Guidelines for training in cardiovascular magnetic resonance (CMR).

These "Guidelines for training in Cardiovascular Magnetic Resonance" were developed by the Certification Committee of the Society for Cardiovascular Magnetic Resonance (SCMR) and approved by the SCMR Board of Trustees.

Edge Sharpness Assessment by Parametric Modeling: Application to Magnetic Resonance Imaging.

In biomedical imaging, edge sharpness is an important yet often overlooked image quality metric. In this work, a semi-automatic method to quantify edge sharpness in the presence of significant noise is presented with application to magnetic resonance imaging (MRI). The method is based on parametric modeling of image edges. First, an edge map is automatically generated and one or more edges-of-interest (EOI) are manually selected using graphical user interface. Multiple exclusion criteria are then enforced to eliminate edge pixels that are potentially not suitable for sharpness assessment. Second, at each pixel of the EOI, an image intensity profile is read along a small line segment that runs locally normal to the EOI. Third, the profiles corresponding to all EOI pixels are individually fitted with a sigmoid function characterized by four parameters, including one that represents edge sharpness. Last, the distribution of the sharpness parameter is used to quantify edge sharpness. For validation, the method is applied to simulated data as well as MRI data from both phantom imaging and cine imaging experiments. This method allows for fast, quantitative evaluation of edge sharpness even in images with poor signal-to-noise ratio. Although the utility of this method is demonstrated for MRI, it can be adapted for other medical imaging applications.

Real-time cardiac cine imaging with SPIDER: steady-state projection imaging with dynamic echo-train readout.

Steady-state projection imaging with dynamic echo-train readout (SPIDER) is a multiecho radial k-space trajectory TrueFISP sequence developed for real-time cine imaging of the heart. This new pulse sequence combines the superior SNR and blood-to-myocardium contrast of TrueFISP with the increased scan time efficiency of EPI and undersampled projection reconstruction. SPIDER sequence RF repetition time (TR) was minimized by limiting the echo-train to a length of three while acquiring the first and third echoes asymmetrically. A temporal resolution of 45 ms was achieved with TR/TE1/TE2/TE3 of 3.24/0.6/1.6/2.6 ms and a factor of 2 view sharing scheme. Phantom experiments showed little difference between the weighting of the signals acquired at each of the echo times but did show considerable off-resonance modulation between them. In vivo experiments demonstrated the feasibility of using the SPIDER sequence for real-time imaging in the cardiac short axis orientation.

Limits of detection of regional differences in vasodilated flow in viable myocardium by first-pass magnetic resonance perfusion imaging.

BACKGROUND: Perfusion imaging techniques intended to identify regional limitations in coronary flow reserve in viable myocardium need to identify 2-fold differences in regional flow during coronary vasodilation consistently. This study evaluated the suitability of current first-pass magnetic resonance approaches for evaluating such differences, which are 1 to 2 orders of magnitude less than in myocardial infarction. METHODS AND RESULTS: Graded regional differences in vasodilated flow were produced in chronically instrumented dogs with either left circumflex (LCx) infusion of adenosine or partial LCx occlusion during global coronary vasodilation. First-pass myocardial signal intensity-time curves were obtained after right atrial injection of gadoteridol (0.025 mmol/kg) with an MRI inversion recovery true-FISP sequence. The area under the initial portion of the LCx curve was compared with that of a curve from a remote area of the ventricle. Relative LCx and remote flows were assessed simultaneously with microspheres. The ratio of LCx and remote MRI curve areas and the ratio of LCx and remote microsphere concentrations were highly correlated and linearly related over a 5-fold range of flow differences (y=0.96 x+/-0.07, P<0.0001, r(2)=0.87). The 95% confidence limits for individual MRI measurements were +/-35%. Regional differences of >/=2-fold were consistently apparent in unprocessed MR images. CONCLUSIONS: Clinically relevant regional reductions in vasodilated flow in viable myocardium can be detected with 95% confidence over the range of 1 to 5 times resting flow. This suggests that MRI can identify and quantify limitations in perfusion reserve that are expected to be produced by stenoses of >/=70%.

