Size estimation and magnification error in radiographic imaging: implications for classification of arteriovenous malformations.

PURPOSETo assess magnification error in digital subtraction angiography as it pertains to arteriovenous malformation (AVM) size.METHODSA rectangular grid phantom with equally spaced markers mounted in a stereotactic frame was imaged with digital angiographic equipment. The location and orientation of the grid was altered relative to the central plane of the phantom. Both linear and area measurements were made according to the perceived location of phantom markers using a standard catheter calibration technique and compared with stereotactically derived estimates. Finally, a single case example of an angiographically imaged rolandic AVM was used to compare linear dimensions obtained with both described techniques.RESULTSThe determination of location and size with standard angiographic imaging is subject to error because of the divergent geometry of the incident x-ray beam. The resulting nonconstant geometric magnification causes errors in linear measurements of 10% to 13% at depths of 7 cm from the calibration plane. Errors in area measurements at the same position increase by 20% to 25%. Measurements of maximum diameter or cross-sectional area may have an additional error when nonspherical objects are inclined to the viewing direction (40% at 45 degrees inclination). These errors are reduced to less than 1 mm using the stereotactic technique. Some commercial angiographic systems have internal software to enable a spatial calibration based on known distances in the image or on the diameter of a catheter. The catheter technique was accurate in the calibration direction (perpendicular to the catheter axis) but had a 12% error in the direction parallel to the catheter because of a nonunity aspect ratio in the video system. Measurement of the dimensions of a rolandic AVM using the catheter calibration technique had an error that ranged from -3% to +26% (standard error, 20%) with respect to the stereotactic technique.CONCLUSIONSNumerous nonstereotactic referential systems for determining linear distances are inherently erroneous by varying degrees compared with the stereotactic technique. Area and volume determinations naturally increase this error further. To the extent that no standardized method for determining linear distances exists, significant variations in estimation of AVM size result. Classification schemes for AVMs have been hampered by this technical error.     

Co-registration of x-ray and MR fields of view in a hybrid XMR system

PURPOSE: To validate one possible function of a real-time x-ray/MR (XMR) interface in a hybrid XMR system using x-ray images as "scouts" to prescribe the MR slices. MATERIALS AND METHODS: The registration process consists of two steps: 1) calibration, in which the system's geometric parameters are found from fiducial-based registration; and 2) application, in which the x-ray image of a target structure and the estimated geometric parameters are used to prescribe an MR slice to observe the target structure. Errors from the noise in the location of the fiducial markers, and MR gradient nonlinearity were studied. Computer simulations were used to provide guidelines for fiducial marker placement and tolerable error estimation. A least-squares-based correction method was developed to reduce errors from gradient nonlinearity. RESULTS: In simulations with both sources of errors and the correction for gradient nonlinearity, the use of 16 fiducial markers yielded a mean error of about 0.4 mm over a 7200 cm(3) volume. Phantom scans showed that the prescribed target slice hit most of the target line, and that the length visualized was improved with the least-squares correction. CONCLUSION: The use of 16 fiducial markers to co-register XMR FOVs can offer satisfactory accuracy in both simulations and experiments.     

Image mottle in abdominal CT

RATIONALE AND OBJECTIVES: To investigate image mottle in conventional CT images of the abdomen as a function of radiographic technique factors and patient size. METHODS: Water-filled phantoms simulating the abdomens of adult (32 cm in diameter) and pediatric (16 cm in diameter) patients were used to investigate image mottle in CT as a function of x-ray tube potential and mAs. CT images from 39 consecutive patients with noncontrast liver scans and 49 patients with iodine contrast scans were analyzed retrospectively. Measurements were made of the mean liver parenchyma Hounsfield unit value and the corresponding image mottle. RESULTS: For a given water phantom and x-ray tube potential, image mottle was proportional to the mAs-0.5. Increasing the phantom diameter from 16 cm (pediatric) to 32 cm increased the mottle by a factor of 2.4, and increasing the x-ray tube potential from 80 kVp to 140 kVp reduced the mottle by a factor of 2.5. All patients were scanned at 120 kVp, with no correlation between patient size and the x-ray tube mAs. The mean mottle level was 7.8 +/- 2.2 and 10.0 +/- 2.5 for the noncontrast and contrast studies, respectively. An increase in patient diameter of 3 cm would require approximately 65% more mAs to maintain the same level of image mottle. CONCLUSIONS: The mottle in abdominal CT images may be controlled by adjusting radiographic technique factors, which should be adjusted to take into account the size of the patient undergoing the examination.     

