Lawrence Dougherty

Tracking and finite element analysis of stripe deformation in magnetic resonance tagging

Magnetic resonance tissue tagging allows noninvasive in vivo measurement of soft tissue deformation. Planes of magnetic saturation are created, orthogonal to the imaging plane, which form dark lines (stripes) in the image. The authors describe a method for tracking stripe motion in the image plane, and show how this information can be incorporated into a finite element model of the underlying deformation. Human heart data were acquired from several imaging planes in different orientations and were combined using a deformable model of the left ventricle wall. Each tracked stripe point provided information on displacement orthogonal to the original tagging plane, i.e., a one-dimensional (1-D) constraint on the motion. Three-dimensional (3-D) motion and deformation was then reconstructed by fitting the model to the data constraints by linear least squares. The average root mean squared (rms) error between tracked stripe points and predicted model locations was 0.47 mm (n=3,100 points). In order to validate this method and quantify the errors involved, the authors applied it to images of a silicone gel phantom subjected to a known, well-controlled, 3-D deformation. The finite element strains obtained were compared to an analytic model of the deformation known to be accurate in the central axial plane of the phantom. The average rms errors were 6% in both the reconstructed shear strains and 16% in the reconstructed radial normal strain.

Dynamic MRI with projection reconstruction and KWIC processing for simultaneous high spatial and temporal resolution

A method for dynamic imaging in MRI is presented that enables the acquisition of a series of images with both high temporal and high spatial resolution. The technique, which is based on the projection reconstruction (PR) imaging scheme, utilizes distinct data acquisition and reconstruction strategies to achieve this simultaneous capability. First, during acquisition, data are collected in multiple undersampled passes, with the view angles interleaved in such a way that those of subsequent passes bisect the views of earlier ones. During reconstruction, these views are weighted according to a previously described k‐space weighted image contrast (KWIC) technique that enables the manipulation of image contrast by selective filtering. Unlike conventional undersampled PR methods, the proposed dynamic KWIC technique does not suffer from low image SNR or image degradation due to streaking artifacts. The effectiveness of dynamic KWIC is demonstrated in both simulations and in vivo, high‐resolution, contrast‐enhanced imaging of breast lesions. Magn Reson Med 52:815–824, 2004.

k -Space weighted image contrast (KWIC) for contrast manipulation in projection reconstruction MRI

A novel technique for manipulating contrast in projection reconstruction MRI is described. The method takes advantage of the fact that the central region of k‐space is oversampled, allowing one to choose different filters to enhance or reduce the amount that each view contributes to the central region, which dominates image contrast. The technique is implemented into a fast spin‐echo (FSE) sequence, and it is shown that multiple T2‐weighted images can be reconstructed from a single image data set. These images are shown to be nearly identical to those acquired with the Cartesian‐sampled FSE sequence at different effective echo times. Further, it is demonstrated that T2 maps can be generated from a single image data set. This technique also has the potential to be useful in dynamic contrast enhancement studies, capable of yielding a series of images at a significantly higher effective temporal resolution than what is currently possible with other methods, without sacrificing spatial resolution. Magn Reson Med 44:825–832, 2000.

Validation of an optical flow method for tag displacement estimation

Presents a validation study of an optical-flow method for the rapid estimation of myocardial displacement in magnetic resonance tagged cardiac images. This registration and change visualization (RCV) software uses a hierarchical estimation technique to compute the flow field that describes the warping of an image of one cardiac phase into alignment with the next. This method overcomes the requirement of constant pixel intensity in standard optical-flow methods by preprocessing the input images to reduce any intensity bias which results from the reduction in stripe contrast throughout the cardiac cycle. To validate the method, SPAMM-tagged images were acquired of a silicon gel phantom with simulated rotational motion. The pixel displacement was estimated with the RCV method and the error in pixel tracking was <4% 1000 ms after application of the tags, and after 30/spl deg/ of rotation. An additional study was performed using a SPAMM-tagged multiphase slice of a canine left ventricle. The true displacement was determined using a previously validated active contour model (snakes). The error between methods was 6.7% at end systole. The RCV method has the advantage of tracking all pixels in the image in a substantially shorter period than the snakes method.

Pretreatment diffusion-weighted and dynamic contrast-enhanced MRI for prediction of local treatment response in squamous cell carcinomas of the head and neck.