Segmented k-space and real-time cardiac cine MR imaging with radial trajectories.

The authors developed and evaluated two cine magnetic resonance (MR) imaging sequences with a radial rather than a rectilinear k-space coordinate frame: segmented k space and real-time true fast imaging with steady-state precession, or FISP. The two radial k-space segmentation (or view sharing) techniques, which were interleaved or continuous, were compared, and the feasibility of their application in cardiac cine MR imaging was explored in phantom and volunteer studies. Images obtained with the radial sequences were compared with those obtained with two-dimensional Fourier transform, or 2DFT, sequences currently used in cine MR imaging. Temporal resolution of 55 msec was achieved with the real-time radial sequences, which allowed acquisition of almost 19 high-quality images per second.

Usefulness of segmented trueFISP cardiac pulse sequence in evaluation of congenital and acquired adult cardiac abnormalities.

OBJECTIVE: The purpose of this study is to compare ultrashort TR, segmented trueFISP (fast imaging with steady-state precession) cine MR imaging with segmented FLASH (fast low-angle shot) cine MR imaging for the detection and characterization of congenital and acquired adult cardiac abnormalities. SUBJECTS AND METHODS: Twenty-five patients with known or clinically suspected cardiac abnormalities were imaged on a 1.5-T scanner. Valve plane movies were obtained in patients with suspected valve morphology or function abnormalities or whose horizontal long-axis images showed jets. For each patient, three radiologists independently compared corresponding matched cine FLASH and trueFISP movies for image quality in evaluating anatomy and function of the great vessels and heart. Image quality was rated on a five-point scale, and data were analyzed using both a Wilcoxon's signed rank test and a repeated-measures analysis of variance. RESULTS: Image quality ratings of trueFISP and FLASH showed a statistically significant difference (F = 58.67; df = 1, 72; p < 0.0001), with the average rating for the trueFISP images being significantly higher (mean rating, 4.1 +/- 0.92) than that for the FLASH images (mean, 3.0 +/- 1.0). However, valve architecture in the aortic valves appeared to be better visualized and was more easily measured in valve plane images with FLASH. No statistically significant differences among the ratings of the interpreters (F = 0.018; df = 2, 72; p = 0.9821) were evident, and, therefore, no suggestion of bias was indicated (F = 0.775; df = 1, 2; p = 0.4645). TrueFISP yielded the correct diagnosis prospectively in 13 (100%) of 13 patients, whereas FLASH yielded the correct diagnosis in 12 (92%) of 13 patients. CONCLUSION: TrueFISP images depict morphologic and functional abnormalities with greater clarity and provide greater diagnostic confidence than FLASH images-and in a fraction of the time. A specific exception is in the assessment of valve leaflet architecture and cross-sectional area calculation (i.e., bicuspid aortic valves); in these evaluations, FLASH maintains a complementary diagnostic imaging role.

3D magnetization-prepared true-FISP: a new technique for imaging coronary arteries.

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 alpha/2 preparation pulse was used to speed up the approach to steady-state following a frequency-selective 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. In conclusion, true-FISP is a promising technique for coronary artery imaging.

Theory of high-speed MR imaging of the human heart with the selective line acquisition mode.

Selective line acquisition mode (SLAM) reduces magnetic resonance imaging time by a factor n relative to conventional techniques. Seventeen patients with cardiac disease and three volunteers were examined with SLAM and two-frame interpolation (2FI). SLAM images were sharper than 2FI images and showed well-defined endocardial borders. SLAM is best suited for fast imaging of moving objects, such as the heart, confined to 1/n of the field of view.

An improved MR imaging technique for the visualization of myocardial infarction.