An option for optimising the radiographic technique for horizontal beam lateral (HBL) hip radiography when using digital X-ray equipment

AimTo investigate the optimum technique for the horizontal beam lateral (HBL) hip projection considering image quality and radiation dose.MethodsUsing digital radiography equipment an anthropomorphic phantom was positioned for a HBL projection of the hip. Radiographic exposures were undertaken across a range of acquisition parameters (tube potentials, source to image distances, object to detector distances, with and without an anti-scatter radiation grid/additional copper filtration). Each acquisition combination was imaged three times and the dose area product (DAP) and post-AEC mAs recorded. 168 images were acquired. A single observer evaluated five anatomical areas on all images using a two-alternative force choice technique. The reference image was selected based on the current locally accepted technique. 50 images out of the original 168 were independently assessed by a further four observers to ensure reliability of the results.ResultsImage quality, when comparing all the images to the reference, was improved on in two cases; however the radiation dose had increased. 18 images had equal image quality with some having an 80% reduction in the DAP. In terms of the diagnostic acceptability, 51 were considered acceptable with a lower radiation dose.ConclusionBy optimising acquisition factors for the HBL hip projection the radiation dose to the patient can be reduced. Based on the findings the factors proposed for HBL hip projections are 90 kVp, 135 cm SID, 45 cm ODD, grid and 0.1 mm copper filtration.     

Transendocardial delivery of extracellular myocardial markers by using combination X-ray/MR fluoroscopic guidance: feasibility study in dogs

PURPOSE: To demonstrate the feasibility of using a combination of x-ray fluoroscopic and magnetic resonance (MR) fluoroscopic (ie, x-ray/MR fluoroscopy) guidance for left ventricular (LV) catheterization and transendocardial delivery of extracellular tissue markers. MATERIALS AND METHODS: Experiments were performed in six dogs by using an x-ray/MR fluoroscopy system. The arterial guide wire and catheter were advanced into the heart with x-ray fluoroscopic guidance. The dogs were injected with 0.5, 1.0, and 2.0 mL of iohexol. For passive catheter tracking, a steady-state free precession MR imaging sequence was used. A steerable dual-lumen catheter was used to transendocardially inject a mixture of gadodiamide (0.05 mol/L) plus Evans blue dye (3%). An electrocardiographically gated dual-inversion-recovery MR imaging sequence was used to visualize the myocardial delivery of the gadodiamide-blue dye mixture. A high concentration of gadodiamide (0.5 mol/L) was used to demarcate the borders of the area of interest, or "hit the target." Blood pressure, heart rate, and oxygen saturation were measured before and after the intervention. Analysis of variance, Scheffé, and paired Student t tests were used for data analysis. RESULTS: LV catheterization via arterial access was feasible with two-dimensional x-ray fluoroscopic and three-dimensional MR fluoroscopic guidance. Delivery of the gadodiamide-blue dye mixture and the consequences of the procedure were monitored with MR imaging. Gadolinium-enhanced regions were bright on T1-weighted MR images, but they varied in size as a function of injectant volume. The mean sizes of these regions were 1.5% +/- 0.6 of the LV after the 0.5-mL injection of the mixture and 7.0% +/- 0.5 of the LV after the 2.0-mL injection (P <.001, Scheffé test). The corresponding mean sizes of the blue dye-enhanced regions were 2.3% +/- 0.6 and 8.3% +/- 0.4, respectively (P <.001). A high concentration of gadodiamide caused signal intensity loss around the gadolinium-enhanced regions. CONCLUSION: Transendocardial delivery of potential therapeutic solutions is feasible with x-ray/MR fluoroscopic guidance. The injection catheter can be navigated with MR imaging guidance to hit the target.     

