Superimposition of computed tomography and single photon emission tomography immunoscintigraphic images in the pelvis: validation in patients with colorectal or ovarian carcinoma recurrence

Computer-aided evaluation of the anatomical accuracy of hybrid SPECT/spiral-CT imaging of lesions localized in the neck and upper abdomen

OBJECTIVE

The purpose of this study was to investigate the anatomical accuracy of hardware-based single-photon emission computed tomography/computed tomography (SPECT/CT) registration in the upper abdomen and neck.

METHODS

The database consisted of 90 patients referred for SPECT/CT for diagnostic workup of either thyroid/parathyroid disease (n=46) or abdominal neuroendocrine tumours (n=44). In the first group, 99mTc-MIBI was used as the tracer and in the second 123I-metaiodobenzylguanidine (n=13), 111In-octreotide (n=28) or 99mTc-octreotide (n=3). For predefined structures represented by both modalities, the distances between the centres of gravity of their CT and SPECT representation were determined in a semiautomated manner. In cervical data sets, this analysis was performed for the submandibular salivary glands (n=92) and in abdominal data sets for 69 neoplastic foci.

RESULTS

The mean distances were 5.7 ± 2.0 mm (range: 1.84-9.67 mm) in the neck and 6.8 ± 3.3 mm (range: 1.4-19.7 mm) in the abdomen. In 42 out of 92 of the cervical and 40 out of 69 of the abdominal data sets at least one of the X-direction-determined, Y-direction-determined, and Z-direction-determined distances was greater than the SPECT pixel width of 4.6 mm.

CONCLUSION

The anatomical accuracy of hardware-based SPECT/CT fusion depends also on the region of the body studied. For example, in the neck and upper abdomen the accuracy is lower than in the lower lumbar spine. In clinical routine, SPECT/CT data sets acquired for the neck and upper abdomen should be regularly checked and corrected for SPECT/CT misalignment. This is, in particular, important when CT-based corrections of SPECT involving pixelwise data integration such as for attenuation correction are made.

The growing development of multimodality imaging in oncology

The first decade of the century has been the beginning of an era of new practice in daily medical imaging, that is the multimodality involving functional or metabolic imaging brought by nuclear medicine techniques directly associated with anatomical information brought by CT (Computed X-Ray Tomography) devices combined with nuclear medicine detectors. PET (Positron Emission Tomography)/CT and SPECT (Single Photon Emission Computed Tomography)/CT are now established to further increase the interest of PET and SPECT, thanks to improved localization of the pathologic processes, and in many instances thanks to a gain in specificity. An even better use of the combined information will necessitate redefining some protocols and indications, and the future will probably see the continued development of multimodality imaging in practice. Besides the combination with CT, another modality is expected in the future: PET/MRI (Magnetic Resonance Imaging).

Semiautomatic nonrigid registration for the prostate and pelvic MR volumes

RATIONALE AND OBJECTIVES

Three-dimensional (3D) nonrigid image registration for potential applications in prostate cancer treatment and interventional magnetic resonance (iMRI) imaging-guided therapies were investigated.

MATERIALS AND METHODS

An almost fully automated 3D nonrigid registration algorithm using mutual information and a thin plate spline (TPS) transformation for MR images of the prostate and pelvis were created and evaluated. In the first step, an automatic rigid body registration with special features was used to capture the global transformation. In the second step, local feature points (FPs) were registered using mutual information. An operator entered only five FPs located at the prostate center, left and right hip joints, and left and right distal femurs. The program automatically determined and optimized other FPs at the external pelvic skin surface and along the femurs. More than 600 control points were used to establish a TPS transformation for deformation of the pelvic region and prostate. Ten volume pairs were acquired from three volunteers in the diagnostic (supine) and treatment positions (supine with legs raised).

RESULTS

Various visualization techniques showed that warping rectified the significant pelvic misalignment by the rigid-body method. Gray-value measures of registration quality, including mutual information, correlation coefficient, and intensity difference, all improved with warping. The distance between prostate 3D centroids was 0.7 +/- 0.2 mm after warping compared with 4.9 +/- 3.4 mm with rigid-body registration.

