Diagnostic accuracy of fracture detection in suspected non-accidental injury: the effect of edge enhancement and digital display on observer performance.

AIM: To compare the effect of varying degrees of edge enhancement and method of digital image display on fracture detection in suspected non-accidental injury (NAI). MATERIALS AND METHODS: Fifty radiographs from post-mortem skeletal surveys in 13 children with suspected NAI were selected. Images were obtained using a Fuji 5000R computed radiography system. Hard copies were printed with edge enhancement factors 0, 0.5 and 1.2. Images (edge enhancement 0.5) were also displayed on a 1K(2) monitor. Six observers independently evaluated all 200 images for the presence of abnormality. Observers also scored each image for visualization of soft tissues, visualization of trabecular markings and overall image quality. The paired Student's t-test and location receiver operating curve (ROC) analysis were used to compare quality scores and diagnostic accuracy of each display method. Individual and pooled true-positive rates (sensitivity) were determined. For the purposes of ROC analysis, histology was taken as the gold standard. RESULTS: There was no difference in duration of hard and soft-copy reading sessions (p=0.76). After image manipulation soft-copy radiographs scored significantly better for image quality than hard copy (p<0.0001). Pooled observer sensitivity (at a specificity of 90%) was below 50% for all display methods. Diagnostic accuracy varied significantly between observers. Diagnostic accuracy of individual observers was not affected by display method. CONCLUSION: In suspected NAI, diagnostic accuracy of fracture detection is generally low. Diagnostic accuracy appears to be affected more by observer-related factors than by the method of digital image display.

Optimal exposure parameters for digital radiography of the infant skull: a pilot study.

AIM: To determine optimal exposure parameters when performing digital skull radiographs in infants with suspected non-accidental injury (NAI). METHOD: Anteroposterior and lateral post-mortem skull radiographs of six consecutive infants with suspected NAI were made at six exposure levels for each projection. Entrance surface doses ranged from 75-351 microGy. Exposures were made with a Fuji 5000R computed radiography system onto a standard resolution imaging plate. In three patients exposures were repeated using a high-resolution imaging plate. Hard copy images with an edge-enhancement factor of 0.5 were produced. Six observers assessed and scored the radiographs from 1=poor to 5=excellent for visualization of five criteria. The criteria scored included outer table of skull vault, inner table of skull vault, suture margins, vascular markings and soft tissues of the scalp. Radiographs were then ranked in order of overall image quality. Film density and sensitivity values were recorded. Local research committee approval was obtained. RESULTS: Current parameters give an average entrance surface dose of 253 microGy and 246 microGy for anteroposterior and lateral radiographs, respectively. The study demonstrated no perceived improvement in image quality above an entrance surface dose of 200 microGy (80% of current dose) or by the use of a high-resolution imaging plate. CONCLUSION: The potential exists to reduce radiation exposure in infants. A study has commenced to determine the effects of dose reduction on diagnostic accuracy in suspected NAI.

Analysis of the distribution of MRI contrast agents in the livers of small animals by means of complementary microscopies.

BACKGROUND: Magnetic resonance imaging (MRI) contrast agents contain magnetic molecules such as iron (Fe) or gadolinium (Gd) that are injected in vivo into rats or mice to study their distribution inside the liver. Fluorescent europium (Eu) can be used as a model of Gd to obtain comparable information of this distribution of corresponding contrast agents. In a similar approach, Fe can be attached to Texas Red and used as a model of ferumoxides and be detected by fluorescence. METHODS: To combine and compare the advantages of different microscopic imaging modes, characterization studies were carried out by means of a confocal laser scanning microscope (CLSM), a secondary ion mass spectrometric (SIMS) microscope, and an electron energy loss spectrometric (EELS) microscope. In the case of CLSM, the locations of fluorescent signals inside preparations were determined by factor analysis of biomedical image sequences (FAMIS) and selection of image sequences at emission. RESULTS: By CLSM and FAMIS, we distinguished chelated Eu and Texas Red attached to Fe. By SIMS microscopy, we distinguished Eu and Gd of chlorides and chelates and Fe of a ferumoxide. By EELS microscopy, we distinguished Eu and Gd of chlorides. CONCLUSIONS: Analysis of compounds inside correlative specimens by means of CLSM, SIMS, and EELS microscopes provided complementary results.

