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A review of cardiac image registration methods

In this paper, the current status of cardiac image registration methods is reviewed. The combination of information from multiple cardiac image modalities, such as magnetic resonance imaging, computed tomography, positron emission tomography, single-photon emission computed tomography, and ultrasound, is of increasing interest in the medical community for physiologic understanding and diagnostic purposes. Registration of cardiac images is a more complex problem than brain image registration because the heart is a nonrigid moving organ inside a moving body. Moreover, as compared to the registration of brain images, the heart exhibits much fewer accurate anatomical landmarks. In a clinical context, physicians often mentally integrate image information from different modalities. Automatic registration, based on computer programs, might, however, offer better accuracy and repeatability and save time.

A 3-D model-based registration approach for the PET, MR and MCG cardiac data fusion

In this paper, a new approach is presented for the assessment of a 3-D anatomical and functional model of the heart including structural information from magnetic resonance imaging (MRI) and functional information from positron emission tomography (PET) and magnetocardiography (MCG). The method uses model-based co-registration of MR and PET images and marker-based registration for MRI and MCG. Model-based segmentation of MR anatomical images results in an individualized 3-D biventricular model of the heart including functional parameters from PET and MCG in an easily interpretable 3-D form.

MRI quality assurance using the ACR phantom in a multi-unit imaging center

Abstract Background. Magnetic resonance imaging (MRI) instrumentation is vulnerable to technical and image quality problems, and quality assurance is essential. In the studied regional imaging center the long-term quality assurance has been based on MagNET phantom measurements. American College of Radiology (ACR) has an accreditation program including a standardized image quality measurement protocol and phantom. The ACR protocol includes recommended acceptance criteria for clinical sequences and thus provides possibility to assess the clinical relevance of quality assurance. The purpose of this study was to test the ACR MRI phantom in quality assurance of a multi-unit imaging center. Material and methods. The imaging center operates 11 MRI systems of three major manufacturers with field strengths of 3.0 T, 1.5 T and 1.0 T. Images of the ACR phantom were acquired using a head coil following the ACR scanning instructions. Both ACR T1- and T2-weighted sequences as well as T1- and T2-weighted brain sequences in clinical use at each site were acquired. Measurements were performed twice. The images were analyzed and the results were compared with the ACR acceptance levels. Results. The acquisition procedure with the ACR phantom was faster than with the MagNET phantoms. On the first and second measurement rounds 91% and 73% of the systems passed the ACR test. Measured slice thickness accuracies were not within the acceptance limits in site T2 sequences. Differences in the high contrast spatial resolution between the ACR and the site sequences were observed. In 3.0 T systems the image intensity uniformity was slightly lower than the ACR acceptance limit. Conclusion. The ACR method was feasible in quality assurance of a multi-unit imaging center and the ACR protocol could replace the MagNET phantom tests. An automatic analysis of the images will further improve cost-effectiveness and objectiveness of the ACR protocol.

MRI quality control: six imagers studied using eleven unified image quality parameters

Quality control of the magnetic resonance imagers of different vendors in the clinical environment is non-harmonised, and comparing the performance is difficult. The purpose of this study was to develop and apply a harmonised long-term quality control protocol for the six imagers in our organisation in order to assure that they fulfil the same basic image quality requirements. The same Eurospin phantom set and identical imaging parameters were used with each imager. Values of 11 comparable parameters describing the image quality were measured. Automatic image analysis software was developed to objectively analyse the images. The results proved that the imagers were operating at a performance level adequate for clinical imaging. Some deficiencies were detected in image uniformity and geometry. The automated analysis of the Eurospin phantom images was successful. The measurements were successfully repeated after 2 weeks on one imager and after half a year on all imagers. As an objective way of examining the image quality, this kind of comparable and objective quality control of different imagers is considered as an essential step towards harmonisation of the clinical MRI studies through a large hospital organisation.

Quality assurance in diagnostic ultrasound

OBJECTIVE To setup a practical ultrasound quality assurance protocol in a large radiological center, results from transducer tests, phantom measurements and visual checks for physical faults were compared. MATERIALS AND METHODS Altogether 151 transducers from 54 ultrasound scanners, from seven different manufacturers, were tested with a Sonora FirstCall aPerio™ system (Sonora Medical Systems, Inc., Longmont, CO, USA) to detect non-functional elements. Phantom measurements using a CIRS General Purpose Phantom Model 040 (CIRS Tissue Simulation and Phantom Technology, VA, USA) were available for 135 transducers. The transducers and scanners were also checked visually for physical faults. The percentages of defective findings in these tests were computed. RESULTS Defective results in the FirstCall tests were found in 17% of the 151 transducers, and in 16% of the 135 transducers. Defective image quality resulted with 15% of the transducers, and 25% of the transducers had a physical flaw. In 16% of the scanners, a physical fault elsewhere than in the transducer was found. Seven percent of the transducers had a concurrent defective result both in the FirstCall test and in the phantom measurements, 8% in the FirstCall test and in the visual check, 4% in the phantom measurements and in the visual check, and 2% in all three tests. CONCLUSION The tested methods produced partly complementary results and seemed all to be necessary. Thus a quality assurance protocol is forced to be rather labored, and therefore the benefits and costs must be closely followed.

