Fast Gradient-recalled Echo Research Articles

Application of a 2D frequency encoding sectoral approach to hyperpolarized 129Xe MRI at low field

Inhaled hyperpolarized 129Xe MRI is a non-invasive and radiation risk free lung imaging method, which can directly measure the business unit of the lung where gas exchange occurs: the alveoli and acinar ducts (lung function). Currently, three imaging approaches have been demonstrated to be useful for hyperpolarized 129Xe MR in lungs: Fast Gradient Recalled Echo (FGRE), Radial Projection Reconstruction (PR), and spiral/cones. Typically, non-Cartesian acquisitions such as PR and spiral/cones require specific data post-processing, such as interpolating, regridding, and density-weighting procedures for image reconstruction, which often leads to smoothing effects and resolution degradation. On the other hand, Cartesian methods such as FGRE are not short-echo time (TE) methods; they suffer from imaging gradient-induced diffusion-weighting of the k-space center, and employ a significant number of radio-frequency (RF) pulses.Due to the non-renewable magnetization of the hyperpolarized media, the use of a large number of RF pulses (FGRE/PR) required for full k-space coverage is a significant limitation, especially for low field (<0.5 T) hyperpolarized gas MRI.We demonstrate an ultra-fast, purely frequency-encoded, Cartesian pulse sequence called Frequency-Encoding Sectoral (FES), which takes advantage of the long T2* of hyperpolarized 129Xe gas at low field strength (0.074 T). In contrast to PR/FGRE, it uses a much smaller number of RF pulses, and consequently maximizes image Signal-to-Noise Ratio (SNR) while shortening acquisition time. Additionally, FES does not suffer from non-uniform T2* decay leading to image blurring; a common issue with interleaved spirals/cones. The Cartesian k-space coverage of the proposed FES method does not require specific k-space data post-processing, unlike PR/FGRE and spiral/cones methods.Proton scans were used to compare the FES sequence to both FGRE and Phase Encoding Sectoral, in terms of their SNR values and imaging efficiency estimates. Using FES, proton and hyperpolarized 129Xe images were acquired from a custom hollow acrylic phantom (0.04L) and two normal rats (129Xe only), utilizing both single-breath and multiple-breath schemes. For the 129Xe phantom images, the apparent diffusion coefficient, T1, and T2* relaxation maps were acquired and generated. Blurring due to the T2* decay and B0 field variation were simulated to estimate dependence of the image resolution on the duration of the data acquisition windows (i.e. sector length), and temperature-induced resonance frequency shift from the low field magnet hardware.

High spatial resolution hyperpolarized 3 He MRI of the rodent lung using a single breath X-centric gradient-recalled echo approach.

Hyperpolarized (HP) gas MRI of the rodent lung is of great interest because of the increasing need for novel biomarkers with which to develop new therapies for respiratory diseases. The use of fast gradient-recalled echo (FGRE) for high-resolution HP gas rodent lung MRI is challenging as a result of signal loss caused by significant diffusion weighting, particularly in the larger airways. In this work, a modified FGRE approach is described for HP 3 He rodent lung MRI using a centric-out readout scheme (ie, x-centric), allowing high-resolution, density-weighted imaging. HP 3 He x-centric imaging was performed in a phantom and compared with a conventional partial-echo FGRE acquisition for in-plane spatial resolutions varying between 39 and 312 µm. Partial-echo and x-centric acquisitions were also compared for high spatial-resolution breath-hold (1 s) imaging of rodent lungs. X-centric provided improved signal-to-noise ratio efficiency by a factor of up to 13/1.7 and 6.7/1.8, compared with the partial-echo FGRE for the airways/parenchyma of mouse and rat, respectively, at high spatial resolutions in vivo (<78 µm). In particular, rodent major airways with less restricted diffusion of 3 He could only be visualized with the x-centric method. The x-centric method significantly reduces diffusion weighting, allowing high spatial and temporal resolution HP 3 He gas density-weighted rodent lung MRI. Magn Reson Med 78:2334-2341, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

T2* Mapping from Multi-Echo Dixon Sequence on Gadoxetic Acid-Enhanced Magnetic Resonance Imaging for the Hepatic Fat Quantification: Can It Be Used for Hepatic Function Assessment?