Eliciting a terminology for mammographic calcifications.

AIM: Two studies were carried out to establish, validate and assess descriptors for use in the differential diagnosis for mammographic calcifications. METHODS: In Study 1, eleven radiologists were asked to 'think out loud' as they interpreted 20 sets of calcifications. Participants used 159 terms to describe calcifications. We used this data to design a scheme with 50 descriptors. In Study 2, ten radiologists used the scheme to describe 40 sets of calcifications. We assessed the capacity of the terms to discriminate between benign and malignant calcifications, testing them against radiologists' assessments of malignancy and follow-up data. RESULTS: All descriptors were used by at least 5 radiologists. Five additional descriptors were required. With some exceptions, properties that discriminated between benign and malignant outcomes were highly correlated with radiologists' assessment of risk. Many descriptors have a fairly low sensitivity but high specificity. CONCLUSIONS: Our data suggest that radiologists consider a wide range of features than is included in existing reporting schemes. Our scheme allows a richer characterization of calcifications, potentially improving the reporting and understanding of these abnormalities.

Solid state detectors in nuclear medicine.

Since Nuclear Medicine diagnostic applications are growing fast, room temperature semiconductor detectors such CdTe and CdZnTe either in the form of single detectors or as segmented monolithic detectors have been investigated aiming to replace the NaI scintillator. These detectors have inherently better energy resolution that scintillators coupled to photodiodes or photomultiplier tubes leading to compact imaging systems with higher spatial resolution and enhanced contrast. Advantages and disadvantages of CdTe and CdZnTe detectors in imaging systems are discussed and efforts to develop semiconductor-based planar and tomographic cameras as well as nuclear probes are presented.

Advances in computers and image processing with applications in nuclear medicine.

The continuing advances in hardware performance had made many previously computationally unattractive methods feasible, an example being iterative reconstruction in tomography, which is now routine. Dynamic SPECT can also be performed. However the aim of image processing is not just to produce pretty pictures, but to extract good clinical information. The methods also need to incorporate clinical knowledge and be defined using clinical constraints. In general data in nuclear medicine are n-D, often 3-D plus time. Data reduction for example by the extraction of physiological information, is important. Such data are in any case hard to visualise without compression, for example some kind of dimensionality reduction, going from n-D to a 2-D "functional" image. Both linear and non-linear operations can be considered. To extract physiological data, we need to fit models. Two classes of method are important: data driven and hypothesis driven. Examples of data driven methods are principal component analysis and factor analysis, where the model is derived form the data. Hypothesis driven methods are all implicitly or explicitly based on model fitting. A preliminary data driven step followed by an hypothesis driven approach could be called constrained statistical image analysis. Examples are shown as used in nuclear medicine and are being extended to MRI. Another important problem considered is that of multi-modality image registration and fusion. Although many methods exist, all based on the minimisation of an appropriate distance functions between 2 image data sets such as mutual information, additional constraints are required when the images are not so similar. Additional constraints can be imposed by means of cluster analysis of the n-dimensional feature space. In the analysis of such data, tests against reference data sets (atlases) are required, normally requiring warping the data sets in space, for example by the use of optic flow, or some kind of diffusion equation. Real time analysis of data during acquisition can lead to optimisation of acquisition procedures. Incorporation of such image analysis into a decision support system is desirable.

Validation of myocardial perfusion reserve measurements using regularized factor images of H(2)(15)O dynamic PET scans.