CONCLUSION

Semiautomatic nonrigid registration works better than rigid-body registration when patient position is changed greatly between acquisitions. It could be a useful tool for many applications in the management of prostate.

Accuracy of Image Fusion Using a Fixation Device for Whole-Body Cancer Imaging

OBJECTIVE

The purpose of this study was to evaluate the clinical feasibility of a simple image fusion technique with PET and CT images acquired separately using a vacuum cushion as a fixation device.

SUBJECTS AND METHODS

Forty-four patients underwent whole-body PET using 18F-fluoro-2-deoxy-D-glucose (FDG) followed by CT with IV contrast material. The patients were carefully fixed in an individually molded cushion to provide the same positioning for both examinations. The PET and CT images were fused on a workstation by using the lower margin of the urinary bladder as a reference. The degree of misregistration was evaluated for the physiologic uptake of the liver and kidneys and for the pathologic uptake of lesions.

RESULTS

The average deviation of the center point of the liver between the two images was 6.6 +/- 8.7 (SD) mm in the craniocaudal direction, 1.9 +/- 5.1 mm in the anteroposterior direction, and 2.3 +/- 7.0 mm in the right-left direction. This value in the craniocaudal direction was 4.7 +/- 8.7 mm in the right kidney and 4.0 +/- 8.8 mm in the left kidney. Above the diaphragm, the deviations of the center point of movable and static lesions were 11.7 +/- 3.4 mm and 10.4 +/- 5.3 mm, respectively. Below the diaphragm, those of movable and static lesions were 9.7 +/- 2.5 mm and 6.9 +/- 2.9 mm, respectively.

CONCLUSION

Our preliminary data indicate that this technique is a simple and practical method for manual image fusion that may be acceptable in clinical settings.

A comparative study of warping and rigid body registration for the prostate and pelvic MR volumes

A three-dimensional warping registration algorithm was created and compared to rigid body registration of magnetic resonance (MR) pelvic volumes including the prostate. The rigid body registration method combines the advantages of mutual information (MI) and correlation coefficient at different resolutions. Warping registration is based upon independent optimization of many interactively placed control points (CP's) using MI and a thin plate spline transformation. More than 100 registration experiments with 17 MR volume pairs determined the quality of registration under conditions simulating potential interventional MRI-guided treatments of prostate cancer. For image pairs that stress rigid body registration (e.g. supine, the diagnostic position, and legs raised, the treatment position), both visual and numerical evaluation methods showed that warping consistently worked better than rigid body. Experiments showed that approximately 180 strategically placed CP's were sufficiently expressive to capture important features of the deformation.

Display of fused images: methods, interpretation, and diagnostic improvements

The use of integrated visualization for medical images aims at assisting clinicians in the difficult task of mentally translating and integrating medical image data from multiple sources into a three-dimensional (3D) representation of the patient. This interpretation of the enormous amount and complexity of contemporary, multiparameter, and multimodal image data demands efficient methods for integrated presentation. This article reviews methods for fused display with the main focus on integration of functional with anatomical images. First, an overview of integrated two-dimensional (2D) and 3D medical image display techniques is presented, and topics related to the interpretation of the integrated images are discussed. Then we address the key issue for clinical acceptance, ie, whether these novel visualization techniques lead to diagnostic improvements. Methods for fused display appear to be powerful tools to assist the clinician in the retrieval of relevant information from multivariate medical image data. Evaluation of the different methods for fused display indicates that the diagnostic process improves, notably as concerns the anatomical localization (typically of functional processes), the registration procedure, enhancement of signal, and efficiency of information presentation (which increases speed of interpretation and comprehension). Consequently, fused display improves communication with referring specialists, increases confidence in the observations, and facilitates the intra- and intersubject comparison of a large part of the data from the different sources, thereby simplifying the extraction of additional, valuable information. In most diagnostic tasks the clinician is served best by providing several (interactive and flexible) 2D and 3D methods for fused display for a thorough assessment of the wealth of image information from multiple sources.

