Software for image registration: algorithms, accuracy, efficacy

Animal-specific positioning molds for registration of repeat imaging studies: comparative microPET imaging of F18-labeled fluoro-deoxyglucose and fluoro-misonidazole in rodent tumors

INTRODUCTION

Comparative imaging of multiple radiotracers in the same animal can be invaluable in elucidating and validating their respective mechanisms of localization. Comparative imaging of PET tracers, particularly in small animals, is problematic, however: such tracers must be administered and imaged separately because simultaneously imaged positron emitters cannot be separated based on energy discrimination.

OBJECTIVE

As part of our ongoing development of hypoxia imaging radiotracers, the intratumoral distributions of sequentially administered F18-fluoro-deoxyglucose (FDG) and the hypoxia tracer F18-fluoromisonidazole (FMiso) were compared in rats by registered microPET imaging with positioning of each animal in a custom-fabricated whole-body mold.

METHODS

Nude rats with a hindlimb R3327-AT anaplastic rat prostate tumor xenograft and a hindlimb FaDu human squamous cell carcinoma (each up to 20 x 20 x 30 mm in size) were studied. Rapid-Foam (Soule Medical, Lutz, FL) was used to fabricate animal-specific molds for immobilization and reproducible positioning. Each rat was injected via the tail vein with approximately 33 MBq (900 microCi) of FDG and imaged in its mold at 1 h postinjection (pi) on the microPET. The next day, each rat was injected with approximately 22 MBq (600 microCi) of FMiso and positioned and imaged in its mold at approximately 2 h pi. Custom-manufactured germanium-68 rods (10 microCi each, 1 x 10 mm) were reproducibly positioned in the mold as fiduciary markers.

RESULTS

The registered microPET images unambiguously demonstrated grossly similar though not identical distributions of FDG and FMiso in the tumors - a high-activity rim surrounding a lower-activity core. There were subtle but possibly significant differences in the intratumoral distributions of FDG and FMiso, however. These may not have been discerned without careful image registration.

CONCLUSION

Animal-specific molds are inexpensive and straightforward to fabricate and use for registration (+/-1 to 2 mm) of sequential PET images and may aid image interpretation.

Automated registration of hip and spine for longitudinal QCT studies: integration with 3D densitometric and structural analysis

To eliminate user interaction in longitudinal quantitative computed tomography (QCT) measurements of bone mineral density (BMD) and geometry, we have developed and optimized an automated registration algorithm for QCT images of the hip and spine and integrated it with a previously developed 3D densitometric and structural analysis program. With registration, the follow-up images are automatically aligned with respect to the baseline scans, and the bone quantification of the aligned follow-up scan is initiated based on the bone morphometric features defined on the baseline scan. To validate the algorithm, we analyzed 20 pairs of repeat QCT images (10 hip pairs and 10 spine pairs) acquired on a modern multi-slice CT scanner, with repositioning between each scan pair to simulate repeat visits. Bone measurements obtained with automatic registration achieved comparable or improved precision errors compared to those obtained by careful manual analysis of the follow-up scans. The algorithm we have developed was based on the mutual information approach, with simplex optimization under a multi-resolution scheme. The average registration time was 2.3 min for a hip pair and 1.1 min for a vertebra pair using a standard desktop computer. Based on the reduced user interaction, high degree of precision, and short execution time, this is a promising technique for monitoring therapy in patients and clinical trials.

Application of 3D-MR image registration to monitor diseases around the knee joint

PURPOSE

To estimate the accuracy and consistency of a method using a voxel-based MR image registration algorithm for precise monitoring of knee joint diseases.

MATERIALS AND METHODS

Rigid body transformation was calculated using a normalized cross-correlation (NCC) algorithm involving simple manual segmentation of the bone region based on its anatomical features. The accuracy of registration was evaluated using four phantoms, followed by a consistency test using MR data from the 11 patients with knee joint disease.

RESULTS

The registration accuracy in the phantom experiment was 0.49+/-0.19 mm (SD) for the femur and 0.56+/-0.21 mm (SD) for the tibia. The consistency value in the experiment using clinical data was 0.69+/-0.25 mm (SD) for the femur and 0.77+/-0.37 mm (SD) for the tibia. These values were all smaller than a voxel (1.25 x 1.25 x 1.5 mm).