UNLABELLED: The use of H(2)(15)O PET scans for the measurement of myocardial perfusion reserve (MPR) has been validated in both animal models and humans. Nevertheless, this protocol requires cumbersome acquisitions such as C(15)O inhalation or (18)F-FDG injection to obtain images suitable for determining myocardial regions of interest. Regularized factor analysis is an alternative method proposed to define myocardial contours directly from H(2)(15)O studies without any C(15)O or FDG scan. The study validates this method by comparing the MPR obtained by the regularized factor analysis with the coronary flow reserve (CFR) obtained by intracoronary Doppler as well as with the MPR obtained by an FDG acquisition. METHODS: Ten healthy volunteers and 10 patients with ischemic cardiopathy or idiopathic dilated cardiomyopathy were investigated. The CFR of patients was measured sonographically using a Doppler catheter tip placed into the proximal left anterior descending artery. The mean velocity was recorded at baseline and after dipyridamole administration. All subjects underwent PET imaging, including 2 H(2)(15)O myocardial perfusion studies at baseline and after dipyridamole infusion, followed by an FDG acquisition. Dynamic H(2)(15)O scans were processed by regularized factor analysis. Left ventricular cavity and anteroseptal myocardial regions of interest were drawn independently on regularized factor images and on FDG images. Myocardial blood flow (MBF) and MPR were estimated by fitting the H(2)(15)O time-activity curves with a compartmental model. RESULTS: In patients, no significant difference was observed among the 3 methods of measurement-Doppler CFR, 1.73 +/- 0.57; regularized factor analysis MPR, 1.71 +/- 0.68; FDG MPR, 1.83 +/- 0.49-using a Friedman 2-way ANOVA by ranks. MPR measured with the regularized factor images correlated significantly with CFR (y = 1.17x - 0.30; r = 0.97). In the global population, the regularized factor analysis MPR and FDG MPR correlated strongly (y = 0.99x; r = 0.93). Interoperator repeatability on regularized factor images was 0.126 mL/min/g for rest MBF, 0.38 mL/min/g for stress MBF, and 0.34 for MPR (19% of mean MPR). CONCLUSION: Regularized factor analysis provides well-defined myocardial images from H(2)(15)O dynamic scans, permitting an accurate and simple measurement of MPR. The method reduces exposure to radiation and examination time and lowers the cost of MPR protocols using a PET scanner.

Mimos: a description framework for exchanging medical image processing results.

Image processing plays increasingly important role in using medical images, both for routine as for research purposes, due to the growing interest in functional studies (PET, MR, etc.). Unfortunately, there exist nearly as many formats for data and results coding as image processing procedures. If Dicom presently supports a kind of structured reporting of image studies, it does not take into account the semantics of the image handling domain. This can impede the exchange and the interpretation of processing results. In order to facilitate the use of image processing results, we have designed a framework for representing image processing results. This framework, whose principle is called an "ontology" in the literature, extends the formalism, which we have used in our previous work on image databases. It permits a systematic representation of the entities and information involved in the processing, that is not only input data, command parameters, output data, but also software and hardware descriptions, and relationships between these different parameters. Consequently, this framework allows the building of standardized documents, which can be exchanged amongst various users. As the framework is based on a formal grammar, documents can be encoded using XML. They are thus compatible with Internet / Intranet technology. In this paper, the main characteristics of the framework are presented and illustrated. We also discuss implementation issues in order to be able to integrate documents, and correlated images, handling these with a classical Web browser.

Adaptive and self-evaluating registration method for myocardial perfusion assessment.

