The purpose of this work was to develop an ECG-triggered, segmented 3D true-FISP (fast imaging with steady-state precession) technique to improve the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of breath-hold coronary artery imaging. The major task was to optimize an appropriate magnetization preparation scheme to permit saturation of the epicardial fat signal. An ␣/2 preparation pulse was used to speed up the approach to steady-state following a frequencyselective fat-saturation pulse in each heartbeat. The application of dummy cycles was found to reduce the oscillation of the magnetization during data acquisition. The fat saturation and magnetization preparation scheme was validated with simulations and phantom studies. Volunteer studies demonstrated substantially increased SNR (55%) and CNR (178%) for coronary arteries compared to FLASH (fast low-angle shot) with the same imaging time. Volume-targeted breath-hold imaging (VCATS) has recently been described for MR angiography of the coronary arteries (1). VCATS employs a 3D steady-state incoherent gradient-echo sequence, FLASH (fast low-angle shot), for data acquisition. In such a sequence, the transverse magnetization is spoiled at the end of each repetition time (TR) to create T 1 -weighted contrast. As TR is shortened and bandwidth increased to speed up data acquisition, the available signal-to-noise ratio (SNR) becomes a limiting factor with FLASH. Injection of contrast media is necessary to shorten the blood T 1 to improve SNR and contrastto-noise ratio (CNR) (2-6).True-FISP (fast imaging with steady-state precession) has recently found important applications in cardiac cine imaging (7,8) for the evaluation of left ventricular function. Significant improvements in blood SNR and blood-myocardium CNR were obtained as compared to FLASH. In true-FISP, the transverse magnetization is maintained between successive RF pulses because the net gradient moments are zero in all three directions and no RF spoiling is implemented. Coherent transverse magnetization continues to contribute to the signal in successive TRs, resulting in a higher SNR than in magnetization-spoiled techniques such as FLASH. For the same reason, large flip angles close to 90°can be used in true-FISP because there are no saturation effects and the magnetization is restored and reused. Analysis using the Bloch equations predicts that the signal intensity in true-FISP is T 2 /T 1 -weighted. The contrast between blood and myocardium is therefore enhanced because blood has a much higher T 2 /T 1 than myocardium.The concept of true-FISP (9) and its applications in brain and spine imaging (10) were proposed years ago. However, because of the zero net applied gradients, there was no averaging of the resonant offsets and, with relatively long minimum TRs achievable at the time, the technique was extremely sensitive to field inhomogeneities and was not used in cardiac imaging. With recent improvements in gradient capabilities, short TRs on the order of 3-4 msec have been achieved. Comb...

They have presented a novel methodology employing dMRI to derive representative 4D-MRI. This set of techniques are practical in that (1) they employ MRI sequences that are standard across commercial vendors; (2) the 2D imaging planes can be oriented onto an arbitrary axis (e.g., sagittal, coronal, axial[ellipsis (horizontal)]); (3) the image processing techniques are relatively simple. Systematically applying this and similar dMRI-based techniques in patients is a crucial next step to demonstrate efficacy beyond CT-only based practice.

An electrocardiogram (ECG)-triggered, magnetization-prepared, segmented, 3D true fast imaging with steady-state precession (true-FISP) sequence with fat saturation was recently proposed for coronary artery imaging. A magnetization preparation scheme consisting of an ␣/2 radiofrequency (RF) pulse followed by 20 constant flip angle dummy RF cycles was used to reduce signal oscillations in the approach to steady state. However, if large resonance offsets on the order of 70 -100 Hz are present, significant magnetization oscillations will still occur during data acquisition, which will result in image ghosting and blurring. The goal of this work was to validate that a linear flip angle (LFA) series can be used during magnetization preparation to reduce these image artifacts. Computer simulations, phantom studies, and coronary artery imaging in healthy volunteers were performed to compare this magnetization preparation scheme with that of an ␣ An electrocardiogram (ECG)-triggered, magnetization prepared, segmented, 3D true fast imaging with steady-state precession (true-FISP) sequence was recently developed for imaging the coronary arteries (1,2). For coronary imaging, the requirement of fat saturation and ECG triggering demands that the data be acquired during the transient period, when the magnetization approaches steady state after a trigger delay time and the fat saturation prepulse in each cardiac cycle. However, if the spins are not at the resonant frequency, magnetization oscillations during data acquisition cause signal fluctuations in k-space, which consequently lead to image artifacts such as ghosting and blurring.Several methods, such as ␣/2 (␣ ϭ data acquisition radiofrequency (RF) pulse flip angle) preparation (3), catalyzation (4), and magnetization preparation using variable flip angles (5), have been proposed to reduce the transient magnetization oscillations in true-FISP and enable data acquisition during the initial RF cycles before establishment of steady state. The ␣/2 preparation works well for spins that are on-resonance, but the magnetization continues to oscillate for spins that are not at the resonant frequency. The catalyzation approach has been shown to improve the signal response over a wide range of offresonance frequencies and to be relatively insensitive to T 1 and T 2 variations. A practical limitation of this method is the requirement of a good slice profile of the frequencyselective RF pulses, because the effectiveness of the method is dependent on B 1 field homogeneity. Nishimura et al. (5) The purpose of this work was to evaluate the effectiveness of a linear flip angle (LFA) preparation scheme in reducing the image artifacts in coronary artery imaging using 3D true-FISP. The LFA preparation was compared to the magnetization preparation method of an ␣/2 pulse followed by constant flip angle preparation cycles (1). Computer simulations, phantom studies, and volunteer studies were performed. Using simulations, the signal oscillations with the LFA preparation were evaluated with varia...

