Pulsatile flow artifacts in fast magnetization-prepared sequences

Fast magnetization-prepared magnetic resonance imaging sequences allow clinical acquisitions in about 1 second, with the preparation phase providing the desired contrast. Pulsatile flow artifacts, although reduced by rapid acquisition, can degrade image quality. The authors explore the causes of aortic pulsatile flow artifacts in inversion-recovery-prepared acquisitions of the abdomen, taking into consideration various parameters. The flow signal within an 8-mm-thick section was simulated and subsequently Fourier transformed to determine the location and extent of flow artifacts. Results of simulations were validated with abdominal images of human subjects. Recording all encodings within one cardiac cycle reduced pulsatile flow artifacts in nonsegmented acquisitions with sequential phase-encoding order, regardless of the location of magnetization preparation within the cardiac cycle. In segmented acquisitions, however, the sequential order always increased flow artifacts. To reduce the artifacts in short TI acquisitions, the magnetization should be prepared during diastole. In clinical acquisitions, flow artifacts were further reduced by modifying the phase-encoding scheme.     

Improved T1-weighted two-dimensional MP-GRE imaging of the liver with variable flip angles for shaping the signal evolution

The recently introduced method of shaping the transient signal evolution in magnetization-prepared gradient-echo (MP-GRE) imaging with variable flip angles has been applied to two-dimensional (2D) MP-GRE imaging of the abdomen. The technique was analyzed by using theoretical models and was implemented on a standard 1.5-T whole-body imager with a segmented acquisition. Theoretical models predicted that the variable-flip-angle 2D MP-GRE sequence would increase liver-spleen signal difference–to-noise ratios by 290%, 110%, and 160% compared with a 2D MP-GRE sequence with a flip angle of 10° and sequential phase encoding, a 2D MP-GRE sequence with a flip angle of 30° and centric phase encoding, and the fast low-angle shot sequence, respectively. Experimental measurements supported the theoretical predictions.     

Tracking motion with tagged rapid gradient-echo magnetization-prepared MR imaging.

A technique for rapid assessment of tissue motion was developed by combining spatially selective radio-frequency tagging pulses with centric phase-encoding view ordering in a T1-weighted, magnetization-prepared gradient-echo acquisition sequence. This sequence allowed labeling and tracking of tissue motion with single-image acquisition times of less than 3 seconds. Multiple tagged bands 3mm thick were superimposed orthogonal to the imaging plane. Motion in the interval between tagging and the start of image acquisition could then be precisely determined. The technique was evaluated with phantom studies and then applied to human volunteers for assessment of skeletal muscle motion, phonation, and pelvic floor motion.     

Single-shot GRASE imaging with short effective TEs

A new phase-encoding scheme for gradient- and spin-echo (GRASE) imaging giving a short effective TE is described. Unlike previous orders, phase encoding is centric rather than sequential. The sequence is a development of k-banded GRASE that uses different time segments of the echo train to encode different bands of k space. This phase-encoding order has been implemented in single-shot sequences on an imager with high performance gradients. Approximately 144 phase-encoding lines can be acquired in an echo train time of 390 ms. With centric phase encoding, the effective TE is 8 ms, compared with 75 ms for sequential encoding, and signal-to-noise ratios (SNRs) in brain tissue are 50 to 70% higher. The sequence can be employed in, for example, diffusion and velocity imaging.     

GRASE imaging at 3 Tesla with template interactive phase–encoding

A new method for ordering the phase-encoding gradient is proposed, and an application for short effective TE gradient-and spin-echo (GRASE) imaging is demonstrated. The proposed method calculates the phase-encoding order from the signal decay of a template scan (hence “template interactive phase-encoding” or TIPE). Computer simulations are used to compare the point spread functions of different phase-encoding orders giving short effective echo times (kb centric GRASE, centric GRASE, centric TIPE). The conventional centric phase-encoding order is also considered for GRASE. The conventional centric method is sensitive to both amplitude and phase modulation of the signal in K-space. The centric TIPE method gives the least amplitude modulation artifacts but is vulnerable to phase artifacts. The TIPE experiment was implemented on a 3 Tesla system. To the best of our knowledge, we present the first in vivo GRASE images at this field strength.     

