Stiffness-weighted magnetic resonance imaging

An imaging method is introduced in which the signal in MR images is affected by the stiffness distribution in the object being imaged. Intravoxel phase dispersion (IVPD) that occurs during MR elastography (MRE) acquisitions decreases the signal in soft regions more than in stiff regions due to changes in shear wave amplitude and wavelength. The IVPD effect is enhanced by lowpass filtering the MR k-space data with a circular Gaussian lowpass filter. A processing method is introduced to take the time series of MRE magnitude images with IVPD and produce a final stiffness-weighted image (SWI) by calculating the minimum signal at each pixel from a small number of temporal samples. The SWI technique is demonstrated in phantom studies as well as in the case of a preserved postmortem breast tissue specimen with a stiff lesion created by focused ultrasound ablation to mimic a breast cancer. When free of significant sources of depth-dependent wave attenuation, interference, and boundary effects, SWI is a simple, fast, qualitative technique that does not require the use of phase unwrapping or inversion algorithms for localizing stiff regions in an object.

Fiducial markers for MR histological correlation in ex vivo or short-term in vivo animal experiments: a screening study

PURPOSE

To test injectable fiducial markers for magnetic resonance (MR) histological correlation in ex vivo or in vivo animal experiments.

MATERIALS AND METHODS

A total of 35 potential markers were tested ex vivo in pork muscle. The end-points were: 1) visibility, size, and shape on MR images and at macroscopic examination; 2) 24-hour stability; and 3) microscopic appearance. Selected markers were injected in vivo (rabbit's muscle and breast tumor tissue) to test their three-hour in vivo stability and their potential toxicity. Finally, different dilutions of the two best markers were assessed again through the same screening tests to determine whether their size on MR images could be customized by dilution.

RESULTS

Two fluid acrylic paints containing inorganic pigments were found to be potentially interesting markers. On MR images, they created well-defined susceptibility artifacts. The markers made with iridescent bronze paint (iron oxide coated mica particles) were readily visible on microscopy and their size on MR images could be customized by dilution. The iridescent stainless steel paint (iron, chromium, nickel) created ex vivo the smallest markers in tissue but needed colloidal iron staining to be visible on microscopy and could not be easily diluted.

CONCLUSION

Fluid acrylic paints are potentially interesting markers for MR histological correlation. Further studies are needed to assess their long-term properties.

MR histological correlation: a method for cutting specimens along the imaging plane in animal or ex vivo experiments

PURPOSE

To assess a method aimed at cutting histological specimens along the magnetic resonance (MR) imaging plane.

MATERIAL AND METHODS

The method is performed in two steps: the imaging plane (defined by three acrylic paint markers) is made horizontal under MR guidance by using a mobile platform that can be rotated in three directions (PlaneFinder device [PFD]); then, the specimen is embedded in wax and cut horizontally. Three-dimensional images parallel to the markers' plane were obtained on 31 pork muscles containing a central hole with a pyramidal shape, with a technique of reference (RT images) and with PFD (PF images), before and after fixation. The last 17 fixed specimens were cut in the markers' plane (tissue section [TS] images). The central hole area (CHA) in the markers' plane was used to compare RT, PF, and TS images. Using a workstation, PF images were rotated and translated to estimate the shift along each direction that could explain the entire CHA difference between RT, PF, and TS images (maximum error, worst-case scenario).

RESULTS

Excellent correlation was found between RT and PF images (r = 0.989, slope = 1.0175), PF and TS images (r = 0.991, slope = 1.0058), and RT images on fresh specimens and TS images (r = 0.979, slope = 1.0732). For each step, the maximum angle error was < or = 3 degrees in 88-95% of the specimens.

CONCLUSION

Our methodology can be used to cut specimens along the imaging plane with high accuracy.

Rapid MR elastography using selective excitations

Rapid MR elastography (MRE) techniques using spatially-selective excitations to reduce acquisition times to a few seconds or less were devised and tested. The techniques included reduced field of view (rFOV) MRE and 1D MRE (beam MRE) using 2D spatially selective RF excitations for gradient-echo (GRE) applications and intersecting 90 degrees and 180 degrees slice-selective excitations for spin-echo (SE) applications. It was shown that scan times could be reduced by a factor of 8 using rFOV MRE, and by 64 using beam MRE, while still obtaining stiffness estimates comparable to full-FOV MRE. Results were shown in gel phantom experiments as well as in the case of a preserved postmortem breast tissue specimen with a stiff lesion. These methods can be used to more rapidly interrogate regions of interest (ROIs) in tissue to quickly obtain information about the viscoelastic properties of that tissue.

