Susanne Dale

Inherent limitations in ectomography

Ectomography, a general tomographic method currently being implemented in nuclear medicine, is described. The object is viewed from different directions from the same projection angle, and the reconstruction process can be described as a two-dimensional filtered back-projection technique. One limitation of this limited-angle approach is that a three-dimensional Fourier representation of the acquired projections has a cone where data are missing. This empty cone can cause distortions in the tomogram by insufficient elimination of details outside the reconstructed tomographic section and by distortion of details in the section. The degree of distortion is dependent on the extension of the object in all directions and on the projection angle. Despite these limitations ectomography can be superior to single-photon-emission computed tomography for imaging sections that are close and parallel to the surface of the object, as in imaging of the hip joint or the heart.

Myocardial perfusion after coronary artery bypass surgery. A study using ectomographic myocardial scintigraphy and adenosine provocation.

A study was conducted to determine the time dependency of myocardial perfusion improvement after coronary artery bypass graft (CABG) surgery. Seventeen 3-vessel diseased patients (16 male, 1 female) scheduled for CABG surgery from a cardiac surgical and intensive-care unit were examined. Ten of the 17 patients returned for examination after 1 year. A titrated adenosine infusion was used to expose reversible ischemia. Tc99m-sestamibi was injected at rest and at maximum adenosine infusion rate, and isotope distribution was determined using ectomographic myocardial scintigraphy. Visually scored percent isotope uptake defect size and percent uptake reduction were assessed. It was found that resting isotope uptake defects were unchanged 1 h after surgery, increased in severity after 1 week, and after 1 year were 24% less than the preoperative scores (p < 0.01) and 55% less than after 1 week (p < 0.001). It was found that adenosine infusion induced a 57% increase in average defect score preoperatively (p < 0.001) but no increase postoperatively. No differences were seen between regions supplied by arterial or venous grafts. Isotope uptake defects increased between 1 h and 1 week after CABG surgery, and after 1 year the scores were less than those recorded preoperatively and after 1 week. Adenosine-induced reversible isotope uptake changes seen preoperatively were eliminated postoperatively in all vessel regions.

748-2 Tomographic Myocardial Perfusion Studies in the Intensive Care Unit Using a Mobile Gamma Camera System

A tomographic technique called Ectomography has been developed to enable bedside evaluation of myocardial perfusion during the acute phase of a myocardial infarction or immediately following coronary by-pass surgery (CABG). Ectomography can be implemented on a mobile gamma camera system using a rotating slant hole collimator, which allows three-dimensional periusion studies within the intensive care unit. A prototype mobile system has been designed and built in our departments. This system is currently undergoing clinical evaluation at the Karolinska hospital. During the first eight months of operation, more than 150 periusion studies have been performed. Currently, a second generation prototype, comparable in size to a modern mobile x-ray cart, is being developed and built. Results from the following three studies will be presented: 1) In an initial comparative study using Tc-99m Cardiolite® and a two day protocol, 19 patients with suspected coronary artery disease were imaged under the same conditions with SPECT and Ectomography. In a blind evaluation of short axis view sections and polar tomograms, the diagnosis were in agreement in 90% of the patients. 2) In an animal study using Tc-99m Myoview ® , 10 pigs were imaged before, during and after occlusion of the left anterior descending artery (LAD); staining and autoradiography were also periormed. Regions of reduced perfusion in Ectomographic section images corresponded well to areas of ischemia and infarction as shown by staining and autoradiogram. 3) The dynamics of reversible myocardial perfusion defects after CABG were studied in 12 patients using Tc-99m Cardiolite ® and adenosine provocation the day before, immediately after and one week after CABG. CABG surgery eliminated coronary steal revealed preoperatively with Ectomography and restored flow reserve in all patients immediately postoperatively, but reversible uptake defects remained after one week in half of the patients. In conclusion, results from studies performed show that a mobile gamma camera system based on Ectomography is an important research instrument and potential clinical tool for the objective evaluation of the effects of intervention on myocardial perfusion.

Cerebral blood flow imaging using ectomography-a feasibility study

The aim of this study was to investigate the feasibility of performing cerebral blood flow (CBF) scintigraphy using the 3D method of Ectomography. Ectomography is a limited view angle method and has been implemented on a mobile system which can be used bedside for acute studies. Simulations (ideal conditions), phantom experiments, and six patient studies have been carried out with the mobile system using a 40/spl deg/ rotating slant hole collimator (general purpose). Reconstructed section images from simulations and patient studies have been compared to SPECT. Ectomographic and SPECT simulations give similar images. Phantom experiments using a 3D brain phantom show that the optimal acquisition geometry is with the detector parallel to the orbito-meatal plane. In patient studies, the Ectomographic images were similar to the SPECT images obtained using a collimator with ultra high resolution. There were however differences which have to be investigated further. In conclusion, the results obtained indicate that it is feasible to use Ectomography for CBF studies, especially if a high resolution collimator is used.