ObjectiveTo evaluate the diagnostic value of T2* mapping using 3D multi-echo Dixon gradient echo acquisition on gadoxetic acid-enhanced liver magnetic resonance imaging (MRI) as a tool to evaluate hepatic function.Materials and MethodsThis retrospective study was approved by the IRB and the requirement of informed consent was waived. 242 patients who underwent liver MRIs, including 3D multi-echo Dixon fast gradient-recalled echo (GRE) sequence at 3T, before and after administration of gadoxetic acid, were included. Based on clinico-laboratory manifestation, the patients were classified as having normal liver function (NLF, n = 50), mild liver damage (MLD, n = 143), or severe liver damage (SLD, n = 30). The 3D multi-echo Dixon GRE sequence was obtained before, and 10 minutes after, gadoxetic acid administration. Pre- and post-contrast T2* values, as well as T2* reduction rates, were measured from T2* maps, and compared among the three groups.ResultsThere was a significant difference in T2* reduction rates between the NLF and SLD groups (−0.2 ± 4.9% vs. 5.0 ± 6.9%, p = 0.002), and between the MLD and SLD groups (3.2 ± 6.0% vs. 5.0 ± 6.9%, p = 0.003). However, there was no significant difference in both the pre- and post-contrast T2* values among different liver function groups (p = 0.735 and 0.131, respectively). A receiver operating characteristic (ROC) curve analysis showed that the area under the ROC curve for using T2* reduction rates to differentiate the SLD group from the NLF group was 0.74 (95% confidence interval: 0.63–0.83).ConclusionIncorporation of T2* mapping using 3D multi-echo Dixon GRE sequence in gadoxetic acid-enhanced liver MRI protocol may provide supplemental information for liver function deterioration in patients with SLD.

Multi‐echo gradient recalled echo imaging of the pelvis for improved depiction of brachytherapy seeds and fiducial markers facilitating radiotherapy planning and treatment of prostatic carcinoma

MR localization of implanted devices for radiotherapy (RT) in prostatic carcinoma is critical for treatment planning. This clinical note studies the application of a multi-echo gradient recalled echo (GRE) pulse sequence with sum of squares echo combination (ME GRE) to enhance detection of seeds and fiducials. Fifteen patients who underwent MRI using fast spin echo (FSE), single-echo and ME GRE over a 9-month period were retrospectively evaluated by two readers who assessed overall image quality, depiction of seeds/fiducials and image sharpness using a 5-point scale (1 = poor, 2 = suboptimal, 3 = adequate, 4 = above average, 5 = excellent). Image scores were compared using the Wilcoxon sign rank test. In all 15 patients, both readers rated the depiction of seeds/fiducials with ME GRE as excellent. In all 15 patients, overall image quality and image sharpness with ME GRE was rated as excellent by reader 1. In 12/15 patients, overall image quality and image sharpness with ME GRE was rated as excellent and in the other patients above average by reader 2. There was a difference in depiction of seeds/fiducials comparing GRE to FSE (P < 0.001) and ME to single echo GRE (P < 0.001). Overall image quality and sharpness was higher with ME compared with single echo GRE (P < 0.001) and similar to FSE (P = 0.26 and P = 0.16). Multi-echo GRE provides better detection of implanted seeds and fiducial markers when compared with both FSE and single-echo GRE potentially improving RT treatment planning for prostate carcinoma.

Precise size evaluation of extraocular muscles using fat-suppressed fast T1-weighted gradient-recalled echo imaging and multiple gaze fixation targets

We studied the feasibility of the precise size measurement of extraocular muscles using fast magnetic resonance imaging sequences and gaze fixation targets. We recruited 20 healthy volunteers and optimized fat-suppressed fast T1-weighted gradient-recalled echo (FS T1-GRE) and single-shot fast spin-echo (SSFSE) imaging for evaluating extraocular muscles. With each eye at 40° abduction and adduction, we scanned the short-axis plane at the maximum diameter of the bilateral lateral and medial rectus muscles and measured the maximal cross-sectional area of the muscles during extension and contraction. We evaluated interobserver reproducibility and variability between the size measurements using the two MR sequences and the measurement reproducibility using FS T1-GRE imaging. The interobserver reproducibility in the muscle size measurements using FS T1-GRE and SSFSE imaging was excellent (r=0.93-0.94) and moderate to good (r=0.54-0.75), respectively. The interobserver variability was less with FS T1-GRE than SSFSE imaging (p<0.01). The reproducibility of the size measurement using FS T1-GRE was good to excellent (r=0.78-0.92). FS T1-GRE imaging with the subject staring at each of multiple targets is useful for evaluating precisely the size of extraocular muscles.