OBJECTIVE The objective of our study was to predict response to chemoradiation therapy in patients with head and neck squamous cell carcinoma (HNSCC) by combined use of diffusion-weighted imaging (DWI) and high-spatial-resolution, high-temporal-resolution dynamic contrast-enhanced MRI (DCE-MRI) parameters from primary tumors and metastatic nodes. SUBJECTS AND METHODS Thirty-two patients underwent pretreatment DWI and DCE-MRI using a modified radial imaging sequence. Postprocessing of data included motion-correction algorithms to reduce motion artifacts. The median apparent diffusion coefficient (ADC), volume transfer constant (K(trans)), extracellular extravascular volume fraction (v(e)), and plasma volume fraction (v(p)) were computed from primary tumors and nodal masses. The quality of the DCE-MRI maps was estimated using a threshold median chi-square value of 0.10 or less. Multivariate logistic regression and receiver operating characteristic curve analyses were used to determine the best model to discriminate responders from nonresponders. RESULTS Acceptable χ(2) values were observed from 84% of primary tumors and 100% of nodal masses. Five patients with unsatisfactory DCE-MRI data were excluded and DCE-MRI data for three patients who died of unrelated causes were censored from analysis. The median follow-up for the remaining patients (n = 24) was 23.72 months. When ADC and DCE-MRI parameters (K(trans), v(e), v(p)) from both primary tumors and nodal masses were incorporated into multivariate logistic regression analyses, a considerably higher discriminative accuracy (area under the curve [AUC] = 0.85) with a sensitivity of 81.3% and specificity of 75% was observed in differentiating responders (n = 16) from nonresponders (n = 8). CONCLUSION The combined use of DWI and DCE-MRI parameters from both primary tumors and nodal masses may aid in prediction of response to chemoradiation therapy in patients with HNSCC.

Alignment of CT lung volumes with an optical flow method

RATIONALE AND OBJECTIVES This study was performed to evaluate an optical flow method for registering serial computed tomographic (CT) images of lung volumes to assist physicians in visualizing and assessing changes between CT scans. MATERIALS AND METHODS The optical flow method is a coarse-to-fine model-based motion estimation technique for estimating first a global parametric transformation and then local deformations. Five serial pairs of CT images of lung volumes that were misaligned because of patient positioning, respiration, and/or different fields of view were used to test the method. RESULTS Lung volumes depicted on the serial paired images initially were correlated at only 28%-68% because of misalignment. With use of the optical flow method, the serial images were aligned to at least 95% correlation. CONCLUSION The optical flow method enables a direct comparison of serial CT images of lung volumes for the assessment of nodules or functional changes in the lung.

Transcytolemmal water exchange in pharmacokinetic analysis of dynamic contrast‐enhanced MRI data in squamous cell carcinoma of the head and neck

To investigate the effect of transcytolemmal water exchange on the dynamic contrast‐enhanced (DCE) T1‐weighted MRI of human squamous cell carcinomas of the head and neck (HNSCC).

In vivo human cervical spinal cord deformation and displacement in flexion

Study Design. In vivo, quasi‐static distortion of the human cervical spinal cord was measured in five volunteers during flexion of the neck using a motion‐tracking magnetic resonance imaging technique. Objectives. To measure cord distortion and movement in living subjects. Summary of Background Data. In situ spinal cord measurements in human and rhesus monkey cadavers taken at full flexion demonstrate that the entire cervical cord elongates approximately 10% of its length at a neutral position, but no data are available at other angles of flexion, or in living subjects. Methods. The spatial modulation of magnetization pulse sequence created a series of parallel lines in the image that deform with the tissue. A custom‐designed device was built to guide the flexion of the neck and enhance motion reproducibility. Midsagittal plane images were acquired before and after flexion. The tagged line pattern in each pair of magnetic resonance images was compared to compute distortion and movement of the cervical spinal cord at varying degrees of flexion. Results. Between a neutral posture and full flexion, the entire cord (C2‐C7) elongated linearly with head flexion, increasing 10% and 6% of its initial length along the posterior and anterior surfaces, respectively. Average displacement was on the order of 1‐3 mm, and varied with region. Specifically, the upper cord showed caudad movement in the spinal canal, and the lower cord moved cephalad, again with larger movements on the posterior surface. Conclusions. The cervical cord elongates and displaces significantly during head flexion in human volunteers, offering valuable information regarding the normal milieu of the cord.

High field body MR imaging:Preliminary experiences

Whole-body magnetic resonance (MR) scanners at high field strengths (> or =3 T) have been introduced in expectation of a larger signal-to-noise ratio (SNR), which would decrease the length of scan time or improve the spatial resolution. In this paper, the advantages and disadvantages of the high field MR imaging are discussed. Although the building of the radio frequency (RF) coil, safety and the specific absorption rate (SAR) are issues, the application of high field MR imaging is promising. The optimization of all parameters including injection rate of Gd-DTPA is necessary for high field MR imaging to obtain maximal results; however, we hope that high field MR imaging can be used in routine clinical applications in the future.

Effects of iodinated contrast and field strength on gadolinium enhancement: Implications for direct MR arthrography

To optimize direct magnetic resonance (MR) arthrography by determining the effect of dilution of gadolinium in iodinated contrast, saline, or albumin on T1‐weighted, T2‐weighted, and gradient‐recalled echo (GRE) images, and the effect of scanner field strength.