MR angiography fusion technique for treatment planning of intracranial arteriovenous malformations

PURPOSE

To develop an image fusion technique using elliptical centric contrast-enhanced (CE) MR angiography (MRA) and three-dimensional (3D) time-of-flight (TOF) acquisitions for radiosurgery treatment planning of arteriovenous malformations (AVMs).

MATERIALS AND METHODS

CE and 3D-TOF MR angiograms with disparate in-plane fields of view (FOVs) were acquired, followed by k-space reformatting to provide equal voxel dimensions. Spatial domain addition was performed to provide a third, fused data volume. Spatial distortion was evaluated on an MRA phantom and provided slice-dependent and global distortion along the three physical dimensions of the MR scanner. In vivo validation was performed on 10 patients with intracranial AVMs prior to their conventional angiogram on the day of gamma knife radiosurgery.

RESULTS

Spatial distortion in the phantom within a volume of 14 x 14 x 3.2 cm(3) was less than +/-1 mm (+/-1 standard deviation (SD)) for CE and 3D-TOF data sets. Fused data volumes were successfully generated for all 10 patients.

CONCLUSION

Image fusion can be used to obtain high-resolution CE-MRA images of intracranial AVMs while keeping the fiducial markers needed for gamma knife radiosurgery planning. The spatial fidelity of these data is within the tolerance acceptable for daily quality control (QC) purposes and gamma knife treatment planning.

CT Fluoroscopy–guided Biopsy of the Lung or Upper Abdomen with a Breath-hold Monitoring and Feedback System: A Prospective Randomized Controlled Clinical Trial

PURPOSE

To prospectively determine the clinical effectiveness of a breath-hold monitoring and feedback system in computed tomographic (CT) fluoroscopy-guided biopsies in which respiratory motion is a problem.

MATERIALS AND METHODS

Institutional review board approval and oral and written informed consent were obtained. This study was HIPAA compliant. A bellows-based system was used to monitor respiration and provide patient feedback. A randomized controlled clinical trial compared intermittent mode CT fluoroscopy-guided biopsies of the lung or upper abdomen performed with (n = 56) and without (n = 57) the bellows system. Inclusion criteria for 113 patients were lesions 6 cm or smaller in maximum dimension that were not affixed to the chest or abdominal wall. Primary outcome measurements were CT fluoroscopy exposure time and patient dose. Wilcoxon rank sum, chi(2), and Fisher exact tests were used for statistical analysis.

RESULTS

Median CT fluoroscopy exposure time was 12.6 seconds (range, 2.4-44.4 seconds) for the bellows group and 18.0 seconds (range, 6.0-118.0 seconds) for the nonbellows group (P = .004). Patient dose was decreased in the bellows group (median dose, 29.5 mGy; range, 4.7-135.8 mGy) versus the nonbellows group (median, 41.3 mGy; range, 11.8-155.9 mGy) (P = .01). Lesions were accessed successfully with one needle puncture attempt in 43 of 56 patients (77%) in the bellows group and 30 of 57 patients (53%) in the nonbellows group (P = .007). Pneumothorax developed in 11 of 50 patients (22%) in the bellows group who underwent lung biopsy compared with 16 of 50 (32%) patients in the nonbellows group.

CONCLUSION

A breath-hold monitoring and feedback system allows depiction of mobile target lesions throughout CT fluoroscopy-guided biopsy of the lung and upper abdomen.

Safety evaluation of titanium middle ear prostheses at 3.0 tesla

OBJECTIVE

To assess the magnetic resonance imaging (MRI) safety of titanium middle ear prostheses at 3.0 tesla (T).

STUDY DESIGN AND SETTING

Titanium middle ear prostheses from 3 commercial vendors were examined for magnetic field interactions at 3 T. Initially, ex vivo studies were performed to test for rotational motion and forward displacement (translational motion) of the prostheses in a static magnetic field. If movement was observed during this screening study, then the prosthesis was tested to determine the translational or rotational force acting upon the prosthesis. In addition to testing for prosthesis displacement, temperature changes of the prostheses were measured to assess for radiofrequency heating during imaging.

RESULTS

Twenty-one of the 24 titanium prostheses tested revealed no movement when tested in the 3 T static magnetic field. Three prostheses revealed minimal movement during the screening study. A translational force test (string test) was performed upon these 3 prostheses, and the measured angle of displacement was used to determine the force. This calculated force acting upon each prosthesis was essentially zero. Therefore, we conclude that the magnetic field interaction is negligible. A positive control with a ferromagnetic stainless steel prosthesis demonstrated obvious displacement during the screening study, as well as deflection of the prosthesis by 90 degrees in the translational force test. Last, heating of the titanium prostheses did not occur in the 7 models tested.