Contrast-detail phantom study for x-ray spectrum optimization regarding chest radiography using a cesium iodide-amorphous silicon flat-panel detector

RATIONALE AND OBJECTIVES: The purpose of this study evaluating a cesium iodide-amorphous silicon-based flat-panel detector was to optimize the x-ray spectrum for chest radiography combining excellent contrast-detail visibility with reduced patient exposure. MATERIALS AND METHODS: A Lucite plate with 36 drilled holes of varying diameter and depth was used as contrast-detail phantom. For 3 scatter body thicknesses (7.5 cm, 12.5 cm, 21.5 cm Lucite) images were obtained at 113 kVp, 117 kVp, and 125 kVp with additional copper filter of 0.2 and 0.3 mm, respectively. For each setting, radiographs acquired with 125 kVp and no copper filter were taken as standard of reference. On soft-copy displays, 3 observers blinded to the exposure technique evaluated the detectability of each aperture in each image according to a 5-point scale. The number of points given to all 36 holes per image was added. The scores of images acquired with filtration were compared with the standard images by means of a multivariate analysis of variance. Radiation burden was approximated by referring to the entrance dose and calculated using Monte Carlo method. RESULTS: All 6 evaluated x-ray spectra resulted in a statistically equivalent contrast-detail performance when compared with the standard of reference. The combination 125 kVp with 0.3 mm copper was most favorable in terms of dose reduction (approximately 33%). CONCLUSION: Within the constraints of the presented contrast-detail phantom study simulating chest radiography, the CsI/a-Si system enables an addition of up to 0.3 mm copper filtration without the need for compensatory reduction of the tube voltage for providing constant image quality. Beam filtration reduces radiation burden by about 33%.     

Amorphous silicon, flat-panel, x-ray detector: reliability of digital image fusion regarding angle and distance measurements in long-leg radiography

RATIONALE AND OBJECTIVES: To evaluate composed long-leg images acquired with a large-area, flat-panel x-ray detector with regard to angle and distance measurements. METHODS: Radiographs of a long-leg phantom were acquired at 13 different angle settings with a 43-cm x 43-cm digital x-ray detector based on cesium iodide (CsI) and amorphous silicon (a-Si) technology. Three overlapping single images of the phantom were reconstructed at a workstation using a generalized correlation method. Four blinded observers were instructed to determine the angle of the axis of the long-legs as well as the length of "femur" and "tibia" on soft-copy displays. For that, the angle and distance measurement software integrated in the workstation was used. The images were analyzed with and without prior manual fine tuning of the primary composition result according to a mapped scale. Standard of reference was angle and distance determination at the phantom. RESULTS: On average, the difference between the observers' angle measurements and the standard of reference was 0.4 degrees for both images with and without prior manual correction. Regarding distance measurements, the average discrepancies to the standard were 0.2 cm (femur) and 0.1 cm (tibia) when analyzing images that had undergone manual fine tuning and 0.5 cm and 0.7 cm, respectively, for images without manual correction. CONCLUSIONS: The evaluated image fusion algorithm in conjunction with a 43-cm x 43-cm flat-panel detector is feasible regarding angle and distance measurements on long-leg images. In the case of inaccurate primary composition, results can be corrected easily by manual fine tuning.     

A novel registration method for interval change detection between two chest X-ray images with different rotation angles

RATIONALE AND OBJECTIVES: Registration is an important process to detect interval changes between two chest x-ray images. However, the conventional registration methods suffer from misregistration because of the difference in rotation angles of human body around an axis parallel to the x-ray films, such as anteroposterior inclination. Such difference causes permutation of the shadows between the two images, which makes registration difficult. This article proposes a novel registration method in cases where two chest x-ray images have different rotation angles. MATERIALS AND METHODS: Twelve x-ray images taken from a chest phantom and four chest photofluorograms of two patients were used to evaluate the performance. First, the proposed algorithm estimates the rotation angles of the body from the pair of two x-ray images based on the function describing the relationship between a point in the current image and that in the previous image, which is derived from a three-dimensional rotational model of the body. Then it aligns two images according to the function. RESULTS: From the results of estimating rotation angles, it was found that proposed method can estimate the angles with an error of less than 1 degrees. Then two physicians evaluated the subtraction images and confirmed that this approach makes it possible to detect the interval changes accurately even if there are permutations of shadows in the x-ray image. CONCLUSIONS: The proposed method is superior to the conventional one when two chest x-ray images have different rotation angles.     

