Preliminary Assessment of an Optical Flow Method (OFM) for Nonrigid Registration and Temporal Subtraction (TS) of Serial CT Examinations to Facilitate Evaluation of Interval Change in Metastatic Lung Nodules.

RATIONALE AND OBJECTIVES: Accurate assessment of size change of lung nodules on chest computed tomography (CT) is important for diagnosis and response assessment. However, manual methods are time-consuming and error-prone. We therefore assessed whether an optical flow method (OFM) with temporal subtraction (TS) can facilitate detection and quantification of lung nodule change on serial CT datasets. MATERIALS AND METHODS: Serial chest CT examinations were selected from 12 patients with multiple pulmonary metastases. Lung nodules were evaluated for change in size using: (1) OFM with TS and (2) reference standard visual and manual assessment. Average time required to assess interval change using both methods was recorded and compared. Concordance of agreement between OFM with TS and reference standard assessment for nodule change was examined. RESULTS: 285 solid pulmonary nodules were evaluated. The average time per nodule to assess interval change in nodule size by OFM with TS (mean 1.15 + 0.5 minutes) was significantly less (P = 0.02) than that the reference standard approach (mean 1.56 + 0.5 minutes). Agreement between OFM with TS and reference standard occurred for 63.2% of nodules overall (kappa = 0.50, standard error 0.35, P< 0.00001), and significantly increased with larger nodule size (kappa = 0.48 for nodules <5 mm; kappa = 0.94 for nodules >20 mm, P < 0.0001). CONCLUSIONS: This preliminary study demonstrates the feasibility of an OFM with TS to assess for interval change in metastatic lung nodules on serial CT examinations with significantly improved reading speed and moderate agreement relative to reference standard assessment. Agreement improved with larger nodule size.

Effect of contrast dose on diagnostic performance in DCE-MR breast imaging.

OBJECTIVE: To assess the diagnostic performance of breast magnetic resonance (MR) imaging as a function of gadolinium contrast dose using a retrospective reader study. MATERIAL AND METHODS: IRB approval was obtained prior to the start of this study and was HIPAA compliant. One-hundred and fifty MR breast examinations were included that were acquired between January 2001 and December 2006. Seventy-five patients received contrast doses (gadopentetate dimeglumine) by weight of 0.10 mmol/kg and 75 patients were imaged using fixed volumes of 20 ml. The images were assessed by two radiologists with performance calculated for each reader as well as a combined assessment. Dose response was measured by comparing performance between cases binned by dose: <=0.10; >0.10; and >0.13 mmol/kg. Statistical significance was calculated using a one-sided Z-test for differences in proportions with interobserver agreement calculated using Cohen's kappa statistics. RESULTS: In the combined reader assessment with equivocal lesions classified as negative, sensitivity rose from 66% (19/29) to 92% (24/26, P < 0.01) and 95% (18/19, P < 0.01) with the specificity also increasing from 65% (32/49) to 87% (40/46, P < 0.01) and 86% (32/37, P = 0.01) corresponding to doses <=0.10, >0.10, >0.13 mmol/kg. With equivocal lesions classified as positive, sensitivity rose from 79% (23/29) to 92% (24/26, P < 0.10) and 95% (18/19, P < 0.10) Specificity also increased from 53% (26/49) to 72% (33/46, P < 0.05) and 70% (26/37, P = 0.05) with increasing dose. Interobserver agreement also improved at the higher doses.

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.

Deformation analysis of 3D tagged cardiac images using an optical flow method.

BACKGROUND: This study proposes and validates a method of measuring 3D strain in myocardium using a 3D Cardiovascular Magnetic Resonance (CMR) tissue-tagging sequence and a 3D optical flow method (OFM). METHODS: Initially, a 3D tag MR sequence was developed and the parameters of the sequence and 3D OFM were optimized using phantom images with simulated deformation. This method then was validated in-vivo and utilized to quantify normal sheep left ventricular functions. RESULTS: Optimizing imaging and OFM parameters in the phantom study produced sub-pixel root-mean square error (RMS) between the estimated and known displacements in the x (RMSx = 0.62 pixels (0.43 mm)), y (RMSy = 0.64 pixels (0.45 mm)) and z (RMSz = 0.68 pixels (1 mm)) direction, respectively. In-vivo validation demonstrated excellent correlation between the displacement measured by manually tracking tag intersections and that generated by 3D OFM (R >or= 0.98). Technique performance was maintained even with 20% Gaussian noise added to the phantom images. Furthermore, 3D tracking of 3D cardiac motions resulted in a 51% decrease in in-plane tracking error as compared to 2D tracking. The in-vivo function studies showed that maximum wall thickening was greatest in the lateral wall, and increased from both apex and base towards the mid-ventricular region. Regional deformation patterns are in agreement with previous studies on LV function. CONCLUSION: A novel method was developed to measure 3D LV wall deformation rapidly with high in-plane and through-plane resolution from one 3D cine acquisition.

