SSFP and least with hEPI (2.00 vs 3.03 P < 0.001). Observers were significantly more confident in reporting hEPI images (1.6 hEPI vs 0.9 SSFP, P < 0.001). Results for GRE were intermediate for all assessments. CONCLUSION: The hEPI sequence scored best for diagnostic performance despite the SSFP sequence having greater CNR. This trial favors hEPI for clinical myocardial perfusion CMR and suggests CNR should not be the sole criterion used to gauge the best candidate sequence.     

Improved correction of spatial inhomogeneities of surface coils in quantitative analysis of first‐pass myocardial perfusion imaging

Purpose:

To test whether image normalization using either a separate 3D proton‐density (PD)‐weighted prescan, or 2D PD‐weighted images prior to the perfusion series, improves correction of differences in spatial sensitivity induced by radiofrequency (RF) surface receiver coils. Originally, this correction was applied using the baseline signal in the myocardium before arrival of the contrast agent. This is of importance, as quantitative analysis of magnetic resonance (MR) myocardial perfusion using deconvolution with the arterial input assumes equal signal sensitivity over the heart.

Materials and Methods:

First‐pass myocardial perfusion measurements were obtained in 13 patients without known coronary artery disease. Absolute perfusion values were assessed for 18 myocardial segments without any normalization and using the three different normalization methods.

Results:

Using 2D or 3D PD‐weighted normalization, similar mean perfusion values were found, but with reduced spatial variance over the 18 segments. The relative dispersion of perfusion at rest was 23% and 35% for the 3D prescan normalization and the baseline normalization, respectively. With 2D and 3D PD‐weighted prescan normalization the relative dispersion was closer to the expected physiological heterogeneity.

Conclusion:

PD‐weighted prescan normalization proved to be a valuable addition to quantitative analysis of myocardial perfusion, and better than baseline‐based normalization. J. Magn. Reson. Imaging 2010;31:227–233. © 2009 Wiley‐Liss, Inc.     

Correction for partial volume errors in MR heart perfusion imaging.

Myocardial MR first-pass perfusion time courses are contaminated by signals from the ventricles (spillover) as a consequence of partial volume effects and motion. An early increase in the signal intensity from the myocardium is an indicator of contamination. This contamination leads to under- or overestimation of perfusion, depending on the amount of contamination. In this work a simple method for contamination correction is proposed: curves proportional to the signal intensity time courses in the ventricles are subtracted from the signal intensity time courses in the myocardium to minimize the variance of signal before the arrival of the contrast medium in the myocardium. The proposed correction is easy to apply, removes the contamination, and leads to more precise perfusion values.     

Off-resonance-dependent slice profile effects in balanced steady-state free precession imaging.

The purpose of this study was the simulation and measurement of balanced steady-state free precession (bSSFP) slice profiles for a detailed analysis of the influence of off-resonance effects on slice profile shape and bSSFP signal intensity. Due to the frequency response function of the bSSFP sequence, measurements that are not on-resonance result in broadened effective slice profiles with different off-resonance-dependent shapes and signal intensities. In this study, bSSFP slice profile effects and their dependence on off-resonance were investigated based on bSSFP signal simulations of phantom data as well as blood and tissue. For a better assessment of the similarity of measured and simulated slice profiles the field map was integrated in the slice profile simulations. The results demonstrate that simulations can accurately predict bSSFP slice profiles. Both measurements and simulations indicate that there is a substantial increase in signal intensity close to the banding artifacts, i.e., at spatial locations with off-resonance frequencies corresponding to a dephasing/TR = +/- pi resulting in signal void (bands). For routine bSSFP imaging, off-resonance-dependent slice broadening may thus result in a substantial difference between nominal and true slice thickness and lead to spatially varying slice thickness and signal intensities across the imaging slice.     

Absolute quantification of myocardial perfusion under adenosine stress.