CONCLUSION

The present method based on an NCC algorithm can be used to register serial MR images of the knee joint with error on the order of a sub-voxel. This method would be useful for precisely assessing therapeutic response and monitoring knee joint diseases; normalized cross-correlation; accuracy.

Dynamic breast MRI: image registration and its impact on enhancement curve estimation

A novel algorithm for performing registration of dynamic contrast-enhanced (DCE) MRI data of the breast is presented. It is based on an algorithm known as iterated dynamic programming originally devised to solve the stereo matching problem. Using artificially distorted DCE-MRI breast images it is shown that the proposed algorithm is able to correct for movement and distortions over a larger range than is likely to occur during routine clinical examination. In addition, using a clinical DCE-MRI data set with an expertly labeled suspicious region, it is shown that the proposed algorithm significantly reduces the variability of the enhancement curves at the pixel level yielding more pronounced uptake and washout phases.

Evaluation of a Rigid Registration Method of Lung Perfusion SPECT and Thoracic CT

OBJECTIVE

The objective of our study was to evaluate a rigid registration method in lung perfusion SPECT using thoracic CT as a standard.

MATERIALS AND METHODS

The reproducibility of markers selection and the robustness of the method were assessed on a torso phantom. The accuracy of registration regarding the number and location of markers and the breathing state during CT was evaluated on eight patients using 10 external markers placed around the thorax before SPECT and CT acquisitions. The accuracy of registration was assessed using the mean errors (ME) between 10 markers after registration.

RESULTS

Registration using external markers on a phantom was accurate (ME, < 3 mm) when rotation was less than 40 degrees (p = 0.02). The accuracy of registration improved markedly from four to six markers for phantom (5.5-3.6 mm) and patients (11.2-9.5 mm) and then remained constant up to 10 markers. The ME was less when using markers that well encompassed the thorax for phantom and patients (p = 0.02 and p = 0.05, respectively). The use of four anatomic markers was not accurate (ME, 20 mm).

CONCLUSION

The registration method is reproducible and accurate, and six external markers were required to get an ME of less than 10 mm in patients.

Radiation dose reduction in four‐dimensional computed tomography

Four-dimensional (4D) CT is useful in many clinical situations, where detailed abdominal and thoracic imaging is needed over the course of the respiratory cycle. However, it usually delivers a larger radiation dose than the standard three-dimensional (3D) CT, since multiple scans at each couch position are required in order to provide the temporal information. Our purpose in this work is to develop a method to perform 4D CT scans at relatively low current, hence reducing the radiation exposure of the patients. To deal with the increased statistical noise caused by the low current, we proposed a novel 4D penalized weighted least square (4D-PWLS) smoothing method, which can incorporate both spatial and phase information. The 4D images at different phases were registered to the same phase via a deformable model, thereby, a regularization term combining temporal and spatial neighbors can be designed for the 4D-PWLS objective function. The proposed method was tested with phantom experiments and a patient study, and superior noise suppression and resolution preservation were observed. A quantitative evaluation of the benefit of the proposed method to 4D radiotherapy and 4D PET/CT imaging are under investigation.

Registration of [18F]FDG microPET and small-animal MRI

This paper describes a voxel-based method for coregistering microPET [(18)F]FDG emission images and MRI data without the need for fiducial markers. [(18)F]FDG has a well-characterized biodistribution in normal mice and thus may be useful for image registration. Female BALB/c mice were implanted with EMT-6 mouse mammary carcinoma 1 week prior to imaging. Three imaging sessions were performed in which a [(18)F]FDG microPET-R4 emission scan was taken followed by small-animal MRI with and without Gd-based contrast agent. MicroPET and MR images were registered using a voxel-based algorithm that computes rigid-body image transformations based on the alignment of intensity gradients. Registration accuracy was assessed on the basis of dual-modality external fiducial line sources incorporated into the mouse bed. The root mean square (rms) registration errors were 0.74 mm translation and 1.44 degrees rotation without contrast and 0.72 mm translation and 0.89 degrees rotation with contrast. Generally, good registration was evident upon inspection of fused microPET/MR images. Accurate automated, voxel-based microPET-MR image coregistration, utilizing image intensity gradients, is feasible. Our technique requires no manual identification of image features and makes no use of surgically implanted or external fiducial markers or stereotactic apparatus.