Modeling upper airway collapse by a finite element model with regional tissue properties.

This study presents a new computational system for modeling the upper airway in rats that combines tagged magnetic resonance imaging (MRI) with tissue material properties to predict three-dimensional (3D) airway motion. The model is capable of predicting airway wall and tissue deformation under airway pressure loading up to airway collapse. The model demonstrates that oropharynx collapse pressure depends primarily on ventral wall (tongue muscle) elastic modulus and airway architecture. An iterative approach that involves substituting alternative possible tissue elastic moduli was used to improve model precision. The proposed 3D model accounts for stress-strain relationships in the complex upper airway that should present new opportunities for understanding pathogenesis of airway collapse, improving diagnosis and developing treatments.

Automatic correction of in-plane bulk motion artifacts in self-navigated radial MRI.

Radial MRI is typically used for scans that are sensitive to unavoidable motion. While the translational motion artifact can be easily removed from the radial trajectory data by phase correction, correction of rotational motion still remains a challenge in radial MRI. We present a novel method to refocus the image corrupted by view-to-view motion in the view-interleaved radial MRI data. In this method, the error in rotational view angles was modeled as a polynomial function of the view order and the model parameters were estimated by minimizing the self-navigator image metrics such as image entropy, gradient entropy, normalized gradient squared and mean square difference. Translational motion correction was conducted by aligning the projection profiles. Simulation studies were conducted to demonstrate the robustness of both translational and rotational motion correction methods in different noise levels. The proposed method was successfully applied to correct for motion of healthy subjects. Substantial motion correction with relative error of less than 5% was achieved by using either first- or second-order model with the image metrics. This study demonstrates the potential of the method for motion-sensitive applications.

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

PURPOSE: To investigate the effect of transcytolemmal water exchange on the dynamic contrast-enhanced (DCE) T(1)-weighted MRI of human squamous cell carcinomas of the head and neck (HNSCC). MATERIALS AND METHODS: Nine patients with HNSCC nodal metastasis underwent pretreatment DCE-MRI with a temporal resolution of 2.5 seconds and a spatial resolution of 1 mm x 1 mm x 5 mm at 1.5T. We used two pharmacokinetic models for data analysis: generalized kinetic model (GKM) without considering transcytolemmal water exchange and the shutter-speed model (SSM), based on a two-site exchange model for transcytolemmal water exchange. The results were compared in three subgroups of voxels in the tumor depending on the level of contrast enhancement. RESULTS: SSM was found to be a better fit for more than 75% of pixels of all subjects (P < 0.01) in terms of residual size and Bayesian information criterion (BIC). For all three subgroups based on the contrast enhancement, the median K trans values of SSM were 42% to 55% higher than those of GKM and the median upsilon e values of SSM were 116% to 176% larger than those of GKM. The median K trans and upsilon e of two models were found significantly different (P < 0.01). The median tau i measured by SSM were from 211 to 364 msec. CONCLUSION: The effect of transcytolemmal water exchange is an important factor that needs to be incorporated for adequate modeling of contrast enhancement dynamics measured by MRI of HNSCC.

Application of an optical flow method to inspiratory and expiratory lung MDCT to assess regional air trapping: a feasibility study.

OBJECTIVE: We describe the application of an optical flow method to inspiratory and expiratory high-resolution volumetric lung MDCT for the assessment of regional air trapping. CONCLUSION: Qualitative and quantitative assessment of regional air trapping is feasible using an optical flow method to align volumetric MDCT data sets.

High frame-rate simultaneous bilateral breast DCE-MRI.

A simultaneous bilateral back-projection method for 3D dynamic contrast-enhanced (DCE)-MRI of the breasts was developed and evaluated. Using a double-side band modulation of the RF slab excitation pulse, discontinuous volumes that included both breasts were simultaneously selected. The number of slice phase-encoding steps was undersampled by a factor of 2, and the resulting signal aliasing from one volume to the other was removed using SENSE processing. In-plane encoding was performed with an interleaved radial acquisition reconstructed using dynamic k-space-weighted image contrast (KWIC) temporal filtering. Image resolution was 0.5 x 0.5 x 3.0 mm(3) with an effective temporal resolution of 15 s for both breast volumes. Combined with the 2x acceleration from SENSE encoding, this is a 16x acceleration factor over a conventional MR bilateral breast scan. An initial evaluation of these methods was performed on a cohort of women who presented with palpable or mammographically visible breast abnormalities. A total of 73 abnormalities were found in 45 of the 54 bilateral examinations that were performed. In 11 of these cases there was a significant finding in the contralateral breast. DCE images of both breasts can be acquired simultaneously, resulting in high-resolution images as well as rapid sampling of the contrast kinetics.