Three‐dimensional dixon fat–water separated rapid breathheld imaging of myocardial infarction

To develop a breathhold three-dimensional (3D) Dixon technique for fat suppressed imaging of myocardial infarction. A pulse sequence was developed that uses a radial fan-beam k-space segmentation scheme for efficient coverage of k-space, enabling 3D scans in a single breathhold. The sequence uses a dual-echo bipolar readout to enable Dixon fat-water separation for improved visualization of epicardial and pericardial delayed enhancement. The 3D Dixon method was compared with a conventional 2D fast gradient recalled echo (FGRE) -based technique in 25 patients. Pericardial visualization scores and confidence were higher while overall image quality and artifacts were slightly worse for 3D Dixon compared with 2D FGRE. Robust fat suppression was achieved in 21 of 25 cases using the 3D Dixon method. A 3D breathhold method for fat-water separated imaging of myocardial delayed enhancement was developed and validated.

Three-dimensional Dixon fat-water separated breath-held imaging of myocardial delayed enhancement

Introduction Myocardial delayed enhancement (MDE) imaging of infarction is commonly performed using an inversion-recovery (IR) two-dimensional (2D) breath-held fast gradient recalled echo (FGRE) pulse sequence. Three-dimensional (3D) imaging can improve scanning efficiency by acquiring the desired volume in a single breath-hold. Fat suppression could greatly improve visualization of epicardial enhancement (which occurs in transmural myocardial infarction and myocarditis) as well as pericardial enhancement.

Poster — Wed Eve—14: Real‐Time MR Imaging for Angioplasty

Angioplasty has been demonstrated as an effective treatment for cardiovascular diseases such as carotid stenosis, having similar patient outcomes to the once dominant endarectomy technique. Angioplasty is an attractive choice, as it is much less invasive. However, angioplasty procedures are hinged on the guidance of catheters through the vascular system, and imaging is required for this process. X‐ray is almost always used for these types of interventions, but has several noted drawbacks, including the exposure of ionizing radiation to both patients and staff. Magnetic resonance (MR) imaging has been used in previous experiments at different centres and overcomes some of the problems associated with X‐ray imaging. Here, we propose a real‐time imaging system, for use in catheter guiding applications, and look at parameters and techniques that will increase the overall frame rate displayed to an in‐room monitor. By modifying a fast gradient recalled echo (FGRE) sequence, and ported data directly to an image reconstruction station, implemented on an iMac computer, images are reconstructed and display in real time. By using algorithms such as variable rate k‐space acquisition (varking), multi‐phase array coils, reducing the number of phase‐encode lines, and reducing the analog to digital converter (ADC) sampling rate, frame rate was improved from ∼1 Hz to ∼5Hz. Analysis of images, pre‐ and post‐optimization, yield comparable quality by inspection, and an improved SNR from 45 to 160. This system has been designed to perform MR angioplasty procedures, which will the next step in our research project using animal models.

Hippocampal Volumes in First-Episode Psychosis

Hippocampal Volumes in First-Episode Psychosis

Hippocampal Volumes in First-Episode Psychosis

Studies have shown that patients with schizophrenia have smaller hippocampi than healthy comparison subjects. There are, however, inconsistencies regarding the relationship between clinical characteristics and topographical differences in hippocampal volumetry. The authors investigated hippocampal volumes in minimally treated patients with first-episode schizophrenia spectrum disorders, relative to comparison subjects. Thirty-nine consecutive patients and 29 matched comparison subjects were scanned using 1.5 tesla MR system. Patients had significantly smaller right anterior, right, and anterior hippocampal volumes than comparison subjects. There was a gender by diagnosis effect: female patients showed significantly smaller anterior and right hippocampal volumes than female comparison subjects, an effect not seen in male patients. Our results suggest that smaller hippocampal volumes are present even in early stages of the illness.