With the advent of ultra-fast MRI, it is now possible to assess non-invasively regional myocardial perfusion with multislice coverage and sub-second temporal resolution. First-pass contrast enhanced studies are acquired with ECG-triggering and breath holding. Nevertheless, some respiratory induced movements still remain. Myocardial perfusion can be assessed locally by parametric imaging methods such as Factor Analysis of Medical Image Sequences (FAMIS), provided that residual motion can be corrected. An a posteriori registration method implemented in the image domain is proposed. It is based on an adaptive registration model of the heart combining three elementary shapes (left ventricle, right ventricle and pericardium). The registration procedure is performed on a potential map derived from the distance map. To evaluate the quality of the registration procedure a superimposition score between the registration model and the contour automatically extracted in the sequence is proposed. Rigid transformation hypotheses and registration analysis provide an efficient and automatic method which allows the rejection of outlier images, such as: out of synchronisation images, out of plane acquisitions. When compared to a manual registration method, this approach reduces processing time and requires a minimal intervention from the operator. The proposed method performs registration with a subpixel accuracy. It has been successfully applied to simulated images and clinical data. It should facilitate the use of MR first-pass perfusion studies in clinical practice.

Confocal image characterization of human papillomavirus DNA sequences revealed with Eu in HeLa cell nuclei stained with Hoechst 33342.

OBJECTIVE: To visualize and localize specific viral DNA sequences revealed with Eu by fluorescence in situ hybridization, confocal laser scanning microscopy (CLSM) and factor analysis of biomedical image sequences (FAMIS). STUDY DESIGN: Human papillomavirus DNA (HPV-DNA) was identified in HeLa cells with biotinylated DNA probes recognizing HPV-DNA types 16/18. DNA-DNA hybrids were revealed by a three-step immunohistochemical amplification procedure involving an antibiotin mouse monoclonal antibody, a biotinylated goat antimouse polyclonal antibody and streptavidin-Eu. Cell nuclei were counterstained with Hoechst 33342. Image sequences were obtained using a CLSM that made possible ultraviolet excitation. The location of fluorescent signals inside cellular preparations was determined by FAMIS and selection of filters at emission. Image sequences were summarized into a reduced number of images, or factor images, and curves, or factors. Factors estimate spectral or temporal patterns and depth emission profiles. Factor images correspond to spatial distributions of the different factors. RESULTS: We distinguished between Eu corresponding to HPV-DNA hybridization signals and nuclear staining by taking into account differences in their spectral and temporal patterns and (using their decay rates). CONCLUSION: FAMIS, together with CLSM and Eu, made possible the detection and characterization of viral papillomavirus DNA sequences in HeLa cells.

Confocal analysis of phosphatidylserine externalization with the use of biotinylated annexin V revealed with streptavidin-FITC, -europium, -phycoerythrin or -Texas Red in oxysterol-treated apoptotic cells.

OBJECTIVE: To analyze externalization of phosphatidylserine via annexin V on apoptotic cells by laser scanning confocal microscopy and factor analysis of biomedical image sequences (FAMIS). STUDY DESIGN: Streptavidin-fluorescein isothiocyanate (FITC), -europium (Eu), -phycoerythrin (PE) and -Texas Red (TR) were chosen to reveal the binding of biotinylated annexin V on apoptotic U937 human leukemic cells and ECV-304 human endothelial cells induced under treatment with 7-ketocholesterol or 7 beta-hydroxycholesterol. Excitation of each fluorochrome was obtained by selection of specific lines (351 + 364 nm, 488 nm) of the argon laser of a confocal microscope. Temporal and spectral series were performed to characterize each fluorochrome. FAMIS was applied to these series to estimate images corresponding to stains. RESULTS: Each fluorochrome was clearly distinguished, and images showed localization of phosphatidylserine, which was improved by image analysis. CONCLUSION: On apoptotic cells it is possible to analyze differences in the improved visualization of phosphatidylserine in series processed by FAMIS with the use of biotinylated annexin V revealed with streptavidin-FITC, -Eu, -PE or -TR.

CADMIUM II: acquisition and representation of radiological knowledge for computerized decision support in mammography.