PURPOSE: To design a segmented inversion-recovery turbo fast low-angle shot (turboFLASH) magnetic resonance (MR) imaging pulse sequence for the visualization of myocardial infarction, compare this technique with other MR imaging approaches in a canine model of ischemic injury, and evaluate its utility in patients with coronary artery disease. MATERIALS AND METHODS: Six dogs and 18 patients were examined. In dogs, infarction was produced and images were acquired by using 10 different pulse sequences. In patients, the segmented turboFLASH technique was used to acquire contrast material-enhanced images 19 days +/- 7 (SD) after myocardial infarction. RESULTS: Myocardial regions of increased signal intensity were observed in all animals and patients at imaging. With the postcontrast segmented turboFLASH sequence, the signal intensity of the infarcted myocardium was 1,080% +/- 214 higher than that of the normal myocardium in dogs-nearly twice that of the next best sequence tested and approximately 10-fold greater than that in previous reports. All 18 patients with myocardial infarction demonstrated high signal intensity at imaging. On average, the signal intensity of the high-signal-intensity regions in patients was 485% +/- 43 higher than that of the normal myocardium. CONCLUSION: The segmented inversion-recovery turboFLASH sequence produced the greatest differences in regional myocardial signal intensity in animals. Application of this technique in patients with infarction substantially improved differentiation between injured and normal regions.

MR angiography of the thoracic aorta with an electrocardiographically triggered breath-hold contrast-enhanced sequence.

An electrocardiographically (ECG) triggered breath-hold contrast material-enhanced magnetic resonance (MR) angiography sequence has been developed for imaging the thoracic aorta. A three-dimensional (3D) gradient-echo sequence is used with a contrast material bolus. Forty-nine patients with various aortic abnormalities and five healthy volunteers underwent imaging with the sequence. All studies were performed in a single breath hold. ECG-triggered breath-hold contrast-enhanced MR angiography was tolerated in 48 of the 49 patients. The images demonstrated no respiratory motion artifacts and diminished pulsation artifacts. The cardiac chambers, aortic root, ascending and descending aorta, aortic arch, proximal arch vessels, and proximal coronary arteries were clearly demonstrated and not obscured by ghost artifacts. The 3D data set allowed excellent multiplanar reformation, permitting orthogonal or oblique views of the vascular anatomy. A variety of congenital and acquired abnormalities were clearly identified. When this sequence is used, it is important to evaluate both the maximum-intensity projection and source images. Delayed imaging should be performed to detect late filling. In conjunction with cine MR and T1-weighted spin-echo imaging, ECG-triggered breath-hold contrast-enhanced MR angiography should be considered the technique of choice for imaging the thoracic aorta.

"Black blood" T2-weighted inversion-recovery MR imaging of the heart.

PURPOSE: To develop a short-inversion-time inversion-recovery (STIR) magnetic resonance imaging pulse sequence for evaluating the myocardium that is relatively free of flow and motion artifact. MATERIALS AND METHODS: The authors implemented a breath-hold, cardiac-triggered STIR sequence with preparatory radio-frequency pulses to eliminate signal from flowing blood. A segmented rapid acquisition with relaxation enhancement (turbo spin echo) readout was used, with the inversion-recovery delay adjusted to null fat. The sequence was implemented at 1.0 and 1.5 T and tested in phantoms, five healthy volunteers, and three patients. RESULTS: Phantom studies confirmed the expected behavior of the sequence. In the volunteers, fat-suppressed images of the heart with STIR contrast were generated in a breath-hold period. Blood in the heart chambers was uniformly nulled, and motion artifacts were effectively suppressed. Focal high signal intensity consistent with edema was seen in two patients with acute myocardial infarction; in a third patient, a paracardiac mass was visualized and sharply demarcated relative to normal myocardium. CONCLUSION: Fast STIR imaging of the heart with effective suppression of flow and motion artifacts was implemented. The approach has much potential for high-contrast imaging in a variety of diseases affecting the heart and mediastinum.

Two-dimensional coronary MRA: limitations and artifacts.

Our purpose was to assess image quality and interpretation problems of two-dimensional (2D) coronary MR angiograms. The coronary arteries of 27 subjects (12 normal volunteers and 15 patients) were evaluated with 2D coronary MR angiography (MRA). Coronary MRA was performed with a fat-suppressed electrocardiographically gated breath-hold gradient-echo sequence with k-space segmentation using a 1.5-T imager. Image quality throughout the study was occasionally degraded by: image ghosting (22%), ringing (19%), and/or blurring (22%) and incomplete fat-suppression (19%). Intermittent difficulties with breathholding were encountered in 44% of subjects. When limiting the analysis to those images with optimal image quality, interpretative difficulties were sometimes found: misregistration due to inconsistent breathholding (37%); difficulty in distinguishing veins from arteries (37%); obscured anatomy due to overlapping structures (26%); and poor visualization of portions of the left main coronary artery (59%). Two-dimensional coronary MRA studies have image quality and interpretive problems which need to be understood and addressed before routine clinical scanning is initiated.