Automatic MR volume registration and its evaluation for the pelvis and prostate

A three-dimensional (3D) mutual information registration method was created and used to register MRI volumes of the pelvis and prostate. It had special features to improve robustness. First, it used a multi-resolution approach and performed registration from low to high resolution. Second, it used two similarity measures, correlation coefficient at lower resolutions and mutual information at full resolution, because of their particular advantages. Third, we created a method to avoid local minima by restarting the registration with randomly perturbed parameters. The criterion for restarting was a correlation coefficient below an empirically determined threshold. Experiments determined the accuracy of registration under conditions found in potential applications in prostate cancer diagnosis, staging, treatment and interventional MRI (iMRI) guided therapies. Images were acquired in the diagnostic (supine) and treatment position (supine with legs raised). Images were also acquired as a function of bladder filling and the time interval between imaging sessions. Overall studies on three patients and three healthy volunteers, when both volumes in a pair were obtained in the diagnostic position under comparable conditions, bony landmarks and prostate 3D centroids were aligned within 1.6 +/- 0.2 mm and 1.4 +/- 0.2 mm, respectively, values only slightly larger than a voxel. Analysis suggests that actual errors are smaller because of the uncertainty in landmark localization and prostate segmentation. Between the diagnostic and treatment positions, bony landmarks continued to register well, but prostate centroids moved towards the posterior 2.8-3.4 mm. Manual cropping to remove voxels in the legs was necessary to register these images. In conclusion, automatic, rigid body registration is probably sufficiently accurate for many applications in prostate cancer. For potential iMRI-guided treatments, the small prostate displacement between the diagnostic and treatment positions can probably be avoided by acquiring volumes in similar positions and by reducing bladder and rectal volumes.

The fusion of anatomic and physiologic imaging in the management of patients with cancer

Imaging is of major clinical importance in the noninvasive evaluation and management of patients with cancer. Computed tomography (CT) and other anatomic imaging modalities, such as magnetic resonance imaging (MRI) or ultrasound, have a high diagnostic ability by visualizing lesion morphology and by providing the exact localization of malignant sites. Nuclear medicine provides information on the function and metabolism of cancer. Over the last decade, there have been numerous attempts to combine data obtained from different imaging techniques. Fused images of nuclear medicine and CT (or to a lesser extent, MRI) overcome the inherent limitations of both modalities. Valuable physiologic information benefits from a precise topographic localization. Coregistered data have been shown to be useful in the evaluation of patients with cancer at diagnosis and staging, in monitoring the response to treatment, and during follow up, for early detection of recurrence. Time-consuming and difficult realignment and computation for fusion of independent studies have, until now, limited the use of registration techniques to pilot studies performed in a small number of patients. The development of the new technology of single photon emission computed tomography/CT and positron emission tomography/CT that allows for combined functional and anatomic data acquisition has the potential to make fusion an everyday clinical tool.

Automatic registration of pelvic computed tomography data and magnetic resonance scans including a full circle method for quantitative accuracy evaluation

The purpose of this study is to develop a method for registration of CT and MR scans of the pelvis with minimal user interaction and to obtain a means for objective quantification of the registration accuracy of clinical data without markers. CT scans were registered with proton density MR scans using chamfer matching on automatically segmented bone. A fixed threshold was used to segment CT, while morphological filters were used to segment MR. The method was tested with transverse and coronal MR scans of 18 patients and sagittal MR scans of 8 patients. The registration accuracy was estimated by comparing (triangulating) registrations of a single CT scan with MR in different orientations in a "full circle." For example, CT is first matched on transverse MR, next transverse MR is matched independently on coronal MR, and finally coronal MR is matched independently on CT. The product of the three transformations is the identity if all matching steps are perfect. Deviations from identity occur both due to random errors and due to some types of systematic errors. MR was registered on MR (to close the "circle") by minimization of rms voxel value differences. CT-MR registration takes about 1 min, including user interaction. The random error for CT-MR registration with transverse or coronal MR was 0.5 mm in translation and 0.4 degree in rotation (standard deviation) for each axis. A systematic registration error of about 1 mm was demonstrated along the MR frequency encoding direction, which is attributed to the chemical shift. In conclusion, the presented algorithm efficiently and accurately registers pelvic CT and MR scans on bone. The "full circle" method provides an estimate of the registration accuracy on clinical data.

