Dual-energy x-ray approach for object/energy-specific attenuation coefficient correction in single-photon emission computed tomography: effects of contrast agent

Purpose: To investigate the influence of radiographic contrast agent on the accuracy of the photon counts arising from the emission of gamma rays of radionuclides in single-photon emission computed tomography (SPECT), when dual-energy x-ray CT (DXCT) is employed for providing object/energy-specific attenuation coefficient correction in SPECT. Approach: Computer simulation was performed for three transmission CT approaches, namely, the conventional (single kVp, unimodal spectrum) x-ray CT, DXCT (single kVp, bimodal spectrum) with basis material decomposition (BMD), and DXCT with BMD followed by basis material coefficients transformation (BMT), to study the effects of these approaches on the accuracy of the photon counts from the SPECT image of a thorax-like phantom. Results: All three CT approaches revealed that the error in the counts was both photon energy and iodine concentration-dependent. Differences in the trending increase/decrease in the errors with the respective increase in iodine concentration and photon energy were observed among the three CT approaches. Of the three, the BMT/SPECT approach resulted in the smallest error in the concentration of radionuclides measured, especially in the contrast agent-filled region, and the optimal level depended on the iodine concentration and photon energy. Conclusion: With a judicious choice of the basis materials and photon energy, it may be possible to take advantage of the benefits of the BMT method to mitigate the accuracy problem in DXCT for quantitative SPECT imaging.

Simple method to extend the range of ECO-BB for waters with high backscattering coefficient

The ECO-BB (Sea Bird Scientific) is a popular instrument used by water optics researchers to measure the backscattering coefficient of waters in the visible to near-infrared wavelengths. The ECO-BB is calibrated by default for oceanic waters where the backscattering coefficient is typically low. In inland and coastal waters however, there is a tendency for the ECO-BB to reach saturation due to the high number of suspended particles. In the paper, a simple method is presented to extend the range of the ECO-BB instrument in such waters using a nephelometric turbidimeter. The method was first tested on powdered calcium carbonate in pure freshwater, followed by collected water samples from the Singapore Straits and the Lupar River in Sarawak. All three tests confirm the correlation between the ECO-BB and turbidimeter and show that the turbidimeter is a good proxy for backscattering coefficient measurements in turbid waters.

Modeling the remote-sensing reflectance of highly turbid waters

In ocean-color remote sensing, subsurface remote-sensing reflectance (r _{r s} ) of optically deep waters can be linked to its absorption (a) and backscattering coefficients (b _b ) by various models. The use of such models allows for quick calculations r _{r s} from such coefficients, eliminating the need to solve the radiative transfer equation. In particular, r _{r s} can be expressed as a function of b_b/(a+b_b). HydroLight and Monte Carlo simulations showed that commonly used models underestimate r _{r s} in waters with high suspended sediment loads. Monte Carlo simulations confirmed that this issue is due to a sharp increase in multiple scattering events at high turbidity levels. A quartic polynomial model is derived relating r _{r s} and inherent optical properties (IOPs) for waters of any turbidity, to avoid significant errors in waters of high turbidity.

Human colorectal cancer initiation is bidirectional, and cell growth, metabolic genes and transporter genes are early drivers of tumorigenesis

The role of stem cells in the development of solid tumors remains controversial. In colorectal cancers (CRC), this is complicated by the conflicting "top-down" or "bottom-up" hypotheses of cancer initiation. We profiled the expressions of genes from the top (T) and bottom (B) crypt fractions of normal-appearing human colonic mucosa (M) at least 20 cm away from the tumor as a baseline and compared this to the genes of matched mucosa adjacent to tumors (M_T) in twenty-three sporadic CRC patients. In thirteen patients, the genetic distance (M-M_T) between the B fractions is smaller than the distance between the T fractions, indicating that the expressions diverge further in the top fractions (B < T). In the remaining patients, the reverse effect is observed (B > T). Assuming that a greater genetic divergence in the top or bottom fractions indicates that position as the initiation site, it is thus equally likely that human CRC initiates from 'top-down' via de-differentiated colonocytes or 'bottom-up' via dysregulated intestinal stem cells. Dysregulated genes that persist until tumor stage are not limited to tumor suppressors or oncogenes but include metabolic and transporter genes such as CA7, PHLPP2, and AQP8.

Fire Distribution in Peninsular Malaysia, Sumatra and Borneo in 2015 with Special Emphasis on Peatland Fires

In this paper, we analyze the spatio-temporal distribution of vegetation fires in Peninsular Malaysia, Sumatra, and Borneo in the severe El Niño year of 2015, concentrating on the distribution of fires between mineral soils and peatland areas, and between land cover types in peatland areas. The results reveal that 53% of all Moderate Resolution Imaging Spectroradiometer (MODIS) fire detections were recorded in peatlands that cover only 12% of the study area. However, fire occurrence in the peatland areas was highly dependent on land cover type. Pristine peat swamp forests (PSF) experienced only marginal fire activity (30 fire detections per 1000 km²) compared to deforested undeveloped peatlands (831-915 fire detections per 1000 km²). Our results also highlight the extreme fire vulnerability of the southern Sumatran and Bornean peatlands under strong El Niño conditions: 71% of all peatland hotspots were detected in the provinces of South Sumatra and Central Kalimantan, which contain 29% of peatlands in the study area. Degraded PSF and all deforested peatland land cover types, including managed areas, in the two provinces were severely affected, demonstrating how difficult it is to protect even managed drained agricultural areas from unwanted fires during dry periods. Our results thereby advocate rewetting and rehabilitation as the primary management option for highly fire prone degraded undeveloped peatland areas, whenever feasible, as a means to reduce fire risk during future dry episodes.

