Effects of inner volume field-of-view reduction on myocardial T2 mapping.

BACKGROUND: Inner volume (IV) excitation was explored with respect to scan time reduction of cardiac gated double inversion recovery multi-echo fast spin echo (MEFSE) to measure the transverse relaxation time (T2 ) in the myocardium. METHODS: The IV imaging was achieved by applying orthogonal slice selection for the excitation and refocusing pulses. The T2 map accuracy was investigated using different excitation and refocusing pulses. The performance of IV-MEFSE was compared with MEFSE on phantoms and eight healthy volunteers, acquiring eight echo times in a single breath-hold. RESULTS: Compared with MEFSE, IV-MEFSE allowed a scan time reduction from 26 s to 16 s, but caused a T2 overestimation of approximately 10% due to stimulated echoes. CONCLUSION: IV successfully reduced the scan time to a single breath-hold feasible for many patients and remarkably facilitated the scan prescription, because there was no image aliasing concern. Care should be taken in using IV for T2 mapping because of T2 relaxation time overestimation.

Motion correction of multi-contrast images applied to T1and T2quantification in cardiac MRI.

OBJECT: The ability to manipulate image contrast and thus to obtain complementary information is one of the main advantages of MRI. Motion consistency within the whole data set is a key point in the context of multi contrast imaging. In cardiac and abdominal MRI, the acquisition strategy uses multiple breath-holds and often relies on acceleration methods that inherently suffer from a signal to-noise ratio loss. The aim of this work is to propose a free-breathing multi-contrast acquisition and reconstruction workflow to improve image quality and the subsequent data analysis. MATERIALS AND METHODS: We extended a previously proposed motion-compensated image reconstruction method for multi-contrast imaging. Shared information throughout the imaging protocol is now exploited by the image reconstruction in the form of an additional constraint based on image gradient sparsity. This constraint helps to minimize the amount of data needed for efficient non-rigid motion correction. T1and T2weighted images were reconstructed from free-breathing acquisitions in 4 healthy volunteers and in a phantom. The impact of multi-contrast motion correction was evaluated in a phantom in terms of precision and accuracy of T1and T2quantification. RESULTS: In the phantom, the proposed method achieved an accuracy of 97.5 % on the quantified parameters against 88.0 % before motion correction. In volunteers, motion inconsistency in T1and T2quantification were noticeably reduced within 5 min of free-breathing acquisition. CONCLUSION: An efficient, free-breathing, multi-contrast imaging method has been demonstrated that does not require prior assumptions about contrast and that is applicable to a wide range of examinations.

Intrinsic contrast for characterization of acute radiofrequency ablation lesions.

BACKGROUND: Both intrinsic contrast (T1 and T2 relaxation and the equilibrium magnetization) and contrast agent (gadolinium)-enhanced MRI are used to visualize and evaluate acute radiofrequency ablation lesions. However, current methods are imprecise in delineating lesion extent shortly after the ablation. METHODS AND RESULTS: Fifteen lesions were created in the endocardium of 13 pigs. A multicontrast inversion recovery steady state free precession imaging method was used to delineate the acute ablation lesions, exploiting T1-weighted contrast. T2 and Mo(*) maps were also created from fast spin echo data in a subset of pigs (n=5) to help characterize the change in intrinsic contrast in the lesions. Gross pathology was used as reference for the lesion size comparison, and the lesion structures were confirmed with histological data. In addition, a colorimetric iron assay was used to measure ferric and ferrous iron content in the lesions and the healthy myocardium in a subset of pigs (n=2). The lesion sizes measured in inversion recovery steady state free precession images were highly correlated with the extent of lesion core identified in gross pathology. Magnetic resonance relaxometry showed that the radiofrequency ablation procedure changes the intrinsic T1 value in the lesion core and the intrinsic T2 in the edematous region. Furthermore, the T1 shortening appeared to be correlated with the presence of ferric iron, which may have been associated with metmyoglobin and methemoglobin in the lesions. CONCLUSIONS: The study suggests that T1 contrast may be able to separate necrotic cores from the surrounding edematous rims in acute radiofrequency ablation lesions.

Flexible cardiac T1 mapping using a modified Look-Locker acquisition with saturation recovery.

