Semi-automated vectorial analysis of anorectal motion by magnetic resonance defecography in healthy subjects and fecal incontinence.

BACKGROUND: Inter-observer variability limits the reproducibility of pelvic floor motion measured by magnetic resonance imaging (MRI). Our aim was to develop a semi-automated program measuring pelvic floor motion in a reproducible and refined manner. METHODS: Pelvic floor anatomy and motion during voluntary contraction (squeeze) and rectal evacuation were assessed by MRI in 64 women with fecal incontinence (FI) and 64 age-matched controls. A radiologist measured anorectal angles and anorectal junction motion. A semi-automated program did the same and also dissected anorectal motion into perpendicular vectors representing the puborectalis and other pelvic floor muscles, assessed the pubococcygeal angle, and evaluated pelvic rotation. KEY RESULTS: Manual and semi-automated measurements of anorectal junction motion (r = 0.70; P < 0.0001) during squeeze and evacuation were correlated, as were anorectal angles at rest, squeeze, and evacuation; angle change during squeeze or evacuation was less so. Semi-automated measurements of anorectal and pelvic bony motion were also reproducible within subjects. During squeeze, puborectalis injury was associated (P ≤ 0.01) with smaller puborectalis but not pelvic floor motion vectors, reflecting impaired puborectalis function. The pubococcygeal angle, reflecting posterior pelvic floor motion, was smaller during squeeze and larger during evacuation. However, pubococcygeal angles and pelvic rotation during squeeze and evacuation did not differ significantly between FI and controls. CONCLUSION & INFERENCES: This semi-automated program provides a reproducible, efficient, and refined analysis of pelvic floor motion by MRI. Puborectalis injury is independently associated with impaired motion of puborectalis, not other pelvic floor muscles in controls and women with FI.

MO-D-213CD-01: Cartesian Methods for Rapid Time-Resolved MR Angiography.

The physics of the MR image formation fundamentally trades off spatial resolution with temporal resolution. Time spent in acquiring data for the second image of a time series can alternatively be spent in sampling higher spatial frequencies for the first image to improve its spatial resolution. Historically this tradeoff has been addressed by making the k-space sampling rate high, such as with very short repetition times, and with methods such as view sharing in which only a portion of k-space is updated from one image to the next in a time series. Over a decade ago the method of parallel acquisition was proposed in which the signals detected by the individual elements comprising a multi-element receiver coil are used to provide further spatial discrimination and reduce acquisition time. These approaches include those based in image space (SENSE) or in k- space (SMASH, GRAPPA). In the last decade these methods have been integrated in contrast-enhanced MRA (CE-MRA) to provide a radical improvement in performance. CE-MRA is an application particularly well suited to these methods. The general desire for MRA images to be three-dimensional allows the use of 2D implementation of parallel acquisition, generally much more robust than 1D implementation. Also, the SNR loss associated with parallel acquisition is tempered in CE-MRA because high, arterial-phase signal is sampled throughout the data acquisition. Cartesian MR data acquisition, performed along a rectilinear sampling pattern in k-space, offers specific advantages in relative ease of implementation of 2D parallel acquisition and in "freezing" the status of the time-varying object at a specific timepoint by use of centric view ordering. This presentation will provide a review of these methods and how they have been effectively developed and integrated within the last decade for improved time-resolved MRA. Cartesian k-space sampling patterns can now be quickly selected on a patient- and anatomy-specific basis for optimum acceleration. Receiver coil arrays have been adapted to allow up to 20× reduction in the number of k-space points sampled for a given spatial resolution. Reconstruction hardware now allows generation of 3D images within only hundreds of msec after data acquisition, permitting real-time generation of diagnostic quality images and their use in interactively guiding other processes. LEARNING OBJECTIVES: 1. Understand recently developed physics techniques which have allowed a 20x improvement in the speed of data acquisition for MR angiography 2. Understand how Cartesian sampling of k-space facilitates the practical and effective implementation of these techniques 3. Show how contemporary implementation of these physics techniques has provided a significant improvement in MRA image quality over the last decade.

Gastric motor disturbances in patients with idiopathic rapid gastric emptying.