T1-weighted snapshot gradient-echo MR imaging of the abdomen

Magnetization-prepared ultrashort-repetition-time (snapshot) gradient-echo imaging is a technique of magnetic resonance (MR) imaging with many potential applications. In the application of this technique to abdominal imaging, the effects on contrast of phase-encoding order, resolution, preparation-phase inversion time, and data-acquisition flip angle were predicted and then demonstrated with images obtained in examinations of 22 patients. In the analysis of 36 liver lesions, snapshot images were compared with corresponding T1-weighted spin-echo images on the basis of signal-to-noise ratio (S/N) of liver and contrast-to-noise ratio (C/N) between liver and lesion. Snapshot MR imaging produced abdominal images with 192 (or 256) x 256 resolution, negligible motion artifact, and C/N 1.29 times (+/- 0.48) higher than that in T1-weighted spin-echo imaging. Acquisition times were 13 seconds or less, short enough for imaging during suspended respiration. Also, use of a phased-array multicoil further improves the S/N in snapshot images without acquisition-time penalty.     

Clinical comparison of three-dimensional MP-RAGE and FLASH techniques for MR imaging of the head.

Three-dimensional (3D) MP-RAGE (magnetization-prepared rapid gradient-echo) imaging was evaluated as a high-resolution 3D T1-weighted brain imaging technique for patients with suspected neurologic disease. Fourteen patients were studied. In five, 3D MP-RAGE images were compared with 3D FLASH (fast low-angle shot) images. Signal difference--to-noise ratios and T1 contrast were not statistically different for 3D MP-RAGE images as opposed to 3D FLASH images. Advantages intrinsic to the application of 3D MP-RAGE sequences include decreased imaging time and decreased motion artifact. With this technique, it is possible to perform a relatively motion-insensitive, T1-weighted screening brain study with voxel resolution of 1.0 x 1.4 x 2.0 mm or smaller, in an imaging time of 5.9 minutes or less--permitting offline (poststudy) reconstruction of high-resolution images in any desired plane.     

Effects of view ordering and dummy pulse rate on two-dimensional and three-dimensional steady-state free precession imaging

RATIONALE AND OBJECTIVE: To assess the effects of view ordering and dummy pulse rate on the image quality of steady-state free precession (SSFP) imaging. MATERIAL AND METHODS: Two- and three-dimensional (2D and 3D) SSFP imaging with various view orderings (eg, centric, sequential, reverse centric) and dummy pulse rates were performed in the phantom including several concentrations of gadolinium and patients. RESULTS: Centric view orderings increased the noise in 3D SSFP images in the phantom. In the patients studied, the centric view ordering increased the signals obtained from abdominal organs in 2D and 3D SSFP and the background signal in 3D images, without changing the vascular signals. An increased dummy pulse rate reduced the background signal in 2D SSFP, but not in 3D SSFP. CONCLUSION: Sequential ordering is recommended for both 2D and 3D SSFP, and centric view ordering combined with an increase in dummy pulse rate can be used for 2D imaging.     

Magnetization-prepared MR angiography with fat suppression and venous saturation.

Magnetization-prepared magnetic resonance (MR) angiography (MPMRA) is an inflow-based two-dimensional (2D) imaging sequence in which a preparation phase precedes rapid image acquisition. For maximal blood/tissue contrast, an inversion-recovery preparation nulls signal from static tissue. If needed, a second inversion suppresses signal from fat. Fully magnetized blood flows in after the inversion pulse(s), providing high signal intensity. The centric phase-encoding order, which ensures that the initial contrast is reflected in the image set, requires the use of a modified venous saturation technique. The sequence is described and its performance assessed with regard to (a) depiction of in-plane flow, (b) fat suppression, and (c) venous saturation. Phantom and volunteer studies showed good performance in all three areas. MPMRA images, acquired in just 2-4 seconds per image, had a blood/tissue contrast-to-noise ratio nearly twice that of standard 2D time-of-flight MR angiograms, acquired in 5-7 seconds. The technique is promising for restless patients and in anatomic areas plagued by motion degradation.     

Multipoint Dixon technique for water and fat proton and susceptibility imaging.