CONCLUSIONS

Middle ear prostheses made from titanium are safe, neither deflecting nor heating during magnetic resonance examinations conducted at 3 T.

EBM RATING

C-1.

Motion-correction techniques for standing equine MRI

Magnetic resonance imaging (MRI) of the distal extremities of the standing, sedated horse would be desirable if diagnostic quality images could be obtained. With the availability of extremity and special purpose magnet designs on the market, a system to safely accommodate the standing horse may gain increasing popularity. This paper considers the issue of motion that will need to be addressed to achieve successful, diagnostic quality images. The motion of the carpus and tarsus of five standing, sedated horses was quantified. The obtained motion records were then used to induce motion in cadaveric joint specimens during several MRI scans. The measured dorsal-palmar/plantar, medial-lateral, and proximal-distal random wobbling motions in the standing sedated horse were several centimeters in magnitude and generated severe motion-artifacts during axial MRI of the cadaveric specimens. Two retrospective motion-correction techniques (autocorrection and navigator-based adaptive correction) were used to correct the corrupted images. The motion artifacts were nearly eliminated with the use of both techniques in series. Although significant hurdles remain, these results suggest promise for allowing diagnostic quality MRI of the carpus and tarsus in the standing horse.

Safety and MRI Artifact Evaluation at 1.5 T of Metallic Mounting Sheath of a Marking Clip Inadvertently Deployed at Stereotactic Biopsy

OBJECTIVE

The purpose of our study was to assess the MRI risk factors for the patient after an unusual case of inadvertent deployment of the mounting sheath of a metallic clip used at stereotactic breast biopsy.

MATERIALS AND METHODS

We evaluated the materials for ferromagnetic properties, heating, and artifacts.

RESULTS

Our analysis showed significant deflection and prominent susceptibility artifacts of the sheath at 1.5 T, although the clip itself showed no deflection and only minimal artifact. Our study shows that a device and its delivery apparatus may have significantly different ferromagnetic properties.

CONCLUSION

In case of inadvertent deployment of the sheath, a delay of 6-8 weeks before MRI is recommended as a conservative approach to ensure tissue ingrowth and to minimize the chance of harm to the patient. In the case of metallic clips used for breast biopsy, caution is warranted when a portion of the device unintended for placement is introduced into the breast.

Shear stiffness estimation using intravoxel phase dispersion in magnetic resonance elastography

Dynamic MR elastography (MRE) is a phase-contrast technique in which the periodic shear motion of an object is encoded as variations in the phase of the reconstructed images. An alternative MRE method is presented whereby waves are depicted as intensity variations in the magnitude images due to intravoxel phase dispersion (IVPD). A theoretical framework is developed to model how the IVPD magnitude data are related to the underlying shear wave motion, and how they can be used to estimate shear stiffness. The results are shown in a series of phantom experiments to demonstrate that IVPD MRE complements phase-contrast MRE.

Self-navigated motion correction using moments of spatial projections in radial MRI

Interest in radial MRI (also known as projection reconstruction (PR) MRI) has increased recently for uses such as fast scanning and undersampled acquisitions. Additionally, PR acquisitions offer intrinsic advantages over standard two-dimensional Fourier transform (2DFT) imaging with respect to motion of the imaged object. It is well known that aligning each spatial domain projection's center of mass (calculated using the 0th and 1st moments) to the center of the field of view (FOV) corrects shifts caused by in-plane translation. In this work, a previously unrealized ability to determine the in-plane rotational motion of an imaged object using the 2nd moments of the spatial domain projections in conjunction with a specific projection angle acquisition time order is reported. We performed the correction using only the PR data itself acquired with the newly proposed projection angle acquisition time order. With the proposed view angle acquisition order, the acquisition is "self-navigating" with respect to both in-plane translation and rotation. We reconstructed the images using the aligned projections and detected acquisition angles to significantly reduce image artifacts due to such motion. The theory of the correction technique is described, and its effectiveness is demonstrated in phantom and in vivo experiments.

Magnetic resonance measurements of renal blood flow as a marker of disease severity in autosomal-dominant polycystic kidney disease11Thomas Andreoli, M.D., served as Guest Editor for this paper

BACKGROUND

Autosomal-dominant polycystic kidney disease (ADPKD) is an inherited disorder characterized by renal cyst growth, early development of hypertension, and late occurrence of renal insufficiency. Despite evidence for the importance of nephroangiosclerosis in the progression of renal insufficiency in ADPKD, evaluation of renal blood flow (RBF) as a surrogate marker of disease severity has received little attention.