Experimental confirmation of phase encoding of instantaneous derivatives of position.

Previous theoretical reports described the dependence of interpretation of the observed phase of the NMR signal on the time origin(s) of moment calculations and position's Taylor series expansion. This work provides experimental confirmation of predictions derived from that theory. For accelerative motion, experimental phase-encoded velocity measurements give instantaneous values at a time corresponding to the origin used for waveform moment calculations. For laminar flow, experimental intensity profiles agree well with theoretical simulations; new findings extend amplitude and spatial distributions of oblique flow profiles beyond previous descriptions. Experiments using sequences with controlled position of the time origins for phase and read axes show that displacement and motion artifacts are reduced when they're coincident (pulsed flow, nongated acquisitions), and virtually eliminated when combined with gating. Potentially significant clinical consequences of coincident and noncoincident time origins are demonstrated in human head MIP MRA images. These results have fundamental implications in waveform design.

MRI gradient waveform design by numerical optimization.

This manuscript describes a method of gradient waveform design by nonlinear constrained optimization. Methods of formulation and solution of the waveform optimization problem are briefly described for minimization of root mean squared current and minimization of waveform moments. Waveforms generated using these objectives are presented and compared with those obtained with other objectives. The method uses waveforms which are defined as a set of discrete amplitudes in order to remove artificial constraints on waveform shape imposed by "multilobe" designs. These point-to-point amplitudes are the parameters determined in the optimization procedure which includes knowledge of the specific imaging conditions and the specific gradient hardware system. Some beneficial results of this design approach are: a) physically realizable waveforms which optimally achieve specific imaging and motion artifact reduction goals, b) waveforms which are guaranteed to be optimal with respect to one of several possible objective, c) less reliance on the experience of the designer, and d) a potential reduction in waveform design time.

An optimal design method for magnetic resonance imaging gradient waveforms.

A method of using nonlinear constrained optimization to design gradient waveforms for magnetic resonance imaging is described. Formulation and solution of the waveform optimization problem are described and example waveforms are presented for a variety of design objectives and constraint sets. Most design objectives can be expressed as linear or quadratic functions of the discrete parameter set, and most constraint functions are linear. Thus, linear and quadratic programming techniques can be utilized to solve the optimization problem. Among the objectives considered are: minimize RMS current; minimize waveform slewing; minimize waveform moments to reduce motion induced dephasing; minimize echo time (TE) for given imaging and motion refocusing conditions; maximize the gradient amplitude during RF application and sampling and the area of the phase encoding waveform to maximize resolution; and minimize or maximize the gradient b factor or diffusion sensitivity. This optimal design procedure produces physically realizable waveforms which optimally achieve specific imaging and motion artifact reduction goals, and it is likely to reduce waveform design time by making it more scientifically (rather than heuristically) based.

Signal-to-noise, resolution, and bias function analysis of asymmetric sampling with zero-padded magnitude FT reconstruction.

This report describes NMR image effects due to sampling asymmetry when using zero-padded magnitude FT reconstruction. With this method, the MTF is not flat over the spatial frequency passband, so resolution cannot be accurately described by a single variable such as voxel size. At small to moderate asymmetry, shortened (reduced window duration) asymmetry provides increased S/N and decreased resolution, whereas shifted (constant window duration) asymmetry yields essentially constant S/N with simultaneously increased and decreased resolution. A bias function expression describes image distortion due to sampling in terms separable from the imaged object. The analyses are consistent with previous descriptions of perceived image differences related to data asymmetry.

Theoretical aspects of motion sensitivity and compensation in echo-planar imaging.