CADMIUM II is a system for the interpretation of mammograms. A novel aspect of the system is that it combines symbolic reasoning with image processing, in contrast with most other approaches, which use only image processing and rely on artificial neural networks (ANNs) to classify mammograms. A problem of ANNs is that the advice they give cannot be traced back to communicable diagnostic inferences. Our approach is to provide advice based on explicit knowledge about the diagnostic process. To this end, we have conducted a knowledge elicitation study which looked at the descriptors used by expert radiologists when making diagnostic decisions about mammograms. The analysis of the radiologists' reports yielded a set of salient diagnostic features. These were used to inform the advice provided by the symbolic decision making component of CADMIUM II.

Should scatter be corrected in both transmission and emission data for accurate quantitation in cardiac SPET?

Ideally, reliable quantitation in single-photon emission tomography (SPET) requires both emission and transmission data to be scatter free. Although scatter in emission data has been extensively studied, it is not well known how scatter in transmission data affects relative and absolute quantitation in reconstructed images. We studied SPET quantitative accuracy for different amounts of scatter in emission and transmission data using a Utah phantom and a cardiac Data Spectrum phantom including different attenuating media. Acquisitions over 180 degrees were considered and three projection sets were derived: 20% images and Jaszczak and triple-energy-window scatter-corrected projections. Transmission data were acquired using gadolinium-153 line sources in a 90-110 keV window using a narrow or wide scanning window. The transmission scans were performed either simultaneously with the emission acquisition or 24 h later. Transmission maps were reconstructed using filtered backprojection and mu values were linearly scaled from 100 to 140 keV. Attenuation-corrected images were reconstructed using a conjugate gradient minimal residual algorithm. The mu value underestimation varied between 4% with a narrow transmission window in soft tissue and 22% with a wide window in a material simulating bone. Scatter in the emission and transmission data had little effect on the uniformity of activity distribution in the left ventricle wall and in a uniformly hot compartment of the Utah phantom. Correcting the transmission data for scatter had no impact on contrast between a hot and a cold region or on signal-to-noise ratio (SNR) in regions with uniform activity distribution, while correcting the emission data for scatter improved contrast and reduced SNR. For absolute quantitation, the most accurate results (bias <4% in both phantoms) were obtained when reducing scatter in both emission and transmission data. In conclusion, trying to obtain the same amount of scatter in emission and transmission data, in addition to being impractical because of the difficulty in knowing the precise scatter components, did not yield such accurate absolute activity quantitation as when emission and transmission scatter were reduced.

Relative impact of scatter, collimator response, attenuation, and finite spatial resolution corrections in cardiac SPECT.

UNLABELLED: We determined the relative effect of corrections for scatter, depth-dependent collimator response, attenuation, and finite spatial resolution on various image characteristics in cardiac SPECT. METHODS: Monte Carlo simulations and real acquisition of a 99mTc cardiac phantom were performed under comparable conditions. Simulated and acquired data were reconstructed using several correction schemes that combined different methods for scatter correction (3 methods), depth-dependent collimator response correction (frequency-distance principle), attenuation correction (nonuniform Chang correction or within an iterative reconstruction algorithm), and finite spatial resolution correction (use of recovery coefficients). Five criteia were considered to assess the effect of the processing schemes: bull's-eye map (BEM) uniformity, contrast between the left ventricle (LV) wall and the LV cavity, spatial resolution, signal-to-noise ratio (SNR), and percent errors with respect to the known LV wall and liver activities. RESULTS: Similar results were obtained for the simulated and acquired data. Scatter correction significantly improved contrast and absolute quantitation but did not have noticeable effects on BEM uniformity or on spatial resolution and reduced the SNR. Correction for the depth-dependent collimator response improved spatial resolution from 13.3 to 9.5 mm in the LV region, improved absolute quantitation and contrast, but reduced the SNR. Correcting for attenuation was essential for restoring BEM uniformity (78% and 89% without and with attenuation correction, respectively [ideal value being 100%]) and accurate absolute activity quantitation (errors in estimated LV wall and liver activity decreased from 90% without attenuation correction to approximately20% with attenuation correction only). Although accurate absolute activity quantitation was achieved in the liver using scatter and attenuation corrections only, correction for finite spatial resolution was needed to estimate LV wall activity within 10%. CONCLUSION: The respective effects of corrections for scatter, depth-dependent collimator response, attenuation, and finite spatial resolution on different image features in cardiac SPECT were quantified for a specific acquisition configuration. These results give indications regarding the improvements to be expected when using a specific processing scheme involving some or all corrections.