The prebolus technique allows one to quantify perfusion in the human heart with a low variability by means of MRI. In this study the prebolus technique was used to determine quantitative perfusion values in the human heart under adenosine stress and to measure the myocardial perfusion reserve (MPR). Twelve healthy volunteers were examined using the multislice prebolus technique with 1/4 cc Gd-BOPTA. Signal intensity (SI) time courses were evaluated in 288 manually segmented sectors at rest and stress. Myocardial perfusion was determined by deconvolution of the SI time courses with the arterial input function (AIF) from the prebolus scan. The mean stress perfusion value was 1.78 +/- 0.53 cc/g/min, and the mean rest perfusion was 0.52 +/- 0.11 cc/g/min, resulting in a mean MPR of 3.59 +/- 1.26. The measured values correlate well with data from animal models and human positron emission tomography (PET) studies.     

Myocardial delayed enhancement imaging using inversion recovery single-shot steady-state free precession: initial experience

PURPOSE: To evaluate the feasibility of using an inversion recovery single-shot steady-state free precession (SS_SSFP) sequence for myocardial delayed enhancement (MDE) imaging, and to compare SS_SSFP with the conventional inversion recovery segmented fast gradient echo (IR_FGRE) technique. MATERIALS AND METHODS: Ten subjects (four volunteers and six patients with suspected or known coronary disease) were included in this study. All subjects were scanned with both IR_FGRE and SS_SSFP sequences 15-25 minutes after gadopentetate dimeglumine injection. Overall image quality, signal-to-noise ratios (SNRs), and contrast-to-noise ratios (CNRs) between the two techniques were compared. RESULTS: Compared to IR_FGRE, SS_SSFP exhibited adequate image quality (average scores = 3.8 for IR_FGRE and 3.9 for SS_SSFP) with much shorter acquisition time (14.4 seconds for IR_FGRE and 1.3 seconds for SS_SSFP). SS_SSFP images showed higher SNRs (P < 0.05) and less motion artifact from breathing. Enhanced myocardium was detected by both techniques in three patients, but the image sharpness is compromised in SS_SSFP images. CONCLUSION: SS_SSFP provides adequate image quality compared to IR_FGRE, while requiring a much shorter acquisition time. It is feasible to use SS_SSFP as an alternative method for MDE imaging, especially in patients who have difficulty with holding their breath.     

Quantitative contrast-enhanced first-pass cardiac perfusion MRI at 3 tesla with accurate arterial input function and myocardial wall enhancement

Purpose:

To develop, and validate in vivo, a robust quantitative first‐pass perfusion cardiovascular MR (CMR) method with accurate arterial input function (AIF) and myocardial wall enhancement.

Materials and Methods:

A saturation‐recovery (SR) pulse sequence was modified to sequentially acquire multiple slices after a single nonselective saturation pulse at 3 Tesla. In each heartbeat, an AIF image is acquired in the aortic root with a short time delay (TD) (50 ms), followed by the acquisition of myocardial images with longer TD values (～150–400 ms). Longitudinal relaxation rates (R₁ = 1/T₁) were calculated using an ideal saturation recovery equation based on the Bloch equation, and corresponding gadolinium contrast concentrations were calculated assuming fast water exchange condition. The proposed method was validated against a reference multi‐point SR method by comparing their respective R₁ measurements in the blood and left ventricular myocardium, before and at multiple time‐points following contrast injections, in 7 volunteers.

Results:

R₁ measurements with the proposed method and reference multi‐point method were strongly correlated (r > 0.88, P < 10^?5) and in good agreement (mean difference ±1.96 standard deviation 0.131 ± 0.317 / 0.018 ± 0.140 s^?1 for blood/myocardium, respectively).

Conclusion:

The proposed quantitative first‐pass perfusion CMR method measured accurate R₁ values for quantification of AIF and myocardial wall contrast agent concentrations in 3 cardiac short‐axis slices, in a total acquisition time of 523 ms per heartbeat. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.