Intracoronary MR imaging using a 0.014‐inch MR imaging‐guidewire: Toward MRI‐guided coronary interventions

To validate the feasibility of using a newly designed MR imaging-guidewire (MRIG) to guide angioplasty balloon placement in coronary arteries. A custom gold/sliver/Nitinol/MP35N-based, 0.014-inch MRIG was manufactured. To test its mechanical performance we used the new MRIG to catheterize the left coronary arteries of three dogs under x-ray fluoroscopy. To further validate the feasibility of using the MRIG to generate intracoronary MR imaging, we positioned the MRIG, along with a dilation-perfusion balloon catheter, into the left coronary arteries of an additional three dogs. Longitudinal and four-chamber views of cine cardiac MR images were obtained using a fast gradient recalled echo (FGRE) sequence (TR/TE/FA = 5.2 msec/1.6 msec/20 degrees , field of view [FOV] = 32 x 32 cm, thickness = 5 mm, space = 2 mm, matrix = 256 x 160, number of excites [NEX] = 0.5, and bandwidth [BW] = 32 kHz). Then three-dimensional (3D) MR coronary angiography of the left coronary arteries was obtained using a fast imaging employing steady-state acquisition (FIESTA) sequence. We subsequently used the MRIG, at a receive-only mode, to generate intracoronary MR images using FGRE (TR/TE/FA = 7.2 msec/3.5 msec/20 degrees , FOV = 18 x 18cm, thickness = 3 mm, space = 0.5 mm, matrix = 256 x 256, NEX = 0.5, and BW = 32 kHz). In all six animals the left main coronary arteries were successfully catheterized. 3D MR imaging displayed left coronary artery branches. Intracoronary MR imaging demonstrated the inflated balloons as a "train track" or a bright, thick ring at different views. This study demonstrates the potential of using this newly designed gold/sliver/Nitinol/MP35N-based, 0.014-inch MRIG to catheterize coronary arteries and, thus, generate intracoronary MR imaging with balloon inflation.

3.0 Tesla High Spatial Resolution Contrast-Enhanced Magnetic Resonance Angiography (CE-MRA) of the Pulmonary Circulation

To evaluate the technical feasibility of high spatial resolution contrast-enhanced magnetic resonance angiography (CE-MRA) with highly accelerated parallel acquisition at 3.0 T using a 32-channel phased array coil, and a high relaxivity contrast agent. Ten adult healthy volunteers (5 men, 5 women, aged 21-66 years) underwent high spatial resolution CE-MRA of the pulmonary circulation. Imaging was performed at 3 T using a 32-channel phase array coil. After intravenous injection of 1 mL of gadobenate dimeglumine (Gd-BOPTA) at 1.5 mL/s, a timing bolus was used to measure the transit time from the arm vein to the main pulmonary artery. Subsequently following intravenous injection of 0.1 mmol/kg of Gd-BOPTA at the same rate, isotropic high spatial resolution data sets (1 x 1 x 1 mm3) CE-MRA of the entire pulmonary circulation were acquired using a fast gradient-recalled echo sequence (TR/TE 3/1.2 milliseconds, FA 18 degrees) and highly accelerated parallel acquisition (GRAPPA x 6) during a 20-second breath hold. The presence of artifact, noise, and image quality of the pulmonary arterial segments were evaluated independently by 2 radiologists. Phantom measurements were performed to assess the signal-to-noise ratio (SNR). Statistical analysis of data was performed by using Wilcoxon rank sum test and 2-sample Student t test. The interobserver variability was tested by kappa coefficient. All studies were of diagnostic quality as determined by both observers. The pulmonary arteries were routinely identified up to fifth-order branches, with definition in the diagnostic range and excellent interobserver agreement (kappa = 0.84, 95% confidence interval 0.77-0.90). Phantom measurements showed significantly lower SNR (P < 0.01) using GRAPPA (17.3 +/- 18.8) compared with measurements without parallel acquisition (58 +/- 49.4). The described 3 T CE-MRA protocol in addition to high T1 relaxivity of Gd-BOPTA provides sufficient SNR to support highly accelerated parallel acquisition (GRAPPA x 6), resulting in acquisition of isotopic (1 x 1 x 1 mm3) voxels over the entire pulmonary circulation in 20 seconds.