METHODS

Flow phantoms and repeat RBF measurements assessed accuracy and reproducibility. One hundred twenty-seven ADPKD subjects with creatinine clearances >70 mL/min underwent measurements of RBF, total, and cyst renal volumes, and % cyst volumes by magnetic resonance (MR) and of glomerular filtration rate (GFR). Renal vascular resistance (RVR) was calculated. MR blood flow sequences utilized a two-dimensional cine phase-contrast breath-hold pulse sequence perpendicular to the renal arteries. Flow rates were calculated utilizing FLOW software. Volumetric analysis was performed using stereology and region-based thresholding.

RESULTS

Excellent accuracy and intraobserver and interobserver reproducibility were demonstrated. Anatomic (total kidney volume, total cyst volume, and % cyst volume), hemodynamic (RBF and RVR), and functional (GFR) parameters were strongly correlated. Left polycystic kidneys were larger and had more severe disease. Regression analysis showed that age, diagnosis of hypertension, anatomic parameters and hemodynamic parameters were significant predictors of GFR. Multiple linear regression analysis identified age and hemodynamic parameters only as separate predictors of GFR. Anatomic, hemodynamic, and functional parameters discriminated between normotensive and hypertensive subjects despite antihypertensive treatments.

CONCLUSION

Renal hemodynamic parameters measured by MR correlate with anatomic and functional indices of disease severity, are the strongest predictors of renal function, and deserve further consideration as an outcome measure in clinical trials to guide therapy in ADPKD.

Intermittent-Mode CT Fluoroscopy–guided Biopsy of the Lung or Upper Abdomen with Breath-hold Monitoring and Feedback: System Development and Feasibility

A bellows-based breath-hold monitoring and feedback system was developed and evaluated for use in intermittent-mode computed tomographic (CT) fluoroscopy-guided biopsy procedures in the lung or upper abdomen. The bellows system is described, and its feasibility is demonstrated in studies with a respiratory phantom and human volunteers. Results are reported for seven patients who underwent bellows-assisted biopsy. Breath-hold monitoring and feedback with the bellows system allow the patient to perform reliable breath holding at a preselected level. This optimizes intermittent-mode CT fluoroscopy-guided biopsies by allowing consistent visualization of the target lesion throughout the procedure.

MRI as a biomarker of disease progression in a therapeutic trial of milameline for AD

OBJECTIVE

To assess the feasibility of using MRI measurements as a surrogate endpoint for disease progression in a therapeutic trial for AD.

METHODS

A total of 362 patients with probable AD from 38 different centers participated in the MRI portion of a 52-week randomized placebo-controlled trial of milameline, a muscarinic receptor agonist. The therapeutic trial itself was not completed due to projected lack of efficacy on interim analysis; however, the MRI arm of the study was continued. Of the 362 subjects who underwent a baseline MRI study, 192 subjects underwent a second MRI 1 year later. Hippocampal volume and temporal horn volume were measured from the MRI scans.

RESULTS

The annualized percent changes in hippocampal volume (-4.9%) and temporal horn volume (16.1%) in the study patients were consistent with data from prior single-site studies. Correlations between the rate of MRI volumetric change and change in behavioral/cognitive measures were greater for the temporal horn than for the hippocampus. Decline over time was more consistently seen with imaging measures, 99% of the time for the hippocampus, than behavioral/cognitive measures (p < 0.001). Greater consistency in MRI than behavioral/clinical measures resulted in markedly lower estimated sample size requirements for clinical trials. The estimated number of subjects per arm required to detect a 50% reduction in the rate of decline over 1 year are: AD Assessment Scale-cognitive subscale 320; Mini-Mental Status Examination 241; hippocampal volume 21; temporal horn volume 54.

CONCLUSION

The consistency of MRI measurements obtained across sites, and the consistency between the multisite milameline data and that obtained in prior single-site studies, demonstrate the technical feasibility of using structural MRI measures as a surrogate endpoint of disease progression in therapeutic trials. However, validation of imaging as a biomarker of therapeutic efficacy in AD awaits a positive trial.

Hybrid phased array for improved internal auditory canal imaging at 3.0-T MR

PURPOSE

To develop and evaluate a hybrid phased array for internal auditory canal (IAC) imaging at 3.0 T.

MATERIALS AND METHODS

A hybrid phased array was designed and built as two circular surface receive-only coils combined with a volume transmit-receive birdcage head coil for simultaneous image acquisition. Phantom and volunteer images were obtained to assess the coil performance.