A volume-targeted contrast agent-enhanced breath-hold coronary magnetic resonance angiographic technique was optimized and evaluated in 16 volunteers. Substantial increases in coronary signal-to-noise ratio, contrast-to-noise ratio, lengths of depiction, and vessel sharpness were observed on enhanced images. The imaging approach with two 20-mL injections of contrast agent covers the left and right coronary arteries in two breath holds and is a promising method for coronary imaging.

Background-The decision to perform coronary revascularization procedures may hinge on assessment of myocardial perfusion reserve. Blood oxygen level-dependent (BOLD) MRI is a potential method to detect the effects of regional variations in myocardial blood flow during vasodilation. Methods and Results-We imaged dogs (nϭ13) on a 1.5-T whole-body MRI scanner using a new T 2 -prepared steady-state free-precession (SSFP) MRI pulse sequence sensitive to BOLD contrast. Images (in-plane resolution Ϸ1 mm 2 ) of 5 short-axis and 2 long-axis slices of the heart were acquired during graded levels of adenosine infusion via a surgically placed left circumflex (LCx) catheter (nϭ11) or via a right atrial catheter in animals with an LCx occluder (nϭ2). Relative myocardial perfusion was measured with the use of fluorescent microspheres. Signal intensity changes in myocardium subtended by the left anterior descending coronary artery were compared with those in the LCx region. Unprocessed T 2 -weighted images revealed changes in signal intensity corresponding to areas of regional vasodilation or reduced myocardial perfusion reserve during systemic vasodilation. At maximal vasodilation, the signal intensity ratio in the LCx versus left anterior descending territories increased by 33Ϯ4% compared with baseline, corresponding to a 3.8Ϯ0.3-fold increase in relative perfusion (PϽ0.01). MR intensity at progressive levels of vasodilation demonstrated good agreement with microsphere flow (Rϭ0.80, PϽ0.01). Conclusions-T 2 -prepared SSFP BOLD imaging is a promising method to determine an index of myocardial perfusion reserve in this animal model.

Purpose:To evaluate the effectiveness of a T2-magnetization preparation scheme for improving coronary artery imaging with true fast imaging with steady-state precession (True-FISP). Materials and Methods:Simulations were performed to compare the blood-myocardium signal difference with no T2-preparation to that with various T2-preparation times (24, 40, and 60 msec) using an electrocardiogram (ECG)-triggered, segmented True-FISP acquisition. Seven volunteers were imaged to evaluate the effectiveness of T2-preparation for coronary artery delineation using True-FISP and to optimize the T2-preparation time.Results: Simulations showed that T2-preparation improved the signal difference between blood and myocardium over that without T2-preparation. The optimal T2-preparation time was determined to be 40 msec. In volunteer studies, a T2-preparation time of 40 msec provided a significant improvement in contrast-to-noise ratio (CNR) between the coronary arteries and myocardium over that without T2-preparation. It also showed a significant improvement in visualizing the distal portions of the coronary arteries. RECENTLY, A THREE-DIMENSIONAL segmented, fatsaturated, magnetization prepared True-FISP (fast imaging with steady-state precession) sequence has been developed to image the coronary arteries (1). Compared to FLASH (fast low angle shot), the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) are significantly higher in the True-FISP sequence due to the use of larger flip angles, the refocused magnetization, and the inherent T2/T1 weighting. In order to preserve the effectiveness of the spectrally selective fat saturation pre-pulse, data was collected during the transient period when the magnetization approached steady-state. Therefore, the myocardial tissue was not suppressed sufficiently, leading to suboptimal contrast between blood and myocardium. ConclusionPrevious research in coronary artery imaging using FLASH has shown that a T2-preparation scheme can effectively suppress the myocardial signal, albeit at the expense of SNR (2,3). This is due to the fact that the T2 of myocardium (ϳ50 msec) (4) is considerably less than that of arterial blood (ϳ250 msec). We hypothesize that T2-preparation can also benefit True-FISP imaging of the coronary arteries. The purpose of this work was to evaluate an ECG-triggered, T2-prepared, fat-saturated, segmented three-dimensional True-FISP sequence for coronary artery imaging within a single breath-hold. T2-preparation was used to enhance visualization of the distal portions of the coronary arteries, as well as improve overall delineation of the arteries. MATERIALS AND METHODS T2-PreparationThe T2-preparation scheme was previously described by Brittain et al (2) and was designed to be insensitive to flow and ␤ 0 and ␤ 1 inhomogeneous field effects. A 90°x excitation pulse was followed by four equally spaced 180°x composite refocusing pulses weighted in a MLEV pattern (5,6). Each 180°x composite pulse consisted of a 90°x, 180°y, and 90°x sequence of pulses (7). The magn...