BACKGROUND: The mechanisms of 'idiopathic' rapid gastric emptying, which are associated with functional dyspepsia and functional diarrhea, are not understood. Our hypotheses were that increased gastric motility and reduced postprandial gastric accommodation contribute to rapid gastric emptying. METHODS: Fasting and postprandial (300kcal nutrient meal) gastric volumes were measured by magnetic resonance imaging (MRI) in 20 healthy people and 17 with functional dyspepsia; seven had normal and 10 had rapid gastric emptying. In 17 healthy people and patients, contractility was analyzed by spectral analysis of a time-series of gastric cross-sectional areas. Logistic regression models analyzed whether contractile parameters, fasting volume, and postprandial volume change could discriminate between health and patients with normal or rapid gastric emptying. KEY RESULTS: While upper gastrointestinal symptoms were comparable, patients with rapid emptying had a higher (P=0.002) body mass index than normal gastric emptying. MRI visualized propagating contractions at ∼3cpm in healthy people and patients. Compared with controls (0.32±0.04, Mean±SEM), the amplitude of gastric contractions in the entire stomach was higher (OR 4.1, 95% CI 1.2-14.0) in patients with rapid (0.48±0.06), but not normal gastric emptying (0.20±0.06). Similar differences were observed in the distal stomach. However, the propagation velocity, fasting gastric volume, and the postprandial volume change were not significantly different between patients and controls. CONCLUSIONS & INFERENCES: MRI provides a non-invasive and refined assessment of gastric volumes and contractility in humans. Increased gastric contractility may contribute to rapid gastric emptying in functional dyspepsia.

Application of magnetic resonance imaging to measure fasting and postprandial volumes in humans.

Our aims were to measure the gastric volume response in excess of ingested meal volume (i.e. gastric accommodation), contribution of swallowed air to this excess, day-to-day variability of gastric volumes measured by MRI and their relationship to volumes measured by single-photon-emission computed tomography (SPECT). In 20 healthy volunteers, fasting and postprandial gastric volumes were measured after technetium(99m)-pertechnetate labeling of the gastric mucosa by SPECT and separately by MRI, using 3D gradient echo and 2D half-Fourier acquisition single-shot turbo spin echo (HASTE) sequences. Ten of these subjects had a second MRI exam to assess intra-individual variation. Thereafter, another 10 subjects had two MRI studies during which they ingested the nutrient in 30 or 150 mL aliquots. During MRI, the postprandial gastric volume change exceeded the ingested meal volume by 106 +/- 12 mL (Mean +/- SEM). The HASTE and gradient echo sequences distinguished air from fluid under fasting and postprandial conditions respectively. This postprandial excess mainly comprised air (61 +/- 5 mL), which was not significantly different when ingested as 30 or 150 mL aliquots. Fasting and postprandial gastric volumes measured by MRI were generally reproducible within subjects. During SPECT, postprandial volumes increased by 158 +/- 18 mL; gastric volumes measured by SPECT were higher than MRI. MRI measures gastric volumes with acceptable performance characteristics; the postprandial excess primarily consists of air, which is not affected by the mode of ingestion. Gastric volumes are technique specific and differ between MRI and SPECT.

Feathering: Vertebral artery pseudostenosis with elliptical centric contrast-enhanced MR angiography.

Elliptical centric contrast-enhanced MR angiography of the cervical vasculature is a well-established technique that in many practices has replaced conventional angiography for several clinical indications, including atherosclerotic disease and dissections. Occasionally blurring or loss of signal intensity occurs in the vertebral arteries, especially in young patients with rapid circulation times. This ringing artifact, which we termed "feathering," results from rapidly changing signal intensity in small vascular structures during the sampling of the center of k-space.

Relationship between symptoms and disordered continence mechanisms in women with idiopathic faecal incontinence.

BACKGROUND AND AIMS: Anal sphincter weakness and rectal sensory disturbances contribute to faecal incontinence (FI). Our aims were to investigate the relationship between symptoms, risk factors, and disordered anorectal and pelvic floor functions in FI. METHODS: In 52 women with "idiopathic" FI and 21 age matched asymptomatic women, we assessed symptoms by standardised questionnaire, anal pressures by manometry, anal sphincter appearance by endoanal ultrasound and magnetic resonance imaging (MRI), pelvic floor motion by dynamic MRI, and rectal compliance and sensation by a barostat. RESULTS: The prevalence of anal sphincter injury (by imaging), reduced anal resting pressure (35% of FI), and reduced squeeze pressures (73% of FI) was higher in FI compared with controls. Puborectalis atrophy (by MRI) was associated (p<0.05) with FI and with impaired anorectal motion during pelvic floor contraction. Volume and pressure thresholds for the desire to defecate were lower, indicating rectal hypersensitivity, in FI. The rectal volume at maximum tolerated pressure (that is, rectal capacity) was reduced in 25% of FI; this volume was associated with the symptom of urge FI (p<0.01) and rectal hypersensitivity (p = 0.02). A combination of predictors (age, body mass index, symptoms, obstetric history, and anal sphincter appearance) explained a substantial proportion of the interindividual variation in anal squeeze pressure (45%) and rectal capacity (35%). CONCLUSIONS: Idiopathic FI in women is a multifactorial disorder resulting from one or more of the following: a disordered pelvic barrier (anal sphincters and puborectalis), or rectal capacity or sensation.