Improved estimation of velocity and flow rate using regularized three-point phase-contrast velocimetry.

We improved the three-point phase-contrast method by regularization of MR velocity data after acquisition of a low velocity-to-noise ratio (VNR) velocity image and a high VNR aliased velocity image. The phase unwrapping algorithm is based on the assumed correlation of the velocity of adjacent flow voxels on the low VNR and the unaliased high VNR images. We used Fourier encoding with eight velocity-encoding gradient steps to obtain reference velocity images of the aorta from five subjects (274 images) and compared them with the phase-contrast and three-point phase-contrast images with and without regularization. The VNR of the regularized velocity image was improved by 9.1 dB and the VNR of the three-point phase-contrast velocity image was improved by 0.7 dB with respect to the low first moment velocity image. Corresponding improvements of 9 dB and 3.7 dB were obtained for the estimations of instantaneous flow rate. Magn Reson Med 44:122-128, 2000.

[Towards the integration of the digital medical image folder within the computerized patient folder: PACS and image networks]. / Vers l'intégration du dossier d'imagerie au dossier patient informatisé : PACS et réseaux d'images.

Medical Images are components of the so-called "Medical Imaging Folder". This folder is a subset of the so-called "Medical Folder", part of the "Patient Folder. The G8 promotes the concept of a "Global Information Society for Health. Within this society, the Patient Folder is seen either from a healthcare organization, from a country or from an international point of view. The Global Patient Folder (Healthcare Folder) is composed of the different Patient Folder instances. Presently, Pacs and Telemedicine are no longer concerned only by storage and transmission issues. The medical images have only meaning associated with their context, the patient healthcare status. This context is rich in information provided by various information systems or healthcare professionals. The different data are linked and time dependant. Therefore, the expert community in the field of patient records argues that the approach must be the integration of Medical Images within the patient folder. It appears clearly that the complete deployment of such an "International Healthcare Folder" needs time and will proceed in several steps. Due to the increase of the people's mobility this deployment is inescapable. Infrastructure must be sized up taking into account the Digital Medical Image spreading and its large data volume which necessitates a large bandwith. In this paper, we detail the Medical Image Folder concept and its position within the Patient Folder and the Healthcare Folder. Then we present PACS, networking and Telemedicine concepts as well as the needs in standards.

Experimental comparison of data transformation procedures for analysis of principal components.

Results of principal component analysis depend on data scaling. Recently, based on theoretical considerations, several data transformation procedures have been suggested in order to improve the performance of principal component analysis of image data with respect to the optimum separation of signal and noise. The aim of this study was to test some of those suggestions, and to compare several procedures for data transformation in analysis of principal components experimentally. The experiment was performed with simulated data and the performance of individual procedures was compared using the non-parametric Friedman's test. The optimum scaling found was that which unifies the variance of noise in the observed images. In data with a Poisson distribution, the optimum scaling was the norm used in correspondence analysis. Scaling mainly affected the definition of the signal space. Once the dimension of the signal space was known, the differences in error of data and signal reproduction were small. The choice of data transformation depends on the amount of available prior knowledge (level of noise in individual images, number of components, etc), on the type of noise distribution (Gaussian, uniform, Poisson, other), and on the purpose of analysis (data compression, filtration, feature extraction).

Spatial regularization of flow patterns in magnetic resonance velocity mapping.