Use of an optical flow method for the analysis of serial CT lung images.

RATIONALE AND OBJECTIVES: Serial CT lung studies are difficult to compare due to misregistration between image sets. An optical flow method (OFM) was adapted for use on CT lung images to register images and visualize changes between studies. Three applications were investigated: lung nodule assessment; evaluation of pulmonary enhancement; and functional changes due to air trapping. MATERIALS AND METHODS: From an initial clinical study, a follow-up study was created by digitally manipulating the images to simulate patient positioning errors and nodule growth. Nodule growth was measured from the temporal subtraction of registered images. In application to the assessment of pulmonary enhancement, pre and postcontrast images from a patient with acute pulmonary embolism (PE) were registered. A map of the perfused blood volume was computed from the ratio of aligned lung volumes. Functional changes in the lung were demonstrated using images from a patient with air trapping. End-inspiratory and end-expiratory volumes were aligned and displacement fields estimated using the OFM. Principal strains were computed from the displacement fields. RESULTS: All image volumes were aligned with at least 0.95 correlation. OFM estimates of displacement showed excellent agreement with the prescribed displacements with 0.33 pixel RMS error. Nodule growth was evident in the presence of significant positioning errors. In the PE case, enhancement ratios indicated a hypoperfused area consistent with an occlusive hypodense filling defect. For the air trapping case, a strain map showed functional changes along the interface of the air trap. CONCLUSIONS: The OFM can facilitate the detection and quantification of changes between serial CT lung studies.

Pharyngeal airway wall mechanics using tagged magnetic resonance imaging during medial hypoglossal nerve stimulation in rats.

To better understand pharyngeal airway mechanics as it relates to the pathogenesis and treatment of obstructive sleep apnoea, we have developed a novel application of magnetic resonance imaging (MRI) with non-invasive tissue tagging to measure pharyngeal wall tissue motion during active dilatation of the airway. Eleven anaesthetized Sprague-Dawley rats were surgically prepared with platinum electrodes for bilateral stimulation of the medial branch of the hypoglossus nerve that supplies motor output to the protrudor and intrinsic tongue muscles. Images of the pharyngeal airway were acquired before and during stimulation using a gated multislice, spoiled gradient recalled (SPGR) imaging protocol in a 4.7 T magnet. The tag pulses, applied before stimulation, created a grid pattern of magnetically imbedded dark lines that revealed tissue motion in images acquired during stimulation. Stimulation significantly increased cross-sectional area, and anteroposterior and lateral dimensions in the oropharyngeal and velopharyngeal airways when results were averaged across the rostral, mid- and caudal pharynx (P < 0.001). Customized software for tissue motion-tracking and finite element-analysis showed that changes in airway size were associated with ventral displacement of tissues in the ventral pharyngeal wall in the rostral, mid- and caudal pharyngeal regions (P < 0.0032) and ventral displacement of the lateral walls in the mid- and caudal regions (P < 0.0001). In addition, principal maximum stretch was significantly increased in the lateral walls (P < 0.023) in a ventral-lateral direction in the mid- and caudal pharyngeal regions and principal maximum compression (perpendicular to stretch) was significantly increased in the ventral walls in all regions (P < 0.0001). Stimulation did not cause lateral displacement of the lateral pharyngeal walls at any level. The results reveal that the increase in pharyngeal airway size resulting from stimulation of the medial branch of the hypoglossal nerve is predominantly due to ventral displacement of the ventral and lateral pharyngeal walls.

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.

A modified logistic model to describe gadolinium kinetics in breast tumors.

A five-parameter modified logistic equation is presented that describes the signal enhancement in magnetic resonance dynamic contrast enhanced imaging (MRI-DCE). In this heuristic model, P(1) approximates the baseline signal, P(2) is related to the magnitude of the peak signal enhancement, P(3) is the approximate time of the maximum rate of increase of signal, P(4) is related to the maximum rate of signal enhancement, and P(5) is the terminal slope of the signal enhancement curve. Six breast tumors were studied that exhibited diverse patterns of signal enhancement, and in each case, estimated model parameters were well identified. Three of the model parameters, P(2), P(4) and P(5) describe attributes of the signal enhancement curve that have previously been shown to have diagnostic value with respect to breast cancer. Procedures for using the primary model parameters to derive a number of secondary parameters that may also have diagnostic value are discussed. Sensitivity analysis shows that the signal enhancement curve is highly sensitive to P(3) in the region of the signal intensity curve associated with rapid uptake of the contrast reagent. Consequently, frequent signal sampling in this time domain is indicated to enable identification of P(3) and sensitive fitting of the signal intensity curve. The advantages of this heuristic model compared to commonly used compartmental modeling approaches are discussed.

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.

In vivo measurements of human brain displacement.