Extensions to a previously described three-point Dixon magnetic resonance imaging technique are presented that use alternative water/fat phase-encoding strategies. The technique is generalized to phase encoding of (-theta, O, theta) or (O, theta, 2 theta) radians, and the signal-to-noise ratio (S/N) performance is evaluated. It was found that a theta of 2 pi/3 radians has optimal S/N but that a theta of pi radians is a good compromise and that phase encoding of (O, pi, 2 pi) radians offer an advantage over the previous method, which used (-pi, O, pi) increments, in that a T2' (intravoxel susceptibility dephasing) image may be obtained in addition to the usual water, fat, and Bo images. A new four-point method with phase encoding of (O, pi, 2 pi, 3 pi) radians that can also provide a measure of the spectral width of the fat resonance is suggested. The disadvantages of the method are the extra imaging time and low S/N efficiency.     

Duration of enhancement and scan timing in three-dimensional contrast-enhanced MR angiography using the elliptical centric phase-encoding technique

In the present study, we quantitatively investigated the relationship between the signal intensity in a vessel and the duration of contrast enhancement as well as scan timing in 3D contrast-enhanced MR angiography using an elliptical centric phase-encoding technique. A tube phantom filled with Gd-DTPA, acting as a vessel, was taken out from the field of view during data acquisition, by using the "pause" function of our MR scanner (GE Signa, 1.5 Tesla), thereby simulating the presence and absence of a vessel. The shortening of the duration of enhancement corresponds to the delay of scan timing from the optimal point in the phase-encoding of the centric-ordering system. The signal intensity in a vessel (1-5 mm in diameter) decreased as the duration of enhancement became shorter and the diameter of the vessel decreased. When the number of partitions was 16 or 32 in a 128-mm-thick slab, the signal intensity obtained by the elliptical centric phase-encoding technique was almost the same as that obtained by the conventional centric phase-encoding technique. However, when the number of partitions was increased (64-124), and if the duration of enhancement was short, the signal intensity obtained by the elliptical centric phase-encoding technique was higher than that obtained by the conventional centric phase-encoding technique. In conclusion, in terms of the duration of enhancement and the delay of scan timing, the elliptical centric phase-encoding technique is superior to the conventional centric phase-encoding technique when the number of partitions in a slab for 3D MR angiography is increased.     

3D magnetization prepared elliptical centric fast gradient echo imaging.

3D magnetization-prepared fast gradient echo MR sequences, such as MP-RAGE and IR-SPGR, provide good spatial resolution and gray-white contrast. The efficiency and image quality of these techniques can be further improved with an interleaved, recessed elliptical centric view order. It is shown that this novel acquisition strategy, along with skipping the acquisition of views in k-space corners can provide images with higher signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR), while reducing artifact level and scan time compared to standard MP-RAGE.     

Three-dimensional MR imaging of the coronary arteries: Preliminary clinical experience

A three-dimensional (3D) magnetization-prepared (MP) rapid gradient-echo (RAGE) and 3D RAGE technique was used to image the coronary arteries in healthy volunteers and patients with known disease. Each sequence produced images of volumes partitioned into 16 thin sections with differing blood-fat-myocardium contrast. The two types of images were subtracted to null fat signal, thus producing a third image set that showed flowing blood. Total imaging time was about 17 minutes. In the volunteers, the 3D MP-RAGE and subtraction images consistently showed the morphology of the right coronary artery. The left main and left anterior descending arteries were also well seen. The circumflex artery was less consistently identified. Of the 17 diseased coronary artery segments identified at catheterization, 16 had altered signal intensity (narrowing, occlusion, reduced contrast-to-noise ratio, irregularity) on the subtraction images, while 13 had altered signal intensity on the 3D MP-RAGE images. The results indicate that this 3D MP-RAGE and 3D RAGE technique has potential utility as a screening method for coronary heart disease.     

Selective versus nonselective preparation pulses in two-dimensional MP-RAGE imaging of the liver.

The use of a section-selective preparation pulse in two-dimensional (2D) T1-weighted magnetization-prepared rapid gradient-echo (MP-RAGE) imaging of the liver was investigated. The images were compared with those obtained with a nonselective pulse. The performances of the sequences were evaluated in 11 patients with 12 focal liver lesions, and lesion-liver and lesion-vessel signal difference-to-noise ratios (SD/Ns) were calculated. With the section-selective preparation pulse, small lesions were better differentiated from vessels, and multiple, consecutive images could be obtained at shorter intervals. The mean lesion-liver SD/N was slightly but not significantly greater for images obtained with a selective pulse, while the lesion-vessel SD/N was significantly greater (P less than .01). It is concluded that a section-selective preparation pulse can improve the clinical utility of the 2D MP-RAGE sequence in the evaluation of focal liver disease.     