Purpose: Current pretreatment, 4D imaging techniques are suboptimal in that they sample breathing motion over a very limited “snapshot” in time. Heretofore, long‐duration, 4D motion characterization for radiotherapy planning, margin optimization, and validation have been impractical for safety reasons, requiring invasive markers imaged under x‐ray fluoroscopy. To characterize 3D tumor motion and associated variability over durations more consistent with treatments, the authors have developed a practical dynamic MRI (dMRI) technique employing two orthogonal planes acquired in a continuous, interleaved fashion. Methods: 2D balanced steady‐state free precession MRI was acquired continuously over 9–14 min at approximately 4 Hz in three healthy volunteers using a commercial 1.5 T system; alternating orthogonal imaging planes (sagittal, coronal, sagittal, etc.) were employed. The 2D in‐plane pixel resolution was 2 × 2 mm2 with a 5 mm slice profile. Simultaneous with image acquisition, the authors monitored a 1D surrogate respiratory signal using a device available with the MRI system. 2D template matching‐based anatomic feature registration, or tracking, was performed independently in each orientation. 4D feature tracking at the raw frame rate was derived using spline interpolation. Results: Tracking vascular features in the lung for two volunteers and pancreatic features in one volunteer, the authors have successfully demonstrated this method. Registration error, defined here as the difference between the sagittal and coronal tracking result in the SI direction, ranged from 0.7 to 1.6 mm (1σ) which was less than the acquired image resolution. Although the healthy volunteers were instructed to relax and breathe normally, significantly variable respiration was observed. To demonstrate potential applications of this technique, the authors subsequently explored the intrafraction stability of hypothetical tumoral internal target volumes and 3D spatial probability distribution functions. The surrogate respiratory information allowed the authors to show how this technique can be used to study correlations between internal and external (surrogate) information over these prolonged durations. However, compared against the gold standard of the time stamps in the dMRI frames, the temporal synchronization of the surrogate 1D respiratory information was shown to be likely unreliable. Conclusions: The authors have established viability of a novel and practical pretreatment, 4D tumor centroid tracking method employing a commercially available dynamic MRI sequence. Further developments from the vendor are likely needed to provide a reliably synchronized surrogate 1D respiratory signal, which will likely broaden the utility of this method in the pretreatment radiotherapy planning context.

The presence of resonance frequency offsets often causes artifacts in images acquired with true fast imaging with steadystate precession (true-FISP). One source of resonance offsets is a suboptimal setting of the synthesizer frequency. The goal of this work was to demonstrate that shifting the synthesizer frequency could minimize the off-resonance related image artifacts in true-FISP. A simple scouting method was developed to estimate the optimal synthesizer frequency for the volume of interest (VOI). To improve fat suppression, a similar scouting method was also developed to determine the optimal frequency offset for the fat saturation pulse. Coronary artery imaging was performed in healthy subjects using a 3D true-FISP sequence to validate the effectiveness of the frequency corrections. Substantial reduction in image artifacts and improvement in fat suppression were observed by using the water and fat frequencies estimated by the scouting scans. Frequency shifting is a useful and practical method for improving coronary artery imaging using true-FISP. With improvements in gradient capabilities in recent years, true fast imaging with steady-state precession (true-FISP) has been successfully used in cardiac cine imaging (1,2) and coronary artery imaging (3). In true-FISP, the zeroth moment of the gradients in each TR are zero so that transverse coherences are maintained in successive radiofrequency (RF) cycles. The resonance frequency offset then dominates the phase behavior of the spins and may cause image artifacts. One of the main sources for the off-resonance frequencies is B 0 field inhomogeneity. Another source is an incorrect setting of the synthesizer frequency (also referred to as the imaging frequency). Careful shimming can minimize these effects.The correct estimation of the optimal imaging frequency is dependent on the field homogeneity. Achieving uniform fields by shim adjustments is particularly challenging in cardiac applications due to heart and respiratory motion, blood flow, chemical shift, and susceptibility variations at air-tissue interfaces. When phase is used to estimate the field distortions, anatomic motion and blood flow may cause errors. Therefore, it is difficult to develop a general shim solution for continually changing heart position and geometry. Suboptimal shimming then gives rise to field inhomogeneity and variations in resonant frequency. Jaffer et al. (4) reported that a peak-to-peak gradient of 62 Hz may be present across the heart at 1.5 T. Also, in a study by Reeder et al. (5), frequency offsets on the order of 100 Hz were found in the vicinity of the cardiac veins. Therefore, no matter what imaging frequency is selected, certain spins will have resonance offsets in the heart. In addition, the frequency estimated by adjustment routines may not be optimal for the heart due to the different volumes used for frequency adjustment and imaging, and/or the presence of tissues other than the heart (chest wall, liver, etc.) in the prescribed adjustment volume when a large field inhom...