Study of CP(N-1) theta-vacua by cluster simulation of SU(N) quantum spin ladders.

D-theory provides an alternative lattice regularization of the 2D CP(N-1) quantum field theory in which continuous classical fields emerge from the dimensional reduction of discrete SU(N) quantum spins. Spin ladders consisting of n transversely coupled spin chains lead to a CP(N-1) model with a vacuum angle theta=npi. In D-theory no sign problem arises and an efficient cluster algorithm is used to investigate theta-vacuum effects. At theta=pi there is a first order phase transition with spontaneous breaking of charge conjugation symmetry for CP(N-1) models with N>2.

Magnetic resonance imaging of anatomic and dynamic defects of the pelvic floor in defecatory disorders.

OBJECTIVE: Endoanal ultrasound identifies anal sphincter anatomy, and evacuation proctography visualizes pelvic floor motion during simulated defecation. These complementary techniques can evaluate obstructed defecation and fecal incontinence. Our aim was to develop a single, nonionizing, minimally invasive modality to image global pelvic floor anatomy and motion. METHODS: We studied six patients with fecal incontinence and seven patients with obstructed defecation. The anal sphincters were imaged with an endoanal magnetic resonance imaging (MRI) coil and endoanal ultrasound (five patients). MR fluoroscopy acquired images every 1.4-2 s, using a modified real-time, T2-weighted, single-shot, fast-spin echo sequence, recording motion as patients squeezed pelvic floor muscles and expelled ultrasound gel; no contrast was added to other pelvic organs. Six patients also had scintigraphic defecography. RESULTS: Endoanal ultrasound and MRI were comparable for imaging defects of the internal and external sphincters. Only MRI revealed puborectalis and/or external sphincter atrophy; four of these patients had fecal incontinence. MR fluoroscopy recorded pelvic floor contraction during squeeze and recorded relaxation during simulated defecation. Corresponding comparisons for angle change and perineal descent during defecation were not significant; only MRI, but not scintigraphy, identified excessive perineal descent in two patients. CONCLUSIONS: Pelvic MRI is a promising single, comprehensive, nonradioactive modality to measure structural and functional pelvic floor disturbances in defecatory disorders. This method may provide insights into mechanisms of normal and disordered pelvic floor function in health and disease.

Embedded MR fluoroscopy: high temporal resolution real-time imaging during high spatial resolution 3D MRA acquisition.

A method termed "embedded fluoroscopy" for simultaneously acquiring a real-time sequence of 2D images during acquisition of a 3D image is presented. The 2D images are formed by periodically sampling the central phase encodes of the slab-select direction during the 3D acquisition. The tradeoffs in spatial and temporal resolution are quantified by two parameters: the "redundancy" (R), the fraction of the 3D acquisition sampled more than once; and the "effective temporal resolution" (T), the time between temporal updates of the central views. The method is applied to contrast-enhanced MR angiography (CE-MRA). The contrast bolus dynamics are portrayed in real time in the 2D image sequence while a high-resolution 3D image is being acquired. The capability of the 2D acquisition to measure contrast enhancement with only a 5% degradation of the spatial resolution of the 3D CE-MR angiogram is shown theoretically. The method is tested clinically in 15 CE-MRA patient studies of the carotid and renal arteries.

Contrast-enhanced magnetic resonance angiography of the cervical vessels: experience with 422 patients.

BACKGROUND AND PURPOSE: Contrast-enhanced magnetic resonance angiography (CEMRA) permits acquisition of high-spatial-resolution, venous-suppressed, 3D MR angiograms of the cervical carotid and vertebral arteries. In this study, an elliptic centric-view ordering with either MR fluoroscopic triggering or test bolus timing was used. The use of CEMRA of the cervical vessels has changed our clinical practice and is replacing conventional angiography for the evaluation of most carotid and vertebral artery diseases. METHODS: We retrospectively reviewed our experience with the use of CEMRA performed in 422 patients from January through December 1999. RESULTS: CEMRA was performed to evaluate transient ischemic attack and ischemic stroke in 239 patients, asymptomatic carotid bruit in 88 patients, and other neurological symptoms in 95 patients. Carotid endarterectomies were performed in 97 patients (103 procedures), and conventional angiography was performed in 12 of these patients. CEMRA was used to evaluate for the presence of an arterial dissection in 85 of the 239 patients with transient ischemic attack and ischemic stroke. Of this group, 32 patients had cervical arterial dissection, and pseudoaneurysm was detected in 11 of these patients. Compared with ultrasonography of the cervical vessels, CEMRA provided additional information in 43 of 422 patients and led to changes in the decision as to whether to perform carotid endarterectomy in 5 patients. CONCLUSIONS: Use of CEMRA permits noninvasive evaluation of patients suspected of having carotid or vertebral disease and avoids the potential complications of conventional angiography.