A technique dedicated to spatial regularization of magnetic resonance (MR) velocity data has been implemented to improve flow image quality. It is assumed that neighboring flow-velocity pixels are partially correlated, although large-velocity discontinuities remain possible. Increasing MR signal magnitude due to the in-flow effect also is used to enhance further reliability of the estimated velocity. By using an eight-step Fourier-encoding approach, 162 "reference" velocity images acquired in the ascending aorta from six healthy volunteers were compared with "raw" and "regularized" images that were computed from only two gradient steps. The mean square error decreased from 0.12 m(2) x s(-2) to 0.06 m(2) x s(-2) (P < 10-9) for velocity pixel values and from 1929 ml(2) x s(-2) to 1336 ml(2) x s(-2) (P < 0.01) for instantaneous flow rates. The regularization of two-step data sets provides the same velocity image quality as that found after using three-step data sets without regularization. The method can be applied to phase-velocity data sets of any MR technique to reduce velocity noise. J. Magn. Reson. Imaging 1999;10:851-860.

Spatial regularization applied to factor analysis of medical image sequences (FAMIS).

Dynamic image sequences allow physiological mechanisms to be monitored after the injection of a tracer. Factor analysis of medical image sequences (FAMIS) hence creates a synthesis of the information in one image sequence. It estimates a limited number of structures (factor images) assuming that the tracer kinetics (factors) are similar at each point inside the structure. A spatial regularization method for computing factor images (REG-FAMIS) is proposed to remove irregularities due to noise in the original data while preserving discontinuities between structures. REG-FAMIS has been applied to two sets of simulations: (a) dynamic data with Gaussian noise and (b) dynamic studies in emission tomography (PET or SPECT), which respect real tomographic acquisition parameters and noise characteristics. Optimal regularization parameters are estimated in order to minimize the distance between reference images and regularized factor images. Compared with conventional factor images, the root mean square error between regularized images and reference factor images is improved by 3 for the first set of simulations, and by about 1.5 for the second set of simulations. In all cases, regularized factor images are qualitatively and quantitatively improved.

Respective roles of scatter, attenuation, depth-dependent collimator response and finite spatial resolution in cardiac single-photon emission tomography quantitation: a Monte Carlo study.

The purpose of this study was to investigate the relative influence of scatter, attenuation, depth-dependent collimator response and finite spatial resolution upon the image characteristics in cardiac single-photon emission tomography (SPET). An acquisition of an anthropomorphic cardiac phantom was performed together with corresponding SPET Monte Carlo simulations. The cardiac phantom and the Monte Carlo simulations were designed so that the effect of scatter, attenuation, depth-dependent collimator response and finite spatial resolution could be studied individually and in combination. The impact of each physical effect and of combinations of effects was studied in terms of absolute and relative quantitative accuracy, spatial resolution and signal-to-noise ratio (SNR) in the resulting images. No corrections for these effects were assessed. Results obtained from Monte Carlo simulations and real acquisitions were in excellent agreement. Attenuation introduced about 90% activity underestimation in a 10-mm-thick left ventricle wall while finite spatial resolution alone introduced about 30% activity underestimation. Scatter had a negligible impact on quantitative accuracy in the recontructed slices when attenuation was present. Neither bull's eye map homogeneity nor contrast between a hot and a cold region were affected by depth-dependent collimator response or finite spatial resolution. Bull's eye map homogeneity was severely affected by attenuation but not by scatter. Attenuation and scatter reduced contrast by about 20% each. Both attenuation and scatter increased the full-width at half-maximum (FWHM) characterizing the spatial resolution of the imaging system by approximately 1 mm each but the main effect responsible for the observed 11-mm FWHM spatial resolution was the depth-dependent collimator response. SNR was reduced by a factor of approximately 2.5 because of attenuation, while scattered counts increased SNR by approximately 10%. In conclusion, the quantification of the relative influence of the different physical effects showed that attenuation is definitely the major phenomenon affecting cardiac SPET imaging accuracy, but that finite spatial resolution, scatter and depth-dependent collimator response also contribute significantly to the errors in absolute and relative quantitation and to the poor spatial resolution.