Comparing pairwise and simultaneous joint registrations of decorrelating interval exams using entropic graphs.

The interest in registering a set of images has quickly risen in the field of medical image analysis. Mutual information (MI) based methods are well-established for pairwise registration but their extension to higher dimensions (multiple images) has encountered practical implementation difficulties. We extend the use of alpha mutual information (alphaMI) as the similarity measure to simultaneously register multiple images. alphaMI of a set of images can be directly estimated using entropic graphs spanning feature vectors extracted from the images, which is demonstrated to be practically feasible for joint registration. In this paper we are specifically interested in monitoring malignant tumor changes using simultaneous registration of multiple interval MR or CT scans. Tumor scans are typically a decorrelating sequence due to the cycles of heterogeneous cell death and growth. The accuracy of joint and pairwise registration using entropic graph methods is evaluated by registering several sets of interval exams. We show that for the parameters we investigated simultaneous joint registration method yields lower average registration errors compared to pairwise. Different degrees of decorrelation in the serial scans are studied and registration performance suggests that an appropriate scanning interval can be determined for efficiently monitoring lesion changes. Different levels of observation noise are added to the image sequences and the experimental results show that entropic graph based methods are robust and can be used reliably for multiple image registration.

Diaphragmatic shape change after lung volume reduction surgery.

Diaphragmatic shape in normal patients was significantly different from shape in emphysema patients. Postoperative diaphragmatic shape in patients with good clinical outcome differed from preoperative shape and was similar to shape in normal patients. In patients with poor clinical outcome, surgery appeared to have little effect on diaphragm shape.

Semiautomatic registration of volumetric ultrasound scans.

We demonstrate the ability to register easily and accurately volumetric ultrasound scans without significant data preprocessing or user intervention. Two volumetric ultrasound breast scan data sets were acquired from two different patients with breast cancer. Volumetric scan data were acquired by manually sweeping a linear array transducer mounted on a linear slider with a position encoder. The volumetric data set pairs consisted of color flow and/or power mode Doppler data sets acquired serially on the same patients. A previously described semiautomatic registration method based on maximizing mutual information was used to determine the transform between data sets. The results suggest that, even for the deformable breast, three-dimensional full affine transforms can be sufficient to obtain clinically useful registrations; warping may be necessary for increased registration accuracy. In conclusion, mutual information-based automatic registration as implemented on modern workstations is capable of yielding clinically useful registrations in times <35 min.

Motion correction in fMRI via registration of individual slices into an anatomical volume.

An automated retrospective image registration based on mutual information is adapted to a multislice functional magnetic resonance imaging (fMRI) acquisition protocol to provide accurate motion correction. Motion correction is performed by mapping each slice to an anatomic volume data set acquired in the same fMRI session to accommodate inter-slice head motion. Accuracy of the registration parameters was assessed by registration of simulated MR data of the known truth. The widely used rigid body volume registration approach based on stacked slices from the time series data may hinder statistical accuracy by introducing inaccurate assumptions of no motion between slices for multislice fMRI data. Improved sensitivity and specificity of the fMRI signal from mapping-each-slice-to-volume method is demonstrated in comparison with a stacked-slice correction method by examining functional data from two normal volunteers. The data presented in a standard anatomical coordinate system suggest the reliability of the mapping-each-slice-to-volume method to detect the activation signals consistent between the two subjects.

Demonstration of accuracy and clinical versatility of mutual information for automatic multimodality image fusion using affine and thin-plate spline warped geometric deformations.