Effect of windowing and zero-filled reconstruction of MRI data on spatial resolution and acquisition strategy.

Standard, MR spin-warp sampling strategies acquire data on a rectangular k-space grid. That method samples data from the "corners" of k-space, i.e., data that lie in a region of k-space outside of an ellipse just inscribed in the rectangular boundary. Illustrative calculations demonstrate that the data in the corners of k-space contribute to the useful resolution only if an interpolation method such as a zero-filled reconstruction is used. The consequences of this finding on data acquisition and data windowing strategies are discussed. A further implication of this result is that the spatial resolution of images reconstructed with zero-filling (but without radial windowing) is expected to display angular dependence, even when the phase- and frequency-encoded resolutions are identical. This hypothesis is experimentally verified with a slit geometry phantom. It is also observed that images reconstructed without zero-filling do not display the angular dependence of spatial resolution predicted solely by the maximal k-space extent of the raw data. The implications of these results for 3D contrast-enhanced angiographic acquisitions with elliptical centric view ordering are explored with simulations.

Dependence of venous enhancement on the field of view in 3D contrast-enhanced MRA using the elliptical centric view order.

The dependence of venous suppression on the acquisition field of view (FOV) in elliptical centric 3D contrast-enhanced magnetic resonance angiography (CE-MRA) is studied theoretically and experimentally. It is hypothesized that a reduced FOV in an arterial phase acquisition results in improved venous suppression. An expression is derived linking the k-space representation of a vein to venous return time and acquisition parameters. For a y x z FOV reduction from 24 cm x 7.2 cm to 18 cm x 3.6 cm, equivalent voxel size, and venous return times ranging from 0-7 s, the mean improvement in venous suppression ranged from 7.0% for the 19-mm-diameter vein to 32.1% in the 6-mm-diameter vein, assuming a step function-shaped venous enhancement profile. Decreased venous enhancement with reduced FOV is also observed for scans with equivalent acquisition times, although the degree of suppression is dependent on the shape of the venous enhancement curve.

Steady-state preparation for spoiled gradient echo imaging.

While spoiled gradient echo sequences provide a rapid means of acquiring T(1)-weighted images, it is often desirable that the magnetization be in the steady state to avoid artifacts. For some applications, this requires many "dummy" repetitions of the pulse sequence prior to data collection, delaying image acquisition. A method is presented in which a saturation pulse, followed by a prescribed recovery period, places longitudinal magnetization levels of all materials near steady state, ready for data acquisition much sooner than when employing only dummy repetitions to achieve steady state. Effects of transverse coherences are studied using configuration theory. The method is shown to be effective in both phantom studies and in vivo applications, including real-time imaging, multiphase cardiac imaging, and triggered contrast-enhanced angiography. Magn Reson Med 45:653-661, 2001.

High-spatial-resolution contrast-enhanced MR angiography of the renal arteries: a prospective comparison with digital subtraction angiography.

PURPOSE: To evaluate a high-spatial-resolution three-dimensional (3D) contrast material-enhanced magnetic resonance (MR) angiographic technique for detecting proximal and distal renal arterial stenosis. MATERIALS AND METHODS: Twenty-five patients underwent high-spatial-resolution small-field-of-view (FOV) 3D contrast-enhanced MR angiography of the renal arteries, which was followed several minutes later by more standard, large-FOV 3D contrast-enhanced MR angiography that included the distal aorta and iliac arteries. For both acquisitions, MR fluoroscopic triggering and an elliptic centric view order were used. Two readers evaluated the MR angiograms for grade and hemodynamic significance of renal arterial stenosis, diagnostic quality, and presence of artifacts. MR imaging results for each patient were compared with those of digital subtraction angiograms. RESULTS: The high-spatial-resolution small-FOV technique provided high sensitivity (97%) and specificity (92%) for the detection of renal arterial stenosis, including all four distal stenoses encountered. The portrayal of the segmental renal arteries was adequate for diagnosis in 19 (76%) of 25 patients. In 12% of the patients, impaired depiction of the segmental arteries was linked to motion. CONCLUSION: The combined high-spatial-resolution small-FOV and large-FOV MR angiographic examination provides improved spatial resolution in the region of the renal arteries while maintaining coverage of the abdominal aorta and iliac arteries.