Digital superimposition of CT and positive SPECT tumor images. Phantom study and clinical applications

Digital superimposition of SPECT and CT data was evaluated in a phantom and then applied to patient data. Seven patients were studied. Six patients had pheochromocytomas as evidenced by I-131 or I-123 MIBG localization and one had ovarian cancer imaged by In-111 OC125 MoAb. Anatomic or skin landmarks identified the level of each SPECT transaxial slice. Both SPECT and CT image data sets were transferred to a minicomputer connected to an image processor. Afterwards, a scaled, rotated and translated realignment was performed. Data for each modality were coded in different primary colors and then superimposed. Superimposition of phantom data was checked for the absence of distortion of pinpoint and large structures. For suspected tumor sites, superimposition of the patients' slices were allowed to check for matching SPECT and CT abnormalities to localize a SPECT abnormality without a corresponding CT lesion or to distinguish SPECT abnormalities from those seen on CT. In one case, the technique failed because of very low I-131 MIBG-tumor uptake. The superimposition decreases false positives in SPECT and both false negatives or false positives in CT.

Tumor imaging with monoclonal antibodies

Immunoscintigraphy offers the possibility of specifically targeting human tumors, but the complexity of the human immune system, as well as tumor-related phenomena, prevent monoclonal antibodies from reaching a large number of tumor cells in which they can interact with the antigen. Possible ways to overcome these problems are the use of small fragments, in particular those of genetically engineered humanized antibodies including single immunoglobulin-variable domains, as well as techniques to label the antibody in vivo after a sufficient amount has been taken up by the tumor and the remainder has been eliminated. Despite the low absolute tumor uptake, results of European studies, presently available radiolabeled monoclonal antibodies in gastrointestinal and ovarian cancers yield an average sensitivity of more than 70% with an average specificity of more than 80%, even in otherwise occult tumors. Because of possible tracer uptake in normal liver, the detection rate of liver metastases varies from less than 10% to more than 90%. For the detection of local recurrence in the pelvis, immunoscintigraphy has been found to be more accurate than methods that are based on the imaging of structural changes. Fusion of morphological and functional images might improve the early detection of recurrent and metastatic disease. In melanoma, another tumor that has been extensively studied in Europe, similar results have been obtained, whereas only few data are presently available for other tumors (especially lung and breast cancer).

Correlative image registration

Image registration in nuclear medicine and radiology refers to the spatial matching or merging of two or more images from the same or different imaging modalities. The coordinates of the corresponding picture elements (pixels) from different images are transformed to align and equate their positions and spatial coordinates. Correlative image registration is a more restrictive term that applies to the matching of spatial coordinates of images coming from different imaging modalities. The registration of correlative images provides a useful approach to combine the best sensitivities and specificities of complementary procedures to detect, locate, monitor, and measure pathological and other physical changes. Here we review the registration of images from nuclear medicine (single-photon emission computed tomography, positron emission tomography and planar imaging) with those from other imaging modalities (magnetic resonance imaging, computed tomography, digital subtraction angiography and ultrasound) to closely correlate changes in metabolism, blood flow, receptor density, and other functional measurements with regional anatomy and morphological changes. The types of image registration applications, techniques, and terminology associated with image registration and examples of application are presented.

Clinical applications of fusion imaging in oncology

Recent developments in tumor imaging, made possible by advances in instrumentation and radiopharmaceuticals, has led to an increasing need for accurate anatomic correlation of single photon emission computed tomography (SPECT) and positron emission tomography (PET) images. Fusion imaging permits the functional strengths of SPECT and PET to be combined with the anatomic resolution of computed tomography (CT) and magnetic resonance imaging (MRI). Clinical applications of fusion imaging include the evaluation of brain tumors, lymphoma, hepatic lesions and monoclonal antibody studies. The continued development of these techniques will eventually allow fusion imaging to become a routine part of nuclear medicine practice.