Quantitative evaluation of nonrepetitive phase-encoding orders for first-pass, 3D contrast-enhanced MR angiography.

In this work, a detailed quantitative comparison was made of many alternative phase-encoding strategies for first-pass 3D MR angiography where each phase encode is only sampled once during the transient passage of contrast agent. A series of standard sequential and centric phase-encoding orders including elliptical centric were tested, as well as a new order called elliptical sequential. The characteristics of the different phase-encoding orders were tested using a computer simulation followed by experimental verification using a variable flip angle scheme. The characteristics to be considered included: arterial intensity, arterial-to-venous contrast, degree of artifact, and the blurring of the point-spread function. By making use of a wide range of start times and a rapidly varying contrast curve, the quantitative results clearly indicate the widely varying merits of each phase-encoding order. In general, when an optimal start time is used techniques that sample the low k-space views most compactly will produce the best results; however, the same methods are more problematic when the bolus arrival time deviates substantially from that expected.     

Rapid three-dimensional T1-weighted MR imaging with the MP-RAGE sequence.

The authors investigated the application of three-dimensional (3D) magnetization-prepared rapid gradient-echo (MP-RAGE) imaging to the acquisition of small (32 x 128 x 256) T1-weighted 3D data sets with imaging times of approximately 1 minute. A theoretical model was used to study the contrast behavior of brain tissue. On the basis of these theoretical results, 3D MP-RAGE sequences were implemented on a 1.5-T whole-body imager. Thirty-two-section 3D data sets demonstrating good signal-to-noise ratios and resolution and strong T1-weighted contrast were obtained in 1 minute. Compared with standard short TR/TE spin-echo sequences with the same imaging times and comparable sequence parameters, the 3D MP-RAGE sequence delivered increases of more than 50% in the white matter/gray matter signal difference-to-noise and white matter signal-to-noise ratios, and provided almost twice as many sections. These sequences may find a clinical role in 3D scout imaging and screening and in patients with claustrophobia or trauma.     

Errors in quantitative dynamic three-dimensional keyhole MR imaging of the breast

Three-dimensional (3D) keyhole magnetic resonance (MR) imaging has been proposed as a means of providing dynamic monitoring of contrast agent uptake by breast lesions, with complete breast coverage and high spatial and temporal resolution. The 3D keyhole technique dynamically samples the central regions of k-space in both phase-encoding directions and provides high-frequency data from a precon-trast acquisition. Errors due to data truncation with two-dimensional and 3D region-of-lnterest measurements are estimated from a numerical simulation of various implementations of the 3D keyhole technique. Errors were found to increase with increasing temporal resolution and reduced object size. Errors of 75% are possible for objects with a diameter approaching 1 pixel when a 3D keyhole implementation that samples 50% of phase-encoding data in each direction is used. Preliminary clinical Images with this approach illustrate artifacts consistent with inadequate k-space sampling.     

Reducing oblique flow effects in interleaved EPI with a centric reordering technique.

Segmented interleaved echo planar imaging offers a fast and efficient approach to magnetic resonance angiography. Unfortunately, this technique is particularly sensitive to oblique flow in the imaging plane. In this work, a mathematical analysis of oblique flow effects for several types of k-space coverage is presented. The conventional linear acquisition scheme, an alternating centric and a nonalternating centric encoding scheme are compared with respect to their flow properties. It is shown both by simulations and imaging experiments that artifacts from oblique in-plane flow are effectively reduced by both centric reordered phase-encoding schemes. The nonalternating centric acquisition scheme is preferred to the alternating centric scheme due to the smoother phase error transition in k-space in the presence of obliquely-angled flow. Magn Reson Med 45:623-629, 2001.     

Improved efficiency in 2DFT magnetization-prepared rapid gradient echo imaging: application to abdominal imaging.

The incorporation of the phase offset multi planar (POMP) technique into breathheld magnetization-prepared gradient echo imaging is discussed as a means for improving imaging efficiency without sacrificing resolution, contrast, or SNR improvement. The phase encoding order necessary to preserve the centric approach is described. When combined with interleaving, the POMP technique enables four 256 x 256 images to be acquired in a 12-s breathhold, doubling the efficiency of the original technique. This scan efficiency is compared with that of other T1-weighted 2DFT methods.