Collimator design and manufacturing for a mobile tomographic gamma camera system based on ectomography

Tomographic scintigraphy such as conventional single photon emission tomography (SPECT), ectomography, fan and cone beam SPECT require high quality collimators to avoid image artifacts and to achieve designs differing from the standard parallel hole collimator. Ectomography is a tomographic method implemented as a mobile system. Improvements in the performance of a mobile gamma camera system based on ectomography can be obtained with segmented collimators. The sensitivity of the gamma camera system is increased with a factor equal to the number of segments of the collimator compared to an non-segmented collimator. This makes it possible to reduce acquisition time or reduce radiation dose to the patient without degrading image quality. Comparison with patient myocardial perfusion data gives that it would, in most cases, be possible to use a slant angle of 40/spl deg/ and a segmentation of the collimator into 4 segments. However, the present manufacturing technology limits the maximum slant angle to approximately 30/spl deg/ and therefore a new manufacturing technique for collimators has been developed. A mould for casting is produced by making slits by wire electrical discharge machining. Accuracy, resolution and sensitivity of such a small scale collimator prototype is higher than or equivalent to a conventional cast collimator. The proposed method is found to be especially suitable for producing complex collimators. Since the new proposed technique results in better accuracy of the final collimator than conventional manufacturing methods does, it holds a promise to less artifacts in nuclear imaging, such as in ectomography and SPECT.

A mobile tomographic gamma camera system for acute studies based on Ectomography

A mobile tomographic gamma camera system, called Cardiotom, has been developed for imaging the myocardium and other small organs. The Cardiotom system is based on a tomographic technique, Ectomography, which is a limited view angle method. Whereas in SPECT, the whole gamma camera detector is rotated around the long axis of the patient, Ectomography uses a rotating slant hole collimator (RSHC) and stationary detector to produce the projection images. This enables the ectomographic system to be implemented as a mobile system, as opposed to SPECT systems which are stationary installations. Furthermore, specially designed collimators, such as segmented collimators, can significantly increase system sensitivity. The mobility of the system and the fact that the examination requires no patient cooperation enable studies of myocardial perfusion in the critically ill patient, either in the intensive care unit or the emergency room. This is to be compared to a SPECT examination, where the patient must be placed on a narrow examination bed, holding one arm above the head. A mobile system can hence offer new possibilities in cardiological research and diagnosis.

Suppression of artifacts due to data truncation when using segmented slant hole collimators in ectomography [myocardial perfusion imaging]

Ectomography is a limited view angle method, which involves rotating a slant hole collimator in front of a stationary gamma camera detector. It has been implemented on a mobile system used for acute perfusion studies. Collimators with multiple segments are used to improve system sensitivity and decrease data acquisition times. For myocardial perfusion imaging, collimators with up to 4 segments and a slant angle of 40/spl deg/ have been used clinically. However, severe circular artifacts may occur, if data are truncated in the projection images. The aim of this study was to investigate the effects of these artifacts, and propose ways of artifact suppression by modifying the current reconstruction algorithm; filtered backprojection. To study artifact suppression, simulations were performed using a computerized phantom. Projection data were extrapolated into the truncated region, using various techniques. Reconstructed images from the extrapolated data showed almost no circular artifacts. Successful artifact suppression was also achieved with data from an animal study. In conclusion, the authors have shown that simple extrapolation can effectively reduce data truncation artifacts when using simulated projection data. However, for clinical data, further investigation is required.

A mobile gamma camera system for 3D acute myocardial perfusion imaging

A mobile tomographic gamma camera has been developed to enable three-dimensional perfusion studies in the emergency room and in the intensive care environment. The system, Cardioatom, is based on the limited view angle method Ectomography and comprises a modern detector head equipped with a rotating slant hole collimator and specially developed hardware and software for data acquisition, processing and display. Two systems have been developed. The first system, is currently in use at the Karolinska Hospital for a blind comparative study with SPECT, for infarction treatment assessment and in animal experiments. The second system is to be used at the Huddinge University Hospital for intraoperative and acute cerebral perfusion studies, and for myocardial infarction treatment assessment.