Accurate geometric calibration in stepping-table digital subtraction angiography

Accurate measurements of vessel dimensions are desirable in many clinical applications. This work uses the known relative motion between X-ray source and the patient in stepping-table digital subtraction angiography (DSA) to provide an accurate geometric calibration for quantitative measurements. The method results in a calibration factor that converts the size of the object measured in pixels on the image to its size in millimetres. The main sources of error relate to: (i) the assessment of relative displacement of a structure in a series of images; (ii) patient motion throughout data acquisition; and (iii) image distortion. Error was evaluated both with a test object consisting of a large grid of ball bearings (2x2 cm spaced) and, in vivo, in five renal DSA examinations performed with identical catheters of known diameter. The calibration factor was calculated with 0.1% accuracy for the test object and at least 2% accuracy in vivo, even with breath holding and pulsatile motion. This demonstrates that the calculation of the calibration factor can be very accurate, and that the method we propose is capable of the submillimetre accuracy required for clinical studies if used in conjunction with an accurate measurement of the vessel size in pixels. In conclusion, accurate geometric measurements can be performed in stepping-table DSA, without the need for external reference objects.     

Digital image composition in long-leg radiography with a flat-panel detector: first clinical experiences

RATIONALE AND OBJECTIVES: The purpose of this study was to evaluate the image quality of composed long-leg examinations with a large-area, flat-panel x-ray detector. METHODS: Thirty-five consecutive patients were included in this study. All images were obtained with a kilovoltage setting identical with conventional radiographies of speed class 400; amperage values were reduced by 50% compared with standard dose. After acquisition, the images were transferred to a workstation where the whole image was reconstructed using a generalized correlation method. Images were presented to 3 observers. Examination quality was ranked on a 3-point scale: 1 = no manual adjustment necessary; 2 = composition required manual correction; 3 = no composition possible. RESULTS: Patient rankings were 31/35 (88.6%) in category 1, 3/35 (8.6%) in category 2, and 1/35 (2.8%) in category 3 (primarily due to an application error). CONCLUSIONS: The analysis of the first clinical examinations of long-leg radiographies with a 43 cm x 43 cm flat-panel detector demonstrates very good reliability of the digital image composition.     

Quantitative digital subtraction arteriography with a calibration catheter

Quantitative intraarterial digital subtraction arteriography (DSA) was performed using a calibration catheter with three distal metallic ring markers. The two outer markers were 50 mm apart, and the third marker was in the middle. Measurements of 54 vessel diameters of the abdominal aorta, renal, lumbar, and iliac arteries were performed in a comparison study with direct film arteriograms in 10 aortofemoral runoff studies. Diameter measurements were made by both the observer on hard copy DSA images and by a computer using modified semiquantitative software. Against measurements on film, which were used as the standard, deviations in measurement on digital images varied from 8 to 13% for arterial diameters <5 mm and from 2 to 6% for diameters ≥5 mm. Projectional artifacts caused 3% or less error. Knowing these variations in measurement is important in order to determine error tolerances for clinical applications. The calibration catheter serves as a convenient internal marker for DSA.     

Radionuclide image minification can compensate for coarse digitization: concise communication

Although it is common practice to digitize radionuclide images onto the finest matrix available, their low count densities and poor spatial resolution suggest that quite large pixels should be adequate. Observers find these large pixels visually obtrusive, but minification of the image can reduce this effect. Experiments reported here have investigated how minification (achieved by increasing viewing distance) affects the perceived quality of images digitized onto different sized matrices. Observers' subjective preference for different pixel sizes was measured at various viewing distances using clinical bone images as test patterns. An objective measure of image quality was made by comparing the detectability of computer-generated focal areas of increased activity both in simple noisy backgrounds and in clinical bone images. The results show that a 128 X 128 matrix is adequate when the image is 8 cm2 and is viewed from 1 and 2 m. A finer matrix failed to produce better results.     

Radiation protection system for interventional procedures of the upper extremity: evaluation in a phantom model

PURPOSE: To design a radiation protection system for interventional procedures of the upper extremity and to evaluate the effectiveness of the system. MATERIALS AND METHODS: The radiation protection system consisted of an image intensifier (I-I) hood and x-ray tube cover. The I-I hood encircled the I-I to protect against scattered radiation from the patient's upper extremity and the table. The I-I hood consisted of four components: a device for attaching the hoods, two acrylate hoods with an 0.50-mm Pb equivalent, and a lead curtain hood with a 0.35-mm Pb equivalent. The x-ray tube cover was constructed of lead curtain to protect against backscattering radiation from the catheter table. An extremity phantom was used to measure the scattered radiation around the angiographic apparatus. The scattering dose rates were measured with an ionization dosimeter with and without a radiation protection system. The heights of the measuring points from the floor were 50 cm (operator's lower limbs), 100 cm (operator's abdomen), and 150 cm (operator's head/neck). RESULTS: The dose rates were reduced most with the combination of the I-I hood and the x-ray tube cover. The x-ray tube cover was effective in reducing scattering when it was set close to the table and the phantom. The maximum percentage decreases in dose rates by the radiation protection system were 99% at 50 cm, 73% at 100 cm, and 100% at 150 cm. Therefore, at 50 cm and 150 cm, the high-dose-rate area around the angiographic apparatus was reduced almost completely by the radiation protection system. CONCLUSION: The radiation protection system for interventional procedures of the upper extremity was effective in reducing scattered radiation around the angiographic apparatus.     

Accurate analysis of blood vessel sizes and stenotic lesions using stereoscopic DSA system

We have developed a technique to determine accurately the magnification factor and three-dimensional orientation of a vessel segment from a stereoscopic pair of digital subtraction angiograms (DSA). Our DSA system includes a stereoscopic x-ray tube with a 25-mm focal spot shift. The magnification and orientation of a selected vessel segment are determined from the distance and direction of the focal spot shift and the stereoscopic discrepancy in image positions for that segment. Our results indicate that the accuracies of determining the magnification and orientation are less than 1% and approximately 5 degrees, respectively. After the magnification and orientation are determined accurately, an iterative deconvolution technique for the measurement of vessel image size is applied to the selected vessel segment. This iterative deconvolution technique provides the best estimate of vessel image size by taking into account the unsharpness of the digital system. With this technique, the vessel image size can be determined to an accuracy of approximately 1.0 mm, which corresponds to one third the pixel size of our DSA system. Information derived from stereoscopic analysis and iterative deconvolution thus allows accurate calculation of actual vascular dimensions from DSA images.     

Contrast and scatter in x-ray imaging

The effect of scattered radiation on x-ray image contrast is reviewed. Without scatter control, the information content of x-ray images is severely compromised. Typical grid performance in general radiography and mammography and grid selection considerations are presented. Contrast can be significantly improved and patient dose reduced if more efficient methods of scatter control can be developed. This can be accomplished in chest radiography with scanning slit and improved grid techniques, in mammography with multiple scanning slit techniques, and in abdominal radiography with scanning grid techniques.     

Newly designed multifunctional coil catheter for gastrointestinal intervention: feasibility determined by experimental study in dogs

RATIONALE AND OBJECTIVE: Conventional vascular catheters or sizing catheters are often inconvenient and time-consuming for gastrointestinal procedures. Our purpose was to evaluate the feasibility of a newly designed coil catheter by experimental study in dogs. MATERIALS AND METHODS: Two catheter models were fabricated using a stainless-steel coil covered by heat shrinkable tube. The distal uncovered coil part was not elongated in model A but was elongated in model B. We developed 3 different types in each model: types II and III had 65-cm and 240-cm nitinol wires on the coil surfaces, respectively, but no wire was attached in type I. On the middle covered coil part, multiple holes were made into which to inject contrast medium, and multiple radiopaque markers were attached. In an experimental study using 4 mongrel dogs, we evaluated the ability of the coil catheter to pass over a guide wire to 30 cm distal from the pylorus, the ability of contrast to pass through the injection holes, the visibility of the radiopaque markers, and the ability of the coil catheter to be pushed without a guide wire to 120 cm distal from the pylorus. RESULTS: All catheters were successfully passed to 30 cm distal from the pylorus. Contrast passage through the injection holes and visualization of the radiopaque markers were excellent. To advance to 120 cm distal from the pylorus, the average success rates of models A and B were 100% and 69%, respectively. In particular, model A type II showed the best results in average success rate and average number of trials to achieve success. CONCLUSION: This experimental study demonstrated the feasibility of the newly designed coil catheter for gastrointestinal intervention.     

Correction of an image size difference between positron emission tomography (PET) and computed tomography (CT) improves image fusion of dedicated PET and CT

AIM: Clinical work in software positron emission tomography/computed tomography (PET/CT) image fusion has raised suspicion that the image sizes of PET and CT differ slightly from each other, thus rendering the images suboptimal for image fusion. The aim of this study was to evaluate the extent of the relative image size difference between PET and CT and the impact of the correction of this difference on the accuracy of image fusion. METHODS: The difference in real image size between PET and CT was evaluated using a phantom study. Subsequently, 13 patients with cancer in the head/neck area underwent both CT and [(18)F]fluorodeoxyglucose PET in a custom-made mask for external beam radiotherapy, with multimodality markers for positional reference. The image size of PET relative to CT was determined by evaluating the distances between the markers in multiple directions in both scans. Rigid-body image fusion was performed using the markers as landmarks, with and without correction of the calculated image size difference. RESULTS: Phantom studies confirmed a difference in real image size between PET and CT, caused by an absolute error in PET image size calibration. The clinical scans demonstrated an average relative difference in image size of 2.0% in the transverse plane and 0.8% along the longitudinal axis, the PET images being significantly smaller. Image fusion using original images demonstrated an average registration error of 2.7 mm. This error was decreased to 1.4 mm after size correction of the PET images, a significant improvement of 48% (P<0.001). CONCLUSIONS: A significant deviation in PET image size may occur, either as a real image size deviation or as a relative difference from CT. Although possibly not clinically relevant in normal diagnostic procedures, correction of such a difference benefits image fusion accuracy. Therefore, it is advisable to calibrate the PET image size relative to CT before performing high-accuracy rigid-body image fusion.     

Truly Hybrid Interventional MR/X-Ray System: Investigation of in Vivo Applications

RATIONALE AND OBJECTIVES: The purpose of this study was to provide in vivo demonstrations of the functionality of a truly hybrid interventional x-ray/magnetic resonance (MR) system. MATERIALS AND METHODS: A digital flat-panel x-ray system (1,024(2) array of 200 microm pixels, 30 frames per second) was integrated into an interventional 0.5-T magnet. The hybrid system is capable of MR and x-ray imaging of the same field of view without patient movement. Two intravascular procedures were performed in a 22-kg porcine model: placement of a transjugular intrahepatic portosystemic shunt (TIPS) (x-ray-guided catheterization of the hepatic vein, MR fluoroscopy-guided portal puncture, and x-ray-guided stent placement) and mock chemoembolization (x-ray-guided subselective catheterization of a renal artery branch and MR evaluation of perfused volume). RESULTS: The resolution and frame rate of the x-ray fluoroscopy images were sufficient to visualize and place devices, including nitinol guidewires (0.016-0.035-inch diameter) and stents and a 2.3-F catheter. Fifth-order branches of the renal artery could be seen. The quality of both real-time (3.5 frames per second) and standard MR images was not affected by the x-ray system. During MR-guided TIPS placement, the trocar and the portal vein could be easily visualized, allowing successful puncture from hepatic to portal vein. CONCLUSION: Switching back and forth between x-ray and MR imaging modalities without requiring movement of the patient was demonstrated. The integrated nature of the system could be especially beneficial when x-ray and MR image guidance are used iteratively.     