Magnetic resonance (MR) imaging can be performed on or below the time scale of most anatomic motion via echo-planar imaging (EPI) techniques and their derivatives. The goal is to image rapidly and reduce artifacts that typically result from view-to-view changes in the spatial distribution of spins due to motion. However, the required time-dependent magnetic field gradient waveforms remain sensitive to the dephasing effects of motion. Sources of motion artifact are simulated for spins moving along the imaging axes and are shown to be an important source of reduced image quality in EPI. A novel method of EPI is proposed that (a) refocuses single or multiple derivatives of motion at all echoes and (b) prevents accumulation of velocity (or higher derivative)--induced dephasing along the phase-encoding axis by moment nulling all phase-encoding-step waveforms about a single instant of time. Theoretical EPI sequences with considerable reductions in ghosts, blurring, and signal loss due to motion sensitivity are produced and compared with other EPI methods. Their time efficiency is presented as a function of available (relative) gradient strength for a variety of sequence waveforms.

Significance of the point of expansion in interpretation of gradient moments and motion sensitivity.

The relationship between magnetic field gradient waveform moments and the motion sensitivity of magnetic resonance imaging was explored analytically and by computer simulation. The analysis and simulations revealed several key points. In general, waveform time moments define sensitivity to the time derivatives of position of moving material only at a single time point: the time about which the moments are computed. A Taylor series description of instantaneous position is expanded about this same time point to compute the phase acquired due to specific derivatives of position. A moment is proportional to phase sensitivity to a particular derivative of position throughout the waveform only when sensitivity to all lower-order derivatives is zero. Under restricted conditions of waveform symmetry and motion characteristics, the phase due to motion may be expressed in terms of the average value of a derivative of position over the duration of the waveform. The choice of the moment center, or point of expansion, adds a degree of freedom that may be used advantageously in the design of motion-compensating and motion phase-encoding gradient waveforms. These results facilitate a more complete understanding of the effects of motion through a magnetic field gradient.

Modified gradients for motion suppression: variable echo time and variable bandwidth.

A linear algebra based deprivation is presented to demonstrate that linearly time scaling an entire gradient waveform by a factor "R" exponentially increases its sensitivity to time derivatives of position by R(i + 1), where i refers to the i-th derivative of position (e.g., i = 1 is velocity). Thus, time scaling will preserve zero valued refocussing moments associated with artifact reduction techniques designed for motion occurring between excitation and detection. Typically, gradient waveforms for artifact reduction techniques are derived for use only at specific echo times. The time scaling described here allows for simple modification of refocussing gradient waveforms for use at variable echo times. Motion sensitivity associated with non-zero moment gradient waveforms can be easily predicted and modified using this technique, with consideration for field of view, resolution, and bandwidth. A clinical example is presented showing the predicted changes in sensitivity to nonrefocussed derivatives of position as the imaging gradients are time scaled. Further, trade-offs and alternatives in sensitivity to motion, slice thickness, image bandwidth, field of view and resolution will be discussed in conjunction with time scaling. This technique will have applicability in many situations involving MRI of moving tissue and a clinical example in cardiac imaging is presented.

Multiecho multimoment refocussing of motion in magnetic resonance imaging: MEM-MO-RE.

Gradient moment nulling techniques for refocussing of spin dephasing resulting from movement during application of magnetic resonance imaging gradients have gained widespread application. These techniques offer advantages over conventional imaging gradients by reducing motion artifacts due to intraview motion, and by recovering signal lost from spin dephasing. This paper presents a simple technique for designing multiecho imaging gradient waveforms that refocus dephasing from the interaction of imaging gradients and multiple derivatives of position. Multiple moments will be compensated at each echo. The method described relies on the fact that the calculation of time moments for nulled moment gradient waveforms is independent of the time origin chosen. Therefore, waveforms used to generate the second echo image for multiple echo sequences with echo times given by TEn = TE1 + (n - 1) * (TE2 - TE1) may also be used for generation of the third and additional echo images. All echoes will refocus the same derivatives of position. Multiecho, multimoment refocussing (MEM-MO-RE) images through the liver in a patient with ampullary adenocarcinoma metastatic to the liver demonstrate the application of the method in clinical scanning.