Incremental Value of Fusion Imaging With Integrated PET-CT in Oncology

PURPOSE

The purpose of this study was to assess the incremental value of integrated F-18 FDG PET-CT imaging compared with either alone for evaluation of patients with body malignancies.

MATERIALS AND METHODS

Images from 173 consecutive patients with body malignancies referred for integrated PET-CT imaging were reviewed. A CT with contrast performed within 2 months of PET-CT was available for 74 patients.

RESULTS

There was agreement between the transmission CT (TrCT) and PET interpreted separately in 65% (112 of 173) of patients. Interpretation of integrated PET-CT had an incremental diagnostic value in 17.9% (31 of 173) of the total patient population. Data was analyzed further excluding patients for whom further analysis was not relevant: 1) 20% (34 of 173) of patients with normal TrCT and PET and 2) 11% (19 of 173) of patients with disseminated metastases (too numerous to count) on both TrCT and PET. Among the 120 other patients, PET interpreted alone was positive in 195 body regions and CT-positive in 178 body regions with agreement for all regions in 49% (59 of 120) of patients and discordance or equivocal lesions in 51% (61 of 120) of patients. Integrated PET-CT had an incremental diagnostic value in 27% (31 of 120) of patients. Contrasted CT scan demonstrated hepatic lesions in 5 and extrahepatic lesions in 3 patients overlooked on TrCT; all 8 of these lesions were PET-positive. There was incremental impact on the management of 12.5% (15 of 120) of patients.

CONCLUSIONS

After excluding patients with a normal PET-CT or disseminated disease, there was an incremental diagnostic value of integrated PET-CT imaging in 27% (31 of 120) and incremental impact on management in 12.5% (15 of 120) of patients. CT with contrast did not demonstrate lesions not appreciated by PET-CT.

Concurrent PET/CT with an Integrated Imaging System: Intersociety Dialogue from the Joint Working Group of the American College of Radiology, the Society of Nuclear Medicine, and the Society of Computed Body Tomography and Magnetic Resonance

Rapid advances in imaging technology are a challenge for health care professionals, who must determine how best to use these technologies to optimize patient care and outcomes. Hybrid imaging instrumentation, combining 2 or more new or existing technologies, each with its own separate history of clinical evolution, such as PET and CT, may be especially challenging. CT and PET provide complementary anatomic information and molecular information, respectively, with PET giving specificity to anatomic findings and CT offering precise localization of metabolic activity. Historically, the acquisition and interpretation of the 2 image sets have been performed separately and very often at different times and locales. Recently, integrated PET/CT systems have become available; these systems provide PET and CT images that are acquired nearly simultaneously and are capable of producing superimposed, coregistered images, greatly facilitating interpretation. As the implementation of this integrated technology has become more widespread in the setting of oncologic imaging, questions and concerns regarding equipment specifications, image acquisition protocols, supervision, interpretation, professional qualifications, and safety have arisen. This article summarizes the discussions and observations surrounding these issues by a collaborative working group consisting of representatives from the American College of Radiology, the Society of Nuclear Medicine, and the Society of Computed Body Tomography and Magnetic Resonance.

Kombinierte Hybridsysteme (PET/CT, SPECT/CT) versus multimodale Bildgebung mit getrennten Systemen

With increasing use of combined PET/CT scanners in the last few years, multimodality imaging (Nuclear Medicine/Radiology) found its way into clinical routine diagnostics. In this overview, necessary components for multimodality imaging, strategies for image analysis and image presentation, and diagnostic goals of combined imaging are demonstrated and discussed. A special focus is on the question, whether combined scanners can be replaced by a software approach with separated modalities. Advantages and limitations of multimodality imaging with combined or separated scanners are shown.