A New Method for the Registration of Cardiac PET and MR Images Using Deformable Model Based Segmentation of the Main Thorax Structures

Integration of magnetic resonance (MR) and positron emission tomography (PET) images of the heart has proved its usefulness for the estimation of the myocardial viability. In this paper, a method for the rigid registration of cardiac MR and PET images is presented. It is based on the matching of the surfaces of thorax structures extracted by a deformable model from PET transmission and MR transaxial images. MR short axis registration with PET emission image is easily derived and allows the study viability in the proper anatomic conditions. The method has been evaluated on ten patients suffering from three vessel coronary artery disease. Qualitative results were good with 9 over the 10 available cases. A quantitative estimation of the registration quality confirmed the nice abilities of this approach.

Transmission imaging for registration of ictal and interictal single-photon emission tomography, magnetic resonance imaging and electroencephalography.

Abstract.A method developed for registration of ictal and interictal single-photon emission tomography (SPET), magnetic resonance imaging (MRI) and electroencephalography (EEG) is described. For SPET studies, technetium-99m ethyl cysteinate dimer (ECD) was injected intravenously while the patient was monitored on video-EEG to document the ictal or interictal state. Imaging was performed using a triple-head gamma camera equipped with a transmission imaging device using a gadolinium-153 source. The images (128×128 pixels, voxel size 3.7×3.7×3.6 mm3) were reconstructed using an iterative algorithm and postfiltered with a Wiener filter. The gold-plated silver electrodes on the patient’s scalp were utilized as markers for registration of the ictal and interictal SPET images, as these metallic markers were clearly seen on the transmission images. Fitting of the marker sets was based on a non-iterative least squares method. The interictal SPET image was subtracted from the ictal image after scaling. The T1-weighted MPRAGE MR images with voxel size of 1.0×1.0×1.0 mm3 were obtained with a 1.5-T scanner. For registration of MR and subtraction SPET images, the external marker set of the ictal SPET study was fitted to the surface of the head segmented from MR images. The SPET registration was tested with a phantom experiment. Registration of ictal and interictal SPET in five patient studies resulted in a 2-mm RMS residual of the marker sets. The estimated RMS error of registration in the final result combining locations of the electrodes, subtraction SPET and MR images was 3–5 mm. In conclusion, transmission imaging can be utilized for an accurate and easily implemented registration procedure for ictal and interictal SPET, MRI and EEG.

Stability study of some neural networks applied to tissue characterization of brain magnetic resonance images

This study investigates the segmentation ability of unsupervised clustering of the image feature space. A self-organizing map, a feed-forward neural network and a k-nearest neighbor classifier were compared in labeling brain slices from magnetic resonance imaging. Qualitative and quantitative tests were carried out using brain images of a patient with an infarction. Five different tissue classes were partitioned: white matter, gray matter, cerebrospinal fluid, fluid in the infarct region and gray matter in the infarct region. The SOM based method performed best in all the cases that were investigated. Especially, the stability of the method concerning the influence of the training set was superior.

Reproducibility of phantom-based quality assurance parameters in real-time ultrasound imaging

Background In a large radiological center, the ultrasound (US) quality assurance (QA) program involves several professionals. Although the operator and the parameters utilized can contribute to the results, the selected QA parameters should still reflect the quality of the US scanner, not the measuring process. Purpose To evaluate the reproducibility of recommended phantom-based US QA parameters in a realistic environment. Material and Methods Six sonographers measured six high-end US scanners with 20 transducers using a general purpose phantom. Every transducer was measured altogether seven times, using one frequency per transducer. The QA parameters studied were homogeneity, visualization depth, vertical and horizontal distance measurements, axial and lateral resolution, and the correct visibility of anechoic and high-contrast masses. The evaluation of the homogeneity was based on visual observations. Inter-observer interquartile ranges were computed for the grading of the masses. For the other QA parameters, the mean inter- and intra-observer coefficients of variation (CoV) were calculated. In addition, the symmetry of the reverberations when imaging air with a clean transducer was checked. Results The mean inter-observer CoVs were: visualization depth 11 ± 4%, vertical distance 1.7 ± 0.4%, horizontal distance 1.4 ± 0.6%, axial resolution 22 ± 7%, and lateral resolution 16 ± 8%. The mean intra-observer values were about half of these values with similar standard deviations. The visual evaluation of the homogeneity and the symmetry of the reverberations produced false-positive findings in 5% of the cases, but were found useful in detecting a defective transducer. The grading of the masses had mean interquartile ranges of 20–30% of the grading scale. Conclusion The inter-observer variability in measuring phantom-based QA parameters can be relatively high. This should be considered when implementing a phantom-based QA protocol and evaluating the results.

PACS: a prerequisite for image fusion in nuclear medicine

Abstract Digital images of nuclear medicine (NM) are produced by a gamma camera. Compared with radiological images, the resolution of NM images is low. However, they reveal regional functional differences, whereas radiological images show high-resolution anatomical details. Combination of anatomical and functional image data from the same part of the body is important and can enhance the understanding of functional abnormalities. The purpose of this study was to introduce a picture archiving and communication system (PACS) for practical clinical use in the divisions of nuclear medicine. The effect of PACS on customizing image fusion in nuclear medicine was also evaluated. For routine use of image fusion, it is essential to have a fast image network for transferring images from different modalities and a PACS for storing them. It is also essential to have a common commitment to Digital Imaging and Communications in Medicine (Dicom) standard. Open-architecture PACS seems to remove the remaining difficulties in customising image fusion. The PACS introduced in the hospital district will be one of the largest in the world when completed in 2003.