Autocorrection of three-dimensional time-of-flight MR angiography of the Circle of Willis.

OBJECTIVE: The purpose of this study was to investigate the efficacy of a retrospective adaptive motion correction technique known as autocorrection for reducing motion-induced artifacts in high-resolution three-dimensional time-of-flight MR angiography of the circle of Willis. MATERIALS AND METHODS: Ten consecutive volunteers were imaged with an unenhanced gradient-recalled echo three-dimensional time-of-flight MR angiography sequence of the circle of Willis. Each volunteer was asked to rotate approximately 2 degrees after completion of one third and one half of the acquisition in the axial, sagittal, and oblique planes (45 degrees to the axial and sagittal planes). A single static data set was also acquired for each volunteer. Unprocessed and autocorrected maximum-intensity-projection images were reviewed as blinded image pairs by six radiologists and were compared on a five-point image quality scale. RESULTS: Mean improvement in image quality after autocorrection was 1.4 (p < 0.0001), 1.1 (p < 0.0001), and 0.2 (p = 0.003) observer points (maximum value, 2.0), respectively, for examinations corrupted by motion in the axial, oblique, and sagittal planes. All three axes had statistically significant improvement in image quality compared with the uncorrected images. Changes in image quality after the application of the autocorrection algorithm to static angiogram data were not statistically significant (mean change in score = -0.13 points; p = 0.29). CONCLUSION: Autocorrection can reduce artifacts in motion-corrupted MR angiography of the circle of Willis without distorting motion-free examinations.

Carotid artery: elliptic centric contrast-enhanced MR angiography compared with conventional angiography.

PURPOSE: To determine the accuracy of elliptic centric contrast material-enhanced magnetic resonance (MR) angiography by using conventional angiography as the reference standard. MATERIALS AND METHODS: Fifty patients were examined prospectively with contrast-enhanced MR angiography and conventional angiography. The two examinations were performed within 1 week of each other. Two patients underwent conventional angiography of only one carotid artery, which yielded 98 arteries for comparison. RESULTS: With conventional angiography as the reference standard and by using a 70% threshold for internal carotid arterial diameter stenosis, maximum intensity projection (MIP) images had a sensitivity of 93.3%, specificity of 85.1%, and accuracy of 87.6%, whereas reformatted transverse source images had a sensitivity of 83.3%, specificity of 97.0%, and accuracy of 92.8%. Interobserver variability for conventional angiograms was 0.97, for MIP images was 0.91, and for source images was 0.90. The contrast-enhanced MR angiographic technique had a sensitivity of 88.9% and specificity of 58.1% for the presence of irregularity and/or ulceration. All 50 examinations were triggered appropriately so that minimal or no venous signal intensity was depicted. CONCLUSION: Contrast-enhanced elliptic centric three-dimensional MR angiography offers high-spatial-resolution, venous-suppressed images of the carotid arteries that appear to be adequate to replace conventional angiography in most patients examined prior to carotid endarterectomy.

Assessment of thermal tissue ablation with MR elastography.

An important part of thermal ablation therapy is the assessment of the spatial extent of tissue coagulation. In this work, the mechanical properties of thermally-ablated tissue were quantitatively evaluated using magnetic resonance elastography (MRE). This study shows that the mechanical properties of focused ultrasound ablated tissue are significantly different from normal tissue, and the difference can be imaged and measured using MRE. Repeated experiments revealed a reproducible pattern of tissue mechanical property change during thermal ablation in ex vivo bovine muscle. This pattern may reflect changes in intrinsic tissue structure and could be used to evaluate tissue coagulation during thermal ablation therapy. Magn Reson Med 45:80-87, 2001.

Simultaneous image acquisition utilizing hybrid body and phased array receiver coils.

In clinical MR imaging the design and selection of receiver coil is an important step in ensuring the highest image quality. Often this choice is based on selecting a receiver coil characterized by high spatial uniformity such as the body and head volume receiver coils or a surface coil (or array of coils) that provide high signal-to-noise ratio (SNR). In the past, it has been difficult to accomplish both high SNR and spatial uniformity as both coil types achieve one of these characteristics at the expense of the other. The purpose of this study was to achieve both high SNR and spatial uniformity through the simultaneous acquisition of the MR signal using the body and a surface coil array. Results indicate that this hybrid system can provide uniformity and SNR values comparable to those achieved by the body and surface coil arrays, respectively.