D. Bone

Total variation norm for three-dimensional iterative reconstruction in limited view angle tomography

An iterative Bayesian reconstruction algorithm for limited view angle tomography, or ectomography, based on the three-dimensional total variation (TV) norm has been developed. The TV norm has been described in the literature as a method for reducing noise in two-dimensional images while preserving edges, without introducing ringing or edge artefacts. It has also been proposed as a 2D regularization function in Bayesian reconstruction, implemented in an expectation maximization algorithm (TV-EM). The TV-EM was developed for 2D single photon emission computed tomography imaging, and the algorithm is capable of smoothing noise while maintaining edges without introducing artefacts. The TV norm was extended from 2D to 3D and incorporated into an ordered subsets expectation maximization algorithm for limited view angle geometry. The algorithm, called TV3D-EM, was evaluated using a modelled point spread function and digital phantoms. Reconstructed images were compared with those reconstructed with the 2D filtered backprojection algorithm currently used in ectomography. Results show a substantial reduction in artefacts related to the limited view angle geometry, and noise levels were also improved. Perhaps most important, depth resolution was improved by at least 45%. In conclusion, the proposed algorithm has been shown to improve the perceived image quality.

A mobile tomographic gamma camera system for acute studies

A mobile tomographic gamma camera system, called Cardiotom Mark 1, 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 using a rotating slant hole collimator (RSHC) and a stationary detector to produce projection images. This enables the ectomographic system to be implemented as a mobile system. With the system developed, almost 200 perfusion studies have been performed. The system is based on a second-hand detector and a 30/spl deg/ RSHC. By segmenting the collimator, total system efficiency is increased and acquisition time can be reduced by a factor equal to the number of segments. The system developed is PC-based and totally self-contained with data acquisition, reconstruction, and image presentation. The mobility of the system and the fact that the examination requires no patient cooperation enable acute studies of myocardial perfusion in the critically ill patient, either in the intensive care unit or the emergency room. A mobile system with three-dimensional imaging can offer new possibilities in cardiological research and diagnosis.

Positron emission tomography shows high specific uptake of racemic carbon-11 labelled norepinephrine in the primate heart

Abstract(−)-Norepinephrine is the predominant neurotransmitter of the sympathetic innervation of the heart. Racemic norepinephrine was labelled with carbon-11 and injected i.v. into Cynomolgus monkeys. Five minutes after injection there was a more than tenfold higher radioactivity in the heart than in adjacent tissue. Pretreatment with the norepinephrine reuptake inhibitor desipramine reduced the uptake by more than 80%. The high specific uptake of racemic [11C]norepinephrine indicates that enatiomerically pure (−)-[11C]norepinephrine has promising potential for detailed mapping of the sympathetic innervation of the human myocardium.

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.

Effects of non-optimal acquisition geometry in myocardial perfusion imaging using Ectomography

Ectomography is a limited view angle method, which has been implemented by rotating a slant hole collimator in front of a stationary gamma camera detector. The system is mobile and can be used for acute perfusion studies. To achieve high image quality in myocardial perfusion imaging, the detector should be positioned perpendicular to the long axis of the left ventricle. The aim of this study is to investigate the effects of (1) misalignment of the detector head with respect to the left ventricle, (2) truncation of the myocardial projection images and (3) external activity. Phantom studies were performed with 5 different slant hole collimators. A heart phantom, containing two defects, was placed in a thorax phantom. Studies with different degree of misalignment were performed using a mobile tomographic gamma camera. Studies with external activity present, and truncation of the apex were also performed. Results show that moderate misalignment causes little influence on size and position of the defect in the reconstructed sections, neither does presence of external activity. Effects of apical truncation do not propagate into nontruncated regions. In conclusion, positioning of a mobile system based on Ectomography does not have to be perfect, allowing rapid positioning in acute situations.

Three-dimensional total variation norm for SPECT reconstruction

Abstract The total variation (TV) norm has been described in literature as a method for reducing noise in two-dimensional (2D) images. At the same time, the TV-norm is very good at recovering edges in images, without introducing ringing or edge artefacts. It has also been proposed as a 2D regularisation function in Bayesian reconstruction, implemented in an expectation maximisation (EM) algorithm, and called TV-EM. The TV-EM was developed for 2D SPECT imaging, and the algorithm is capable of smoothing noise while maintaining edges without introducing artefacts. We have extended the TV-norm to take into account the third spatial dimension, and developed an iterative EM algorithm based on the three-dimensional (3D) TV-norm, which we call TV3D-EM. This takes into account the correlation between transaxial sections in SPECT, due to system resolution. We have compared the 2D and 3D algorithms using reconstructed images from simulated projection data. Phantoms used were a homogeneous sphere, and a 3D head phantom based on the Shepp-Logan phantom. The TV3D-EM algorithm yielded somewhat lower noise levels than TV-EM. The noise in the TV3D-EM had similar correlation in transaxial and longitudinal sections, which was not the case for TV-EM, or any 2D reconstruction method. In particular, longitudinal sections from TV3D-EM were perceived as less noisy when compared to TV-EM. The use of 3D reconstruction should also be advantageous if compensation for distant dependent collimator blurring is incorporated in the iterative algorithm.

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.

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.

Data Acquisition and Image Reconstruction Systems from the miniPET Scanners to the CARDIOTOM Camera

Nuclear imaging devices play an important role in medical diagnosis as well as drug research. The first and second generation data acquisition systems and the image reconstruction library developed provide a unified hardware and software platform for the miniPET‐I, miniPET‐II small animal PET scanners and for the CARDIOTOM™.