Fast iterative algorithm for metal artifact reduction in X-ray CT

RATIONALE AND OBJECTIVES: The reduction of metal artifacts in x-ray computed tomography (CT) has important clinical applications. An iterative method adapted from the expectation maximization (EM) formula for emission CT was shown to be effective for metal artifact reduction, but its computational speed is slow. The goal of this project was to accelerate that iterative method for metal artifact reduction. MATERIALS AND METHODS: Using the row-action/ordered-subset (EM) formula for emission CT as a basis, the authors developed a fast iterative algorithm for metal artifact reduction. In each iteration of this algorithm, both reprojection from an intermediate image and backprojection from discrepancy data are performed. RESULTS: The feasibility of the fast iterative algorithm was demonstrated in numerical and phantom experiments. In comparison with the nonaccelerated iterative algorithm, the speed of iterative metal artifact reduction is improved by an order of magnitude given image quality in terms of visual inspection, I-divergence in the projection domain, and the euclidean distance in the image domain. CONCLUSION: The fast iterative algorithm corrects intermediate reconstruction according to subsets of projections and produces satisfactory image quality at a much faster speed than the previously published iterative algorithm. This algorithm has important potential in clinical applications, such as orthopedic, oncologic, and dental imaging.     

Correction of photon attenuation and collimator response for a body-contouring SPECT/CT imaging system.

(111)In-Capromab pendetide imaging is indicated for postprostatectomy patients at risk for residual or recurrent disease. However, this study is complicated by relatively long times for tumor uptake and background washout that require imaging to be performed several days after radiopharmaceutical administration. In addition, (111)In-capromab pendetide demonstrates uptake in normal structures that produce images that are interpreted best using correlation with anatomic imaging. Finally, the visual quality of radionuclide imaging can be improved with corrections for photon attenuation and for the geometric response of the radionuclide collimator. Therefore, we have evaluated the advantages of using a commercially available dual-modality SPECT/CT system. In this article, we evaluate a novel iterative reconstruction algorithm using the SPECT/CT data obtained from phantoms and (111)In-capromab pendetide patient studies. METHODS: Phantom data acquired with the dual-head SPECT camera were reconstructed using both filtered backprojection (FBP) and an iterative maximum-likelihood expectation maximization (MLEM) algorithm incorporating corrections for (a) attenuation coefficient at the effective energy of the radionuclide (either (99m)Tc or (111)In) and (b) collimator response based on experimentally measured depth-dependent spatial resolution of the camera. The collimator response model used the coregistered CT image to estimate the source-target distances produced by the patient-contouring logic of the SPECT camera. Spatial resolution was measured using SPECT images of 2 line sources and uniformity from a uniform cylindric tank. Clinical (111)In-capromab pendetide SPECT/CT data were acquired according to the radiopharmaceutical manufacturer's protocol. Region-of-interest (ROI) analysis of a transverse slice at the level of the sacral base produced mean, median, maximum, and minimum counts per pixel for bone marrow and surrounding soft-tissue ROIs. Ratios of the mean capromab pendetide uptake within marrow to uptake within soft tissue were compared for images reconstructed with FBP versus that obtained from the MLEM method with photon attenuation and collimator response corrections. RESULTS: The source-target distances reconstructed from the patient-specific CT image agreed well with the corresponding values recorded manually from the camera display unit. This information was incorporated into the iterative reconstruction algorithms and improved the quality of SPECT images from phantoms and patients versus SPECT images reconstructed without the depth-dependent collimator response model. Qualitatively, SPECT images reconstructed with corrections for photon attenuation and collimator response showed less background activity and improved target contrast compared with those images reconstructed with FBP. The target-to-background ratio (marrow uptake-to-soft-tissue uptake) was significantly better using MLEM reconstruction than with FBP when mean uptake values were measured. CONCLUSION: A priori anatomic data can be used to enhance the quality of the SPECT image when reconstructed using iterative techniques (e.g., MLEM) that use the CT data to produce a patient-specific attenuation map and a collimator response model based on the body contour produced during the SPECT acquisition.