Magnetic resonance imaging of pelvic floor relaxation: dynamic analysis and evaluation of patients before and after surgical repair.

OBJECTIVE: To evaluate structures involved in pelvic support using conventional and snapshot magnetic resonance imaging (MRI). METHODS: We used conventional spin-echo MRI and dynamic snapshot GRASS MRI at various levels of the Valsalva maneuver to describe and quantitate the anatomy of pelvic floor relaxation and to assess anatomical changes produced by surgical repair. Ten female volunteers were evaluated to define normal anatomy and reference measurements. Five women with pelvic floor relaxation were evaluated before and after surgical repair. RESULTS: Static and dynamic MRI were more sensitive than clinical pelvic examination in assessing and grading pelvic floor relaxation. Quantitative results showed widening of the levator hiatus and more vertical lie of the levator plate postoperatively. Descent of the pelvic organs on maximal straining postoperatively was the same as that in normal volunteers. The posterior urethrovesical angle on MRI was more than 110 degrees in 14 of 15 continent subjects. CONCLUSIONS: Magnetic resonance imaging may be valuable in analyzing and assessing pelvic floor relaxation and in understanding anatomical changes occurring before and after surgical repair. The increased sensitivity of MRI in grading prolapse may make it useful in evaluating women with symptoms of pelvic floor relaxation but who have negative findings on clinical examination.

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.

Altered phase-encoding order for reduced sensitivity to motion in three-dimensional MR imaging.

A method of reordering phase and slab encoding that can be used to address some of the inherent problems due to motion in three dimensional imaging is described and implemented. The method is shown to be more robust with respect to reducing artifacts resulting from several fundamental types of motion. It can be readily implemented on a standard magnetic resonance imager with essentially no increase in total imaging time. Results of simulations and phantom and in vivo experiments are presented.

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.

Centric phase-encoding order in three-dimensional MP-RAGE sequences: application to abdominal imaging.

Three-dimensional (3D) magnetization-prepared rapid gradient-echo imaging has been proposed as a method for improving signal-to-noise ratio (S/N) and contrast-to-noise ratio (C/N) in rapid abdominal imaging. Originally, a standard sequential phase-encoding order was proposed. In the present study, two approaches to a 3D centric phase-encoding order are presented: (a) application of the two-dimensional (2D) centric order to one of the 3D encoding directions, and (b) an interleaved square spiral order, which is the segmented 3D analog of the 2D centric order. With use of simulation, phantom, and volunteer results, the proposed 3D centric methods are compared in terms of S/N, C/N, and artifacts to the 3D sequential method and 2D magnetization-prepared methods. The second centric approach was found to be superior to the first; however, in general, the 3D technique was found to be inferior to the 2D technique for abdominal imaging because of motion artifact in the 3D image set caused by misregistration among the multiple breath holds required.

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.

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.

Real-time interactive color flow MR imaging.

Real-time interactive color flow magnetic resonance (MR) imaging is a combination of real-time MR imaging and color encoding of velocity-induced phase angle. Flow-compensated (FC) and flow-encoded (FE) images are acquired continuously by using gradient echoes and a 12-msec repetition time. Each image is reconstructed within 200 msec of acquisition, and the FC magnitude image is displayed in gray-scale format. The phase difference between the reconstructed FC and FE images, a difference proportional to velocity along the flow-encoding direction, is encoded in color and superimposed on the gray-scale FC image. Magnitude and phase information are thus presented simultaneously. The viewer may interactively adjust many acquisition parameters during data acquisition. Experimental results of phantom and in vivo human studies validate the method. Characteristics of the color flow MR imaging technique are compared with those of duplex color ultrasound.