Myocardial viability is a better than qrs duration in predicting clinical benfit from cardiac resynchronisation therapy

Background Although the benefits of cardiac resynchronisation therapy (CRT) are well established, they are difficult to predict from clinical, ECG or echocardiographic parameters. Gadolinium DTPA-enhanced magnetic resonance (Gd- MR) imaging is the gold-standard for the in vivo identification of myocardial scarring. This study hypothesised that the clinical benefit of CRT can be predicted from the assessment of left ventricular (LV) myocardial viability using Gd-MR. Methods 29 patients with heart failure due to coronary heart disease or dilated cardiomyopathy, aged 71.3 (1.8) yrs [mean (SEM)], in NYHA class II (n=1), III (n=21) and IV (n=7) and with a QRS of 154 (12) ms underwent a clinical assessment, including a 6-min walk test, before and after CRT. A Gd-MR scan (1.5 TGE Signa scanner) was obtained at baseline,10 min following 0.1 mmol/Kg Gd-DTPA i.v., using a multiphase inversion recovery fast gradient recalled-echo (IR-FGRE) sequence. Scar volume was calculated by planimetry from short axis sections and expressed as a % of total LV myocardial volume. Non-responders (NR) were defined as patients who died or in whom, at the most recent follow-up, NYHA class and/or 6 min walking distance had either not changed or decreased. Results After a follow-up period of 418.0 (44.9) days, 6 NR were identified. % scar volume was 30.7 (8.9) cm3 in NR and 12.5 (2.0) cm3 in responders(R) (ANOVA, p=0.0041). In logistic regression analyses, % scar volume emerged as a predictor of NR (p=0.0194), whilst LV ejection fraction, baseline QRS duration and aetiology did not. In survivors, % scar volume emerged as a predictor of change in walking distance from baseline to follow-up (p=0.0337). Conclusion Myocardial viability assessed by Gd-MR is a predictor of outcome in patients undergoing CRT. Further studies are needed to determine the role of Gd-MR in selecting patients for CRT.

OA24 Magnetic resonance imaging of the trabecular bone microarchitecture at 3 Tesla - comparison with 1.5 Tesla

Objectifs High-resolution MRI has currently evolved as a promising technique to depict trabecular bone architecture. However, one drawback is a limited spatial resolution and signal-to-noise ratio (SNR). With the advent of 3T MRI it is possible to improve spatial resolution and SNR in a clinical setting. The purpose of this study was to compare standard 1,5T MR imaging of trabecular bone structure with imaging at 3T. Materiels et methodes Ten human calcaneus specimens were imaged with a wrist coil at 1,5T (Fast Gradient Recalled Echo (FGRE) - sequence with TR = 19 ms, TE = 4,6 ms, flip angle 20°) and at 3 T (FGRE-sequence with TR = 19,6 ms, TE = 4,7 ms, flip angle 20°) using the same spatial resolution. Structural analysis was performed after an alignment procedure between MR images at 1,5 T and at 3 T. Structural parameters and SNR determined in the corresponding sections, were compared. Resultats SNR was substantially higher in the images obtained at 3T. Qualitative analysis showed a better depiction of the trabecular bone structure at 3T and the quantitative analysis demonstrated significantly higher apparent trabecular number and trabecular thickness, thus also increasing bone volume fraction. Conclusion These preliminary results show that imaging at 3T may substantially improve the depiction of trabecular bone microarchitecture though suceptibility artifacts also increase.

FAST sequences optimization for contrast media pharmacokinetic quantification in tissue.

The purpose of this study was to investigate the influence of the fast gradient-recalled echo (GRE) sequence parameters on the contrast dynamic range and signal sensitivity, to optimize the magnetic resonance (MR) sequence for contrast media pharmacokinetic assessment. Effects of the fast low-angle shot (FLASH), Fast acquisition at steady rate (FAST), and radiofrequency-spoiled (RF)-FAST sequence parameters were studied in vitro. The FAST sequence had the highest sensitivity in low gadolinium (Gd) concentration. The FLASH and RF-FAST sequences had a larger contrast dynamic range, but the FLASH images contained side band artifacts. Increasing the flip angle to 90 degrees raised the sensitivity of the FAST sequence and the contrast dynamic range of the RF-FAST sequence. The shortest possible TE was optimal for both contrast dynamics and imaging time. TI had an influence on the sensitivity of the FAST sequence only for small acquisition matrices. This study indicates the optimal parameters for contrast dynamics (RF-FAST, 90 degrees flip angle, shortest possible TE) and sensitivity (FAST, 90 degrees flip angle, long TI(eff)).

MR imaging-guided prostate biopsy with surgical navigation software: device validation and feasibility.

Magnetic resonance (MR) imaging--guided prostate biopsy in a 0.5-T open imager is described, validated in phantom studies, and performed in two patients. The needles are guided by using fast gradient-recalled echo and T2-weighted fast spin-echo images. Surgical navigation software provided T2-weighted images critical to targeting the peripheral zone and the tumor. MR imaging can be used to guide prostate biopsy.

Interventional and intraoperative magnetic resonance imaging.

The goal of the Image Guided Therapy Program, as the name implies, is to develop the use of imaging to guide minimally invasive therapy. The program combines interventional and intraoperative magnetic resonance imaging (MRI) with high-performance computing and novel therapeutic devices. In clinical practice the multidisciplinary program provides for the investigation of a wide range of interventional and surgical procedures. The Signa SP 0.5 T superconducting MRI system (GE Medical Systems, Milwaukee, WI) has a 56-cm-wide vertical gap, allowing access to the patient and permitting the execution of interactive MRI-guided procedures. This system is integrated with an optical tracking system and utilizes flexible surface coils and MRI-compatible displays to facilitate procedures. Images are obtained with routine pulse sequences. Nearly real-time imaging, with fast gradient-recalled echo sequences, may be acquired at a rate of one image every 1.5 s with interactive image plane selection. Since 1994, more than 800 of these procedures, including various percutaneous procedures and open surgeries, have been successfully performed at Brigham and Women's Hospital (Boston, MA).

Normal in-plane respiratory motion of the bilateral hemidiaphragms evaluated by sequentially subtracted fast magnetic resonance images.

The purpose of this study was to demonstrate hemidiaphragmatic motion using magnetic resonance imaging. The subjects were 13 healthy, nonsmoking volunteers. The sequence consisted of a fast gradient-recalled echo for coronal images and a single-shot fast spin echo with half Fourier transformation for sagittal images. The coronal and sagittal acquisition times were 0.83 msec and 0.58 msec per image, respectively. Thirty sequential images were obtained during quiet, deep respiration. Two series of subtraction images were made from these original sequential images (a given image was subtracted from the preceding or the next image in the original series). Hemidiaphragmatic motion was visualized as white or black bands on subtraction images. In the coronal plane through the left ventricle, there was a difference in motion between the right and left hemidiaphragms in 10 subjects during tidal respiration and in 12 subjects during deep respiration. On the sagittal images, the dorsal part of the hemidiaphragm moved more rapidly during quiet, deep respiration, especially on the left side.

Design, clinical suitability and future aspects of a 0.5 T MRI special system for interventional use

To develop a superconducting magnetic resonance (MR) imager that provides direct access to the patient and permits interactive MR-guided interventional procedures. A 0.5-T superconducting magnet that allows a region of vertical access to the patient was designed and constructed. This magnet was integrated with newly designed shielded gradient coils, flexible surface coils, nonmagnetic displays, position-monitoring probes and device-tracking instrumentation. The magnet homogeneity was 12.3 ppm or better, and the gradient field was linear to within 1% over an imaging region of 30 cm in diameter. The signal-to-noise ratio was 10% higher than in a comparable 0.5-T superconducting imager. Images were obtained in several anatomic regions with use of routine pulse sequences. Interactive image plane selection and near real-time imaging, with use of fast gradient-recalled echo sequences were demonstrated at a rate of one image every 1.5 s. More than 240 procedures (including open brain surgery) have been successfully performed within this environment. MR-guided interventional and surgical procedures can be performed with full patient access using an open-configuration, superconducting MR magnet with near real-time imaging and interactive image plane control.

Contrast-modified gradient echo imaging using rotary echo preparatory pulses.

The use of on-resonance 121 binomial composite pulses in two- or three-dimensional magnetization-prepared gradient-recalled echo magnetic resonance imaging experiments generates rotary echoes, leading to an increase in contrast range that is, in part, determined by the ratio of T2 to T1. In comparison with other fast gradient-recalled echo imaging techniques designed for enhanced T2 contrast, this method is more robust with respect to radiofrequency field inhomogeneity and less sensitive with respect to motion artifacts. Three-dimensional parametric images may be calculated using least-squares fitting based on a simple model for steady-state longitudinal magnetization during the imaging sequences.