     

Characterization of focal hepatic lesions with ferumoxides-enhanced MR imaging: utility of T1-weighted spoiled gradient recalled echo images using different echo times

PURPOSE: To evaluate the different signal characteristics of focal hepatic lesions on ferumoxides-enhanced MR imaging, including T1-weighted spoiled gradient recalled echo (GRE) images using different echo times (TE) and T2- and T2*-weighted images. MATERIALS AND METHODS: Ferumoxides-enhanced MR imaging was performed using a 1.5-T system in 46 patients who were referred for evaluation of known or suspected hepatic malignancies. One hundred and seven lesions (42 hepatocellular carcinomas [HCC], 40 metastases, 13 cysts, eight hemangiomas, three focal nodular hyperplasias [FNHs], and one cholangiocarcinoma) were evaluated. Postcontrast MR imaging included 1) T2-weighted FSE; 2) T2*-weighted GRE; 3) T1-weighted spoiled GRE using moderate (TE = 4.2-4.4 msec) TE; and 4) minimum (TE = 1.8-2.1 msec) TE. Signal intensities of the focal lesions were rated by two radiologists in conference as follows: hypointense, isointense or invisible, hyperintense, and markedly hyperintense. Lesion-to-liver contrast-to-noise ratio (C/N) was measured by one radiologist for a quantitative assessment. RESULTS: On ferumoxides-enhanced FSE images, 92% of cysts were "markedly hyperintense" and most of the other lesions were "hyperintense", and the mean C/N of cysts was significantly higher than that of other focal lesions. T2*-weighted GRE images showed most lesions with similar hyperintensities and the mean C/N was not significantly different between any two types of lesion. T1-weighted GRE images using moderate TE showed all FNHsand hemangiomas, 29 (69%) HCCs and eight (20%) metastases as "hyperintense". On T1-weighted GRE images using minimum TE, however, all HCCs and metastasis except one were iso- or hypointense, while all of the FNHs and hemangiomas were hyperintense. Ring enhancement was highly suggestive of malignant lesions, and was more commonly seen on the minimum TE images than on the moderate TE images. CONCLUSION: Addition of T1-weighted GRE images using minimum and moderate TE is helpful for characterizing focal lesions in ferumoxides-enhanced MR imaging.     

Simultaneous myocardial strain and dark‐blood perfusion imaging using a displacement‐encoded MRI pulse sequence

The purpose of this study is to develop and evaluate a displacement‐encoded pulse sequence for simultaneous perfusion and strain imaging. Displacement‐encoded images in two to three myocardial slices were repeatedly acquired using a single‐shot pulse sequence for 3 to 4 min, which covers a bolus infusion of Gadolinium contrast. The magnitudes of the images were T₁ weighted and provided quantitative measures of perfusion, while the phase maps yielded strain measurements. In an acute coronary occlusion swine protocol (n = 9), segmental perfusion measurements were validated against microsphere reference standard with a linear regression (slope 0.986, R² = 0.765, Bland‐Altman standard deviation = 0.15 mL/min/g). In a group of ST‐elevation myocardial infarction patients (n = 11), the scan success rate was 76%. Short‐term contrast washout rate and perfusion are highly correlated (R² = 0.72), and the pixelwise relationship between circumferential strain and perfusion was better described with a sigmoidal Hill curve than linear functions. This study demonstrates the feasibility of measuring strain and perfusion from a single set of images. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Fast mapping of myocardial blood flow with MR first-pass perfusion imaging.

Accurate and fast quantification of myocardial blood flow (MBF) with MR first-pass perfusion imaging techniques on a pixel-by-pixel basis remains difficult due to relatively long calculation times and noise-sensitive algorithms. In this study, Zierler's central volume principle was used to develop an algorithm for the calculation of MBF with few assumptions on the shapes of residue curves. Simulation was performed to evaluate the accuracy of this algorithm in the determination of MBF. To examine our algorithm in vivo, studies were performed in nine normal dogs. Two first-pass perfusion imaging sessions were performed with the administration of the intravascular contrast agent Gadomer at rest and during dipyridamole-induced vasodilation. Radiolabeled microspheres were injected to measure MBF at the same time. MBF measurements in dogs using MR methods correlated well with the microsphere measurements (R2=0.96, slope=0.9), demonstrating a fair accuracy in the perfusion measurements at rest and during the vasodilation stress. In addition to its accuracy, this method can also be optimized to run relatively fast, providing potential for fast and accurate myocardial perfusion mapping in a clinical setting.