PET and PET-CT for evaluation of colorectal carcinoma

The evaluation of patients with known or suspected recurrent colorectal carcinoma is now an accepted indication for positron emission tomography using (18)F-fluorodeoxyglucose (FDG-PET) imaging. FDG-PET does not replace imaging modalities such as computed tomography (CT) for preoperative anatomic evaluation but is indicated as the initial test for diagnosis and staging of recurrence and for preoperative staging (N and M) of known recurrence that is considered to be resectable. FDG-PET imaging is valuable for the differentiation of posttreatment changes from recurrent tumor, differentiation of benign from malignant lesions (indeterminate lymph nodes, hepatic and pulmonary lesions), and the evaluation of patients with rising tumor markers in the absence of a known source. The addition of FDG-PET to the evaluation of these patients reduces overall treatment costs by accurately identifying patients who will and will not benefit from surgical procedures. Although initial staging at the time of diagnosis is often performed during colectomy, FDG-PET imaging is recommended for a subgroup of patients at high risk (with elevated CEA levels) and normal CT and for whom surgery can be avoided if FDG-PET shows metastases. Screening for recurrence in patients at high risk has also been advocated. FDG-PET imaging seems promising for monitoring patient response to therapy but larger studies are necessary. The diagnostic implications of integrated PET-CT imaging include improved detection of lesions on both the CT and FDG-PET images, better differentiation of physiologic from pathologic foci of metabolism, and better localization of the pathologic foci. This new powerful technology provides more accurate interpretation of both CT and FDG-PET images and therefore more optimal patient care. PET-CT fusion images affect the clinical management by guiding further procedures (biopsy, surgery, radiation therapy), excluding the need for additional procedures, and changing both inter- and intramodality therapy.

Metabolic Imaging with FDG

The rapid advances in imaging technologies are a challenge for both radiologists and clinicians who must integrate these technologies for optimal patient care and outcomes at minimal cost. Multiple indications for functional imaging using fluorode-oxyglucose (FDG) are now well accepted in the fields of neurology, cardiology, and oncology, including differentiation of benign from malignant lesions, staging of malignant lesions, detection of malignant recurrence, and monitoring of therapy. The fusion of anatomic and molecular images (computed tomography [CT] and FDG) obtained with integrated positron emission tomography (PET)-CT systems, sequentially in time but without moving the patient from the imaging table, allows optimal co-registration of anatomic and molecular images, leading to accurate attenuation correction and precise anatomic localization of lesions with increased metabolism. This powerful technology provides a valuable new tool for diagnostic and therapeutic applications. The diagnostic accuracy is improved in approximately 50% of patients because of improvement of lesion detection on both CT and FDG PET images, better differentiation between physiologic and pathological foci of FDG uptake, and better localization of malignant foci of FDG uptake. This new technology affects the management of 10%-20% of cases by guiding further therapy. Promising clinical applications include guiding biopsy to the metabolically active sites of tumors, guiding surgery, and planning intensity-modulated radiation therapy. In addition, new PET radiopharmaceuticals are emerging for indications for which FDG has limitations. Some of the new PET tracers are labeled with (18)F, which has a practical half-life for commercial distribution. In the past few years, the clinical indications for FDG have broadened dramatically, and the rapid technical developments of integrated multimodality imaging systems and new PET tracers further extend the horizon.

Fusion Imaging in Nuclear Medicine—Applications of Dual-Modality Systems in Oncology

Medical imaging has become of the utmost importance in evaluating patients with cancer. Single-photon emission computed tomography (SPECT) and positron emission tomography (PET) are accurate methods for detecting cancer and related metabolic abnormalities, but they often do not provide the anatomical landmarks needed to precisely localize lesions. Magnetic resonance imaging (MRI) and computed tomography (CT) scan, on the other hand, offer excellent anatomic detail but are less sensitive because they do not provide functional detail. Fusion imaging combines functional studies with morphological ones, so overcoming the drawbacks of both modalities. Software-based fusion of independently performed scintigraphic and radiological images has proven time consuming and impractical for routine use. Recently, dual-modality integrated imaging systems (SPECT/CT and PET/CT) have been developed: the acquired images are coregistered by means of the hardware in the same session. These new devices can be particularly useful for tumour imaging. The anatomical images provide precise localization and allow the exclusion of disease in sites of physiologic tracers' accumulation for SPECT and PET findings. Hybrid imaging in oncological applications has been very encouraging, indicating that these systems are suited for routine use in clinical practice. In fact, fused images provide additional information that improves diagnostic accuracy and impacts on patient management.