This paper applies and evaluates an automatic mutual information-based registration algorithm across a broad spectrum of multimodal volume data sets. The algorithm requires little or no pre-processing, minimal user input and easily implements either affine, i.e. linear or thin-plate spline (TPS) warped registrations. We have evaluated the algorithm in phantom studies as well as in selected cases where few other algorithms could perform as well, if at all, to demonstrate the value of this new method. Pairs of multimodal gray-scale volume data sets were registered by iteratively changing registration parameters to maximize mutual information. Quantitative registration errors were assessed in registrations of a thorax phantom using PET/CT and in the National Library of Medicine's Visible Male using MRI T2-/T1-weighted acquisitions. Registrations of diverse clinical data sets were demonstrated including rotate-translate mapping of PET/MRI brain scans with significant missing data, full affine mapping of thoracic PET/CT and rotate-translate mapping of abdominal SPECT/CT. A five-point thin-plate spline (TPS) warped registration of thoracic PET/CT is also demonstrated. The registration algorithm converged in times ranging between 3.5 and 31 min for affine clinical registrations and 57 min for TPS warping. Mean error vector lengths for rotate-translate registrations were measured to be subvoxel in phantoms. More importantly the rotate-translate algorithm performs well even with missing data. The demonstrated clinical fusions are qualitatively excellent at all levels. We conclude that such automatic, rapid, robust algorithms significantly increase the likelihood that multimodality registrations will be routinely used to aid clinical diagnoses and post-therapeutic assessment in the near future.

Robust three-dimensional object definition in CT and MRI.

This work describes the application of an object definition algorithm to the medical imaging environment for the task of automated detection of anatomical boundaries in three dimensions in the presence of low spatial frequency nonstationarities. We have chosen the Liou-Jain algorithm and have modified it for use with 3D medical image datasets and extended it by including a recruitment operator that corrects for the algorithm's inherent volume underestimation. The algorithm avoids problems in both traditional statistical segmentation and 2D techniques and elegantly bridges the gap between traditional gradient-based edge finding and regression-based segmentation techniques. Results are shown for MRI datasets from the human abdomen and brain and for a CT dataset of a liver tumor, as well as an MRI scan of a glioma in a rat brain. For comparison, the human abdomen dataset was processed by a multivariate, statistical classifier. The results demonstrate the statistical technique's susceptibility to low spatial frequency nonstationarities due to rf field inhomogeneity; the Liou-Jain algorithm is shown to be immune to this effect. Further, the results show spatial consistency as a result of inherent characteristics of the algorithm. Volumes identified by the algorithm are visualized and assessed qualitatively in three dimensions. Quantitative accuracy of the algorithm's volume estimates is assessed by the use of a phantom. This work demonstrates that this technique is effective in automatically detecting anatomical organ and lesion surfaces in 3D medical datasets that are corrupted by low spatial frequency nonstationarity and in obtaining volume estimates.

Retrospective correction of intensity inhomogeneities in MRI.

Medical imaging data sets are often corrupted by multiplicative inhomogeneities, often referred to as nonuniformities or intensity variations, that hamper the use of quantitative analyses. The authors describe an automatic technique that not only improves the worst situations, such as those encountered with magnetic resonance imaging (MRI) surface coils, but also corrects typical inhomogeneities encountered in routine volume data sets, such as MRI head scans, without generating additional artifact. Because the technique uses only the patient data set, the technique can be applied retrospectively to all data sets, and corrects both patient independent effects, such as rf coil design, and patient dependent effects, such as attenuation of overlying tissue experienced both in high field MRI and X-ray computed tomography (CT). The authors show results for several MRI imaging situations including thorax, head, and breast. Following such corrections, region of interest analyses, volume histograms, and thresholding techniques are more meaningful. The value of such correction algorithms may increase dramatically with increased use of high field strength magnets and associated patient-dependent rf attenuation in overlying tissues.

Generating a normalized geometric liver model using warping.

RATIONALE AND OBJECTIVES: Automated liver surface determination in abdominal computed tomography scans, currently difficult to achieve, is of interest to determine liver location and size for various medical applications, including radiation therapy treatment planning, surgical planning, and oncologic monitoring. The authors propose to facilitate automation by the addition of a priori shape information in the form of a liver model. METHODS: The normalized geometric liver model is generated by averaging outlines from a set of normal liver studies previously registered using thin-plate spline warping. The model consists of an averaged liver surface, a set of anatomic landmarks, and a deformation function. RESULTS: A liver model is presented and its ability to represent normal liver shapes is demonstrated. CONCLUSIONS: Liver surface warping provides a means of data normalization for model construction and a means of model deformation for representation of liver organ shapes.

Determination of liver volume from CT scans using histogram cluster analysis.

The histogram cluster analysis procedure (HICAP), which was developed by NASA for processing satellite images, classifies images into discrete clusters of pixels according to one or more arbitrary imaging variables. We incorporated this nonparametric, multivariate procedure in a semiautomatic computer algorithm for calculating total liver volume from CT scans and compared its performance with that of a human observer. Total liver volumes were calculated from CT scans in adult patients by the algorithm and by an experienced radiologist using the trackball controlled cursor at the CT console. Variability in the computer calculated volumes was determined by repeating calculations three times over the course of 3-12 months. Using HICAP in the univariate mode, we calculated total liver volumes from 28 contrast enhanced CT scans in 27 patients. Liver volumes calculated by the semiautomatic and manual methods had a median absolute difference of 3.6% (Vcomputer = 1.08 * Vmanual - 99.52 cc; r2 = 0.99). Median day-to-day variability of the computer calculated volumes was 1.9% (95% confidence interval: 1.3-2.7%). Using HICAP in a bivariate mode to illustrate its ability to incorporate two image features in one analysis, we studied an additional patient and compared total liver volume calculated from the univariate data set defined by the contrast enhanced CT scan with that calculated from the bivariate data set defined by nonenhanced and contrast enhanced CT scans. The HICAP errors were 4.1% in the univariate analysis and 0.4% in the bivariate analysis. It is concluded that this statistical clustering algorithm provides a clinically accurate, repeatable, and feasible method of in vivo liver volume determination.

Quantitative assessment of cartilage surface roughness in osteoarthritis using high frequency ultrasound.

Osteoarthritis (OA) is a common disease which affects nearly 50% of people over age 60. Histologic evaluation suggests that fibrillations approximately 20-150 microns are among the earliest changes in the articular cartilage. We propose a technique to quantify these surface fibrillatory changes in osteoarthritic articular cartilage by considering the angular distribution of the envelope-detected backscattered pressure field from an incident 30-MHz focused transducer. The angular distribution of the scattered acoustic field from an inosonifying source will directly relate to the distribution of surface fibrillatory changes. Data are presented for three different grades (400, 500 and 600 grit) of commercially available emory paper and three samples of osteoarthritic femoral head articular cartilage, which were visually assessed as having smooth, intermediate and rough surfaces, respectively. Our preliminary results indicate a probable monotonic relationship between articular cartilage roughening and the degree of broadening in the angle-dependent pressure amplitude. When applied to the emory paper, the technique indicates sensitivity to differences as small as approximately 5-10 microns in mean roughness. This procedure may provide an extremely sensitive and reproducible means of quantifying and following the cartilage changes observed in early osteoarthritis.

Prostate cancer: correlation of MR images with tissue optical density at pathologic examination.

To explain the variability in detection of prostate cancer with magnetic resonance (MR) imaging, the authors correlated preoperative MR findings in 28 patients with tissue optical density (TOD) measurements on whole-mount pathologic slides prepared from radical prostatectomy specimens. TOD was used as an indicator of the degree of tissue compactness or openness. TOD measurements from proved cancers and from pathologic regions corresponding to MR lesions (areas of low signal intensity seen at T2-weighted MR imaging) were compared with TOD measurements from adjacent, nonmalignant tissue. TOD measurements corresponding to MR lesions were higher than noncancerous tissue measurements in all cases (P less than .005). Although most of these lesions represented cancers (21 of 30), nine of 30 represented benign tissue that was composed mainly of densely packed fibromuscular stroma (30% false-positive results). Thus, signal intensity appeared to be related to TOD rather than to a specific histologic tissue type, and the finding of a peripheral zone lesion with low signal intensity did not necessarily indicate the presence of a cancer.

Carcinoma of the prostate: MR images obtained with body coils do not accurately reflect tumor volume.