A modified Look-Locker acquisition using saturation recovery (MLLSR) for breath-held myocardial T(1) mapping is presented. Despite its reduced dynamic range, saturation recovery enables substantially higher imaging efficiency than conventional inversion recovery T(1) mapping because it does not require time for magnetization to relax to equilibrium. Therefore, MLLSR enables segmented readouts, shorter data acquisition windows, and shorter breath holds compared with inversion recovery. T(1) measurements in phantoms using MLLSR showed a high correlation with conventional single-point inversion recovery spin echo. In vivo T(1) measurements from normal and infarcted myocardium in 41 volunteers and patients were consistent with previously reported values. Twenty subjects were also scanned with MLLSR using an accelerated sampling scheme that required half the scan time (eight vs. 16 heartbeats) but yielded equivalent results. The flexibility afforded by the improved imaging efficiency of MLLSR allows the acquisition to be tailored to particular clinical needs and to individual patient's breath-holding abilities.

Noninvasive quantitative measurement of myocardial and whole-body oxygen consumption using MRI: initial results.

The purpose of this study was to investigate the feasibility of a noninvasive approach that combines magnetic resonance imaging (MRI) oximetry and flow measurement to obtain the oxygen consumption in the myocardium and in the whole body. Thirteen healthy male volunteers [mean (+/-S.D.) age: 35+/-7 years] underwent this MR study, which included myocardial oxygen consumption (MVO(2)) measurements in 11 subjects and whole-body oxygen consumption (VO(2)) measurements in 8 subjects. In six subjects, both measurements were obtained. Five subjects had repeated MRI measurements of global MVO(2) in order to verify the reproducibility of this approach. The protocol included in vitro blood sample T(2)-%O(2) calibration, coronary sinus (CS) and main pulmonary artery (MPA) T(2) and phase contrast flow measurement and left ventricular (LV) mass calculation. Based on Fick's law, a global measurement of LV MVO(2) and whole-body VO(2) using MRI was feasible. The MVO(2) values were 11+/-3 ml/min per 100 g LV mass. For repeated measurements, differences in MVO(2) of 1 ml/min per 100 g LV mass appear detectable. The whole-body VO(2) values were 3.8+/-0.8 ml/min/kg body weight. MRI techniques that combine CS and MPA T(2), flow and LV mass measurements to quantify MVO(2) and whole-body VO(2) noninvasively in healthy subjects appear feasible, based on their correspondence to previously published work.

Free-breathing, nongated real-time delayed enhancement MRI of myocardial infarcts: a comparison with conventional delayed enhancement.

PURPOSE: To compare a free-breathing, nongated, and black-blood real-time delayed enhancement (RT-DE) sequence to the conventional inversion recovery gradient echo (IR-GRE) sequence for delayed enhancement MRI. MATERIALS AND METHODS: Twenty-three patients with suspected myocardial infarct (MI) were examined using both the IR-GRE and RT-DE imaging sequences. The sensitivity and specificity of RT-DE for detecting MI, using IR-GRE as the gold standard, was determined. The contrast-to-noise ratios (CNR) between the two techniques were also compared. RESULTS: RT-DE had a high sensitivity and specificity (94% and 98%, respectively) for identifying MI. The total acquisition time to image the entire left ventricle was significantly shorter using RT-DE than IR-GRE (5.6+/-0.9 versus 11.5+/-1.9 min). RT-DE had a slightly lower infarct-myocardium CNR but a higher infarct-blood CNR than IR-GRE imaging. Compared with IR-GRE, RT-DE accurately measured total infarct sizes. CONCLUSION: RT-DE can be used for delayed enhancement imaging during free-breathing and without cardiac gating.

Validation of supervised automated algorithm for fast quantitative evaluation of organ motion on magnetic resonance imaging.