RESULTS

The phantom data show that significant signal-to-noise ratio (SNR) improvement was achieved in the region corresponding to the inner ear, i.e., by a factor of 2.5 compared to the standard head coil data. Volunteer IAC image quality was deemed superior as compared to images acquired at 3.0 T using a standard head coil.

CONCLUSION

This hybrid array combined with three-dimensional fast spin-echo (FSE) acquisition resulted in improved high spatial resolution IAC imaging.

Motion correction using the k-space phase difference of orthogonal acquisitions

Rigid body translations of an object in MRI create image artifacts along the phase-encode (PE) direction in standard 2DFT imaging. If two images are acquired with swapped PE direction, it is possible to determine and correct for arbitrary in-plane translational interview motions in both images directly from phase differences in the k-space acquisitions by solving a large system of linear equations. For example, if one assumes two N x N 2D acquisitions with in-plane translational interview motion, 4N unknown motions may corrupt the two images, but the phase difference at each point in k-space yields a system of N(2) equations in these 4N unknowns. If the acquisitions have orthogonal PE directions, this highly overdetermined system of equations can be solved to provide the motion records, which in turn can be used to correct the motion artifacts in each image. The theory of this orthogonal k-space phase difference (ORKPHAD) technique is described, and results are presented for synthetic and in vivo motion-corrupted data sets. In all cases, the data showed clear improvement of translation-induced artifacts. These methods do not require special pulse sequences and are theoretically generalizable to partial Fourier imaging and 3D acquisitions.

Wrist: improved MR imaging with optimized transmit-receive coil design

The optimized wrist coil was designed and built as a transmit-receive birdcage coil for 1.5-T magnetic resonance (MR) imaging. Phantom studies were conducted to compare the optimized design with custom-designed and commercially available phased-array wrist coils and showed a 50%-90% improvement in signal-to-noise ratio (SNR). Blinded review of wrist images obtained in six volunteers showed that the optimized birdcage coil was preferred in 75% of the comparisons. An optimized birdcage coil designed for wrist imaging has improved both SNR and uniformity compared with those with a phased-array coil with the same geometry.

Exertional compartment syndrome and the role of magnetic resonance imaging

A 47-yr-old woman presented with a 1-yr history of progressive bilateral anterior lower leg pain and swelling with walking, which resolved 10 min after activity. Postexercise (forced dorsiflexion) magnetic resonance imaging revealed increased T2 signal intensity in the entire anterior muscle compartment, and anterior compartment pressures were elevated at rest and postexercise. Chronic exertional compartment syndrome can occur in some patients after even minimal physical activities, and magnetic resonance imaging without the use of radioisotopes was a useful adjunct for diagnosis.

A quality control program for MR-guided focused ultrasound ablation therapy

In this study, we propose a quality control program for MR-guided focused ultrasound (FUS) ablation treatment to assess FUS beam positioning accuracy, FUS power delivery accuracy, MR imaging quality, and FUS ablation system safety. A total of 353 sonication points in Lucite cards were measured, the average placement errors were -0.06 mm in the SI direction and -0.04 mm in the LR direction. Temperature elevation was calculated from MR phase difference images and the measured water proton chemical shift (WPCS) temperature coefficient. WPCS temperature calibration for phantoms yielded a temperature coefficient of 0.011 ppm/degree C. Sixteen experiments were conducted using six different phantoms to test the reliability of FUS power delivery. SNR and RF power calculated from phantom images were analyzed and stored at the MR console. A computer program was developed to integrate the system power delivery and the MR image quality control into one automated process. In the clinical trial at our institution, we expect this quality control program to be carried out before each patient treatment. If measured quality control values exceeds or below the preset values, a system service and retest should be conducted before the treatment.

Magnetic resonance elastography: non-invasive mapping of tissue elasticity

Magnetic resonance elastography (MRE) is a phase-contrast-based MRI imaging technique that can directly visualize and quantitatively measure propagating acoustic strain waves in tissue-like materials subjected to harmonic mechanical excitation. The data acquired allows the calculation of local quantitative values of shear modulus and the generation of images that depict tissue elasticity or stiffness. This is significant because palpation, a physical examination that assesses the stiffness of tissue, can be an effective method of detecting tumors, but is restricted to parts of the body that are accessible to the physician's hand. MRE shows promise as a potential technique for 'palpation by imaging', with possible applications in tumor detection (particularly in breast, liver, kidney and prostate), characterization of disease, and assessment of rehabilitation (particularly in muscle). We describe MRE in the context of other recent techniques for imaging elasticity, discuss the processing algorithms for elasticity reconstruction and the issues and assumptions they involve, and present recent ex vivo and in vivo results.