MR imaging with a body coil is unreliable in directly demonstrating tumor spread through the prostatic capsule. However, the likelihood of extracapsular spread of prostatic cancer rises with increasing tumor volume. The aim of our study was to assess the accuracy of MR with a body coil in diagnosing capsular penetration indirectly via an estimation of prostatic tumor volumes. Twenty-six patients with proved prostatic cancer that was clinically confined to the gland underwent MR imaging before radical prostatectomy and whole-mount pathologic sectioning of the specimen. Twenty of 31 lesions prospectively outlined on the MR images corresponded to cancers outlined on the pathology slides, and tumor volumes were calculated by using a voxel summation technique. On MR, tumor volume was underestimated in 11 of 20 cases and overestimated in nine of 20 cases. Only two of 20 size estimates based on MR findings were within 10% of actual tumor volume. Overlap in MR tumor volumes was significant between lesions with and without capsular penetration at microscopy. Factors contributing to inaccuracies in measurements of tumor volume on MR images included the variable histologic make-up of the tumors. Our results show that, although knowledge of the size of a prostatic lesion is important in predicting the behavior of the tumor, MR imaging with a body coil is not reliable for accurate estimation of tumor volume.

Quantitative tissue motion analysis of digitized M-mode images: gestational differences of fetal lung.

Quantitative analysis of transmitted cardiac motion in fetal lung is evaluated by applying correlation techniques to digitized M-mode images in 21 patients, subdivided into two subgroups by gestational age: (I) 25-30 weeks (11 patients), and (II) greater than or equal to 35 weeks (10 patients). The corresponding numbers of M-mode images analyzed for each group are 23 and 18, respectively. This partition is expected to reflect functionally "immature" and "mature" lungs. The estimated maximum mean radial deformation per unit epicardial excursion, (r), is calculated from the two-time, spatially averaged correlation function obtained between diastolic and systolic M-mode lines. The collective results for each subgroup are (r) I = 0.79 +/- 0.11 (sem) and (r) II = 0.62 +/- 0.13 (sem). The analysis presented, albeit in a limited population, is indicative of a trend in accordance with qualitative observations of Birnholz and Farrell (1985). M-mode analysis, as indicated by Adler et al. (1989) is a potentially useful technique to quantify such tissue motion.

Radiologic evaluation of the subcarinal lymph nodes: a comparative study.

Subcarinal lymph nodes are commonly involved by metastases from cancers of both the right and left lungs. No data exist on the relative accuracy of radiologic methods for evaluating subcarinal nodes. We prospectively studied lung cancer patients who were surgical candidates with CT, MR imaging (0.35 T), esophagography, and anteroposterior tomography. Forty-six patients who subsequently underwent thoracotomy had excision or sampling of subcarinal nodes at mediastinoscopy. All 46 had CT scans, 27 had MR imaging, 23 had esophagography, and 21 had anteroposterior tomography. Receiver-operating characteristic curves were constructed for each technique, and the area under each curve was calculated. MR and CT were nearly identical in subcarinal evaluation, with areas under the receiver-operating characteristic curves of 0.90 and 0.86, respectively; both were superior to esophagography (0.55) and anteroposterior tomography (0.61). The size threshold at which subcarinal nodes were considered abnormally enlarged in this lung cancer population was 11 mm in short axis for CT, agreeing with the size threshold previously reported for a normal population. The size threshold for abnormal nodal enlargement with MR imaging was 18 mm in short axis. We conclude that CT and MR imaging are comparable in the detection of subcarinal lymphadenopathy and are superior to both tomography and esophagography. Different size thresholds for metastatic subcarinal nodes are needed for CT and MR imaging to be comparable in overall performance.

Tissue characterization of the testes using ultrasonic CT. Work in progress.

Ultrasonic computed tomography (UCT) can aid in characterizing tissue for the detection and diagnosis of leukemic infiltration of the testes. Preliminary studies in 6 healthy adults and 26 patients (3-20 years old) with leukemia or non-Hodgkin lymphoma suggest that elevated speed of sound in the testis may be an indicator of leukemic infiltration. UCT may become an important screening method for detecting testicular involvement. In long-term follow-up, UCT can be performed more frequently and easily than biopsy, which is the current screening method.

Ultrasonic computed tomography of the breast. Improvement of image quality by use of cross-correlation time-of-flight and phase-insensitive attenuation measurements.

The authors present two methods of reducing refraction artifacts in ultrasonic computed tomography of the breast. Measuring the time of flight of sonic pulses by cross-correlation instead of leading-edge detection significantly reduces distortion and improves resolution in speed-of-sound images. Phase-insensitive reception of pulse energy across a large aperture array is shown to be superior to a conventional single-element transducer for attenuation imaging.