PURPOSE: To validate a correlation coefficient template-matching algorithm applied to the supervised automated quantification of abdominal-pelvic organ motion captured on time-resolved magnetic resonance imaging. METHODS AND MATERIALS: Magnetic resonance images of 21 patients across four anatomic sites were analyzed. Representative anatomic points of interest were chosen as surrogates for organ motion. The point of interest displacements across each image frame relative to baseline were quantified manually and through the use of a template-matching software tool, termed "Motiontrack." Automated and manually acquired displacement measures, as well as the standard deviation of intrafraction motion, were compared for each image frame and for each patient. RESULTS: Discrepancies between the automated and manual displacements of > or =2 mm were uncommon, ranging in frequency of 0-9.7% (liver and prostate, respectively). The standard deviations of intrafraction motion measured with each method correlated highly (r = 0.99). Considerable interpatient variability in organ motion was demonstrated by a wide range of standard deviations in the liver (1.4-7.5 mm), uterus (1.1-8.4 mm), and prostate gland (0.8-2.7 mm). The automated algorithm performed successfully in all patients but 1 and substantially improved efficiency compared with manual quantification techniques (5 min vs. 60-90 min). CONCLUSION: Supervised automated quantification of organ motion captured on magnetic resonance imaging using a correlation coefficient template-matching algorithm was efficient, accurate, and may play an important role in off-line adaptive approaches to intrafraction motion management.

Quantitative apparent diffusion coefficients and T2 relaxation times in characterizing contrast enhancing brain tumors and regions of peritumoral edema.

PURPOSE: To investigate the potential value and relationship of in vivo quantification of apparent diffusion coefficients (ADCs) and T2 relaxation times for characterizing brain tumor cellularity and tumor-related edema. MATERIALS AND METHODS: A total of 26 patients with newly diagnosed gliomas, meningiomas, or metastases underwent diffusion-weighted and six-echo multisection T2-preparation imaging. Regions of interest (ROIs) were drawn on conventional MR images to include tumor (as defined by contrast agent enhancement) and immediate and peripheral edema. Areas of necrosis were excluded. Median values of ADCs and T2 in the ROIs were calculated. RESULTS: ADCs for gliomas were similar to those for meningiomas or metastases in all regions. Tumor T2 values for gliomas (159.5+/-30.6 msec) were significantly higher than those for meningiomas or metastases (125.0+/-31.1 msec; P=0.005). Immediate-edema T2 values for meningiomas or metastases (226.0+/-44.1 msec) were significantly higher than those for gliomas (203.5+/-32.8 msec; P=0.033). Peripheral-edema T2 values for gliomas (219.5+/-41.9 msec) were similar to those for meningiomas or metastases (202.5+/-26.5 msec; P=0.377). Both immediate- and peritumoral-edema ADCs and T2 values were significantly higher than those in tumor for both tumor types. ADCs and T2 values from all regions correlated significantly for gliomas (r=0.95; P<0.0001) and for meningiomas or metastases (r=0.81; P<0.0001). CONCLUSION: The higher immediate-edema T2 values for nonglial tumors than for gliomas suggest tumor-related edema (vasogenic vs. infiltrated) can be further characterized by using T2 values. There were significant correlations between ADC and T2 values.

Real-time magnetic resonance with physiologic monitoring for improved scan localization.

Imaging of the coronary arteries at diagnostic resolutions is made difficult due to cardiac and respiratory motion during data acquisition. Cardiac gating and respiratory gating or breath holding are effective ways to reduce the effects of motion. The optimal cardiac and respiratory timings vary widely across individuals. This work presents a real-time magnetic resonance imaging approach with physiologic monitoring that can be used to predict the optimal timings on a subject-by-subject basis during a brief real-time prescan. The feasibility of this approach at determining the optimal cardiac trigger delay and respiratory phase is demonstrated.

Variable-density adaptive imaging for high-resolution coronary artery MRI.

Variable-density (VD) spiral k-space acquisitions are used to acquire high-resolution (0.78 mm), motion-compensated images of the coronary arteries. Unlike conventional methods, information for motion compensation is obtained directly from the coronary anatomy itself. Specifically, periods of minimal coronary distortion are identified by applying the correlation coefficient template matching algorithm to real-time images generated from the inner, high-density portions of the VD spirals. Combining the data associated with these images together, high-resolution, motion-compensated coronary images are generated. Because coronary motion is visualized directly, the need for cardiac-triggering, breath-holding, and navigator echoes is eliminated. The motion compensation capability of the technique is determined by the inner-spiral spatial and temporal resolution. Results indicate that the best performance is achieved using inner-spiral images with high spatial resolution (1.6-2.9 mm), even though temporal resolution (four to six independent frames per second) suffers as a result. Image quality within the template region in healthy volunteers was found to be comparable to that achieved with cardiac-triggered breath-hold scans, although extended acquisition times of around 5 min were needed to overcome reduced SNR efficiency.