Finite element models are increasingly important in understanding head injury mechanisms and designing new injury prevention equipment. Although boundary conditions strongly influence model responses, only limited quantitative data are available. While experimental studies revealed some motion between brain and skull, little data exists regarding the base of the skull. Using magnetic resonance images (MRI) of the caudal brain regions, we measured in vivo, quasi-static angular displacement of the cerebellum (CB) and brainstem (BS) relative to skull, and axial displacement of BS at the foramen magnum in supine human subjects (N=5). Images were obtained in flexion (7 degrees - 54 degrees ) and neutral postures using SPAMM tagging technique (N=47 pairs). Rigid body skull rotation angle from neutral posture (theta, degrees) was determined by extracting the edge feature points of the skull, and rotating and displacing the coordinates in one image until they matched those in the other. Tissue rotation was obtained by comparing tag lines in image pairs before and after flexion, and the motion of BS and CB were expressed relative to skull rotation and displacement. During flexion, the CB rotated in the flexion direction, exceeding the skull rotation, but relative BS rotations were negligible. Meanwhile, the BS moved caudally toward the foramen magnum. With a flexion angle of 54 degrees , the 95% confidence interval for the relative CB rotation was 2.7 degrees - 4.3 degrees , and 0.8 - 1.6mm for the relative BS axial displacement. Albeit quasi-static, this study provides important data that can be implemented to create more life-like boundary conditions in human finite element models.

Abdominal imaging at 4 T MR system: a preliminary result.

In this study, the potential for abdominal MR images at 4 T using a tailored coil was demonstrated using healthy volunteers. These images were compared with those obtained in the same subject at 1.5 T to discuss whether 4 T would be superior to 1.5 T in abdominal imaging. MR images at 4 T were characterized by high contrast and signal-to-noise ratios, more than 2.95 times higher than those at 1.5 T. In conclusion, abdominal MR imaging at 4 T was feasible. Furthermore, abdominal MR imaging at 4 T was superior to that at 1.5 T in qualitative and quantitative analyses.

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.

A direct comparison of signal behavior between 4.0 and 1.5 T: a phantom study.

INTRODUCTION: Higher magnetic fields (>or=3 T) afford higher spatial and/or temporal resolution in MR imaging with contrast agents, however, studies containing direct comparisons of signal intensity among different magnetic fields are substantially sparse. Our aim was to quantify the differences in terms of signal-to-noise ratios (SNRs) and contrast-to-noise ratios (CNRs) between higher and lower (

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

PURPOSE: 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. MATERIALS AND METHODS: Gadopentetate dimeglumine was diluted into normal saline, albumin, or iodinated contrast (0.625 mmol/liter to 40 mmol/liter). Samples were scanned at 1.5T and 0.2T. Signal intensity was measured using T1-weighted spin-echo (SE), T2-weighted SE, and two- and three-dimensional GRE (20 degrees-75 degrees flip angle) sequences. Graphical analysis of signal intensity vs. gadolinium concentration was performed. RESULTS: Albumin had no effect on gadolinium contrast. Dilution of gadolinium in iodinated contrast decreased signal intensity on all sequences compared to samples of identical concentration diluted in saline at both 1.5T and 0.2T: with a 2 mmol/liter gadolinium solution at 1.5T, signal was decreased by 26.1% on T1-weighted images, 31.7% on GRE20 images, and 28.9% on GRE45 images, and the T2 value decreased by 71.1%; at 0.2T, signal was decreased by 23.5% on T1-weighted images. On all sequences, the peak signal shifted to the left (lower gadolinium concentration) when diluted in iodinated contrast. Peak signal was also seen at different gadolinium concentrations on different sequences and field strength: at 1.5T, peak in saline/iodine was 2.5/0.625 mmol/liter on T1-weighted images, and 2.5/1.25 mmol/liter on GRE20 and GRE45 sequences. At 0.2T, peak in saline/iodine was 0.625-2.5/1.25 mmol/liter on T1-weighted images, 0.625-2.5/1.25 on GRE45 images, 2.5-10.0/1.25-5.0 mmol/liter on GRE65 images, and 1.25-5.0/0.625-1.25 mmol/liter on GRE75 images. CONCLUSION: Dilution of gadolinium in iodinated contrast results in decreased signal on T1-weighted, T2-weighted, and GRE images compared to dilution in saline or albuminfor both 1.5-T and 0.2-T scanners; if gadolinium is diluted in iodinated contrast for MR arthrography, a lower concentration should be used because the peak is shifted to the left. The use of iodinated contrast should be minimized, as it may diminish enhancement and lower the sensitivity and specificity of MR arthrography. Optimal gadolinium concentration for MR arthrography is dependent on scanner field strength and a broader range of gadolinium concentration can be used to provide maximal signal at low field strength.