Håkan Elmqvist

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.

Lessons from the first patient with an implanted pacemaker : 1958-2001

On October 8, 1958 the world’s first pacemaker implantation was performed at the Karolinska Hospital in Stockholm, Sweden. The system had been developed by the surgeon A ◦ ke Senning and the physician inventor Rune Elmqvist. The patient was a 43-year-old engineer suffering from Stokes-Adams syndrome. His name was Arne H.W. Larsson (Fig. 1). At the time Senning was in charge of the experimental laboratory at the newly inaugurated Department of Thoracic Surgery at the Karolinska Hospital in Stockholm. Elmqvist was head of the Electromedical Division at Elema Schonander in Stockholm. He had studied medicine in Lund, Sweden but had not pursued a medical practice. Rather, he had become a self-taught engineer. In 1931 he had designed the first portable optical electrocardiographic (ECG) recorder and in 1948 had introduced the well-known direct writing Mingograf ink jet recorders. When Senning and Elmqvist met in 1950 they began a close cooperation in the development and testing of fibrillators and defibrillators for use in conjunction with open heart surgery, then an experimental procedure. Senning had studied and been inspired by the experimental work of Bigelow and Callaghan1,2 in stimulation for hypothermic cardiac standstill and Zoll who used external chest wall stimulation for management of Stokes-Adams attacks.3,4 During a visit to Minneapolis in 1957, Senning observed C. Walton Lillehei during open heart surgery suture stainless steel electrodes to the ventricle to manage surgically caused atrioventricular (AV) block, after closure of a ventricular septal defect. The wires were led through the thoracic wall and connected to an external pulse generator. This method avoided the discomfort and placement problems in the postoperative patient associated with Zoll’s external pacemaker. Senning considered this to be the beginning of the era of clinical pacing.5,6 On returning to Stockholm, Senning studied Lillehei’s technique experimentally and clini-

Topical humidified carbon dioxide to keep the open surgical wound warm: the greenhouse effect revisited.

Background:Perioperative hypothermia is common in open surgery and is associated with increased rates of wound infection. This is a result of decreased wound tissue oxygenation, which can be normalized by local warming. Recently, a technique has been developed to establish a carbon dioxide atmosphere in an open surgical wound. Therefore, the authors studied the possible “greenhouse effect” of carbon dioxide insufflation and operation lamps on wound temperature. Methods:In a fully ventilated operating room surface temperature was measured at steady state in a model of an open surgical wound containing blood agar. The wound model was randomized to either no insufflation or insufflation of dry and humidified carbon dioxide or air, respectively, at a flow of 5 l/min via a gas diffuser. The surface temperature was measured with operation lamps switched on and off, respectively. Evaporation rates were also measured. Results:With the operation light off, the surface temperature in the control was 31.8°C, and with the operation light on, the temperature increased by 1.5°C (P < 0.001). Additional insufflation of dry carbon dioxide increased the surface temperature another 1.9°C (P < 0.001). When the carbon dioxide was humidified, the evaporation rate was lowest and the surface temperature increased further to 35.6°C (P = 0.002). In contrast, insufflation of dry and humidified air did not have a significant effect on the evaporation rate and only marginally increased the wound temperature in comparison with the control. Conclusions:Insufflation of humidified carbon dioxide in combination with light from the operation lamps may help to keep the open wound warm during surgery.

Initial experience with a thin single segment pressure and conductance catheter for measurement of left ventricular volume

A thin and soft multifunctional catheter was evaluated for the simultaneous real time monitoring of left ventricular volume and pressure with special consideration to side effects such as interference with normal cardiac electrophysiology. In four pigs, pressure and volume were simultaneously recorded by using the thin single segment pressure and conductance catheter. Measurements were done under varied cardiac conditions: at baseline, during preload reduction and afterload increase. Volumes were calibrated with intracardiac ultrasound measurements. During preload reduction the pressure and volume decreased as expected. A cautious afterload increase resulted in a corresponding pressure and volume increase, the maximum of the pressure curve changed from early to late. Both SV and EDV increased. The very few arrhythmias were mainly caused by surgical interference. The present study demonstrates that our thin single segment conductance catheter for the simultaneous measurement of LV volume and pressure has a performance that warrants further development with the goal of making the method available for human use. In particular, the catheter did not cause any arrhythmias.

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.

Design of a single segment conductance catheter for measurement of left ventricular volume

The objective of this paper is to investigate how to best position the sensing electrodes on a single segment conductance catheter, and to calculate the expected performance. Using electrode potential data, obtained with the present five segment conductance catheter in pig experiments, we have interpolated the electrical field at any given point of time, and calculated what volume curve to expect with only two sensing electrodes. Comparison shows that the deviation between our calculated method and the present one is stable and small. Mean deviation with optimized electrode positions was 0.05% per sample, and the maximum deviation found for a single time sample was 2.57%. This indicates that it is possible to build a thin single segment catheter with as good performance as for the present five segment conductance catheter.

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.

A 3D-model for computer simulation of atrial electrophysiology

The presented implementation is a three-dimensional isotropic monodomain reaction-diffusion model with a realistic geometry coupled with the atrial ion model proposed by Nygren et al. (1998). The partial differential equations are solved by a Garlekin finite element method in space and a forward Euler approximation in time. Simulations yields the expected results and the computational performance of the model is good considering the size of the problem.