Status of peatland degradation and development in Sumatra and Kalimantan

Peatlands cover around 13 Mha in Sumatra and Kalimantan, Indonesia. Human activities have rapidly increased in the peatland ecosystems during the last two decades, invariably degrading them and making them vulnerable to fires. This causes high carbon emissions that contribute to global climate change. For this article, we used 94 high resolution (10-20 m) satellite images to map the status of peatland degradation and development in Sumatra and Kalimantan using visual image interpretation. The results reveal that less than 4% of the peatland areas remain covered by pristine peatswamp forests (PSFs), while 37% are covered by PSFs with varying degree of degradation. Furthermore, over 20% is considered to be unmanaged degraded landscape, occupied by ferns, shrubs and secondary growth. This alarming extent of degradation makes peatlands vulnerable to accelerated peat decomposition and catastrophic fire episodes that will have global consequences. With on-going degradation and development the existence of the entire tropical peatland ecosystem in this region is in great danger.

Bilateral uveal effusion associated with scleral thickening due to amyloidosis

A 45-year-old man with primary systemic amyloidosis was found to have bilateral uveal effusions secondary to thickened sclera according to magnetic resonance imaging of the orbits. The patient was treated with bilateral sclerectomies and vortex vein decompression, and had an excellent outcome. Light microscopy of excised sclera revealed severe infiltration of the tissue by amyloid. To our knowledge, this is the first report of amyloid infiltration of the sclera leading to uveal effusion. Arch Ophthalmol. 2000;118:1293-1295

Topical negative pressure in wound management

OBJECTIVE

To investigate the role of topical negative pressure (TNP) therapy in the management of difficult wounds.

DESIGN

Prospective consecutive patient series.

PATIENTS AND SETTING

30 patients referred to our tertiary plastic and reconstructive surgical service with wounds deemed unsuitable for reconstructive surgery were treated between November 1997 and the end of December 1998. The mean pretreatment duration of the wounds was 418 days (range, 8-1650 days). All wounds were at least Grade III pressure sores.

INTERVENTION

Topical negative pressure therapy (TNP) using the VAC device (KCI Medical, San Antonio, USA). Suction (75-125 mmHg) was continuous for the first 48 hours, then intermittent (2 min on, 5 min off).

MAIN OUTCOME MEASURES

Achievement of wound healing endpoints: (1) complete healing of the wound; (2) obliteration of the wound cavity to allow surface dressings; or (3) closure of the wound by suture or skin graft.

RESULTS

TNP was successful in 26 out of 30 patients with mean therapy time of 35 days (range, 3-124 days). Healing was more rapid in acute (less than six weeks old) wounds. A reduction in the number of bacterial species and colonies was also observed during therapy.

CONCLUSION

TNP can, in some circumstances, promote rapid secondary wound healing. A further randomised trial of TNP versus more traditional wound management modalities is justified.

Laparoscopic resection of an omental cyst with pedicle torsion

Omental cysts are the least-common variety of all types of intra-abdominal cystic lesions. In the past, transabdominal laparotomy with excision of the cyst was the treatment of choice. With the advent of laparoscopic surgery, it has become possible to resect the cyst without the need for a large incision in the abdomen. We report a case of a 15-year-old girl who underwent diagnostic laparoscopy for recurring abdominal pain of 2 years' duration. The procedure revealed a huge cyst lying above the omentum with its pedicle rising from the greater curvature of the stomach. The pedicle was noted to have twisted eight times in a clockwise direction. The cyst was resected by laparoscopic means using three trocars. The postoperative course was uneventful. As presented in this case, we believe that a laparoscopic approach is an attractive alternative for the management of omental cystic lesions.

Small bowel perforation caused by a sharp bone: laparoscopic diagnosis and treatment

Small bowel perforation caused by sharp or pointed foreign bodies rarely is diagnosed preoperatively because clinical symptoms usually are nonspecific and can mimic other surgical conditions such as appendicitis and cecal diverticulitis. We report the case of a 57-year-old man who underwent diagnostic laparoscopy for abdominal pain of unknown etiology. Swelling and erythema were noted in a segment of small bowel in the right lower abdomen. A tiny sharp-pointed object was found penetrating the inflamed portion of the bowel. This foreign body was removed and the perforation immediately repaired under laparoscopic guidance. The postoperative course was uneventful. We believe that in cases of lower abdominal pain with peritoneal irritation, laparoscopy can provide not only a wide field of examination to localize intraabdominal pathology, but in some instances it can aid in case management.

Anterior ischaemic optic neuropathy in a patient with optic disc drusen

BACKGROUND

Although visual field defects are well-known complications of optic disc drusen, reduction in visual acuity with this condition is rare.

METHOD/RESULTS

We report on a 68-year-old male with bilateral optic disc drusen who presented with monocular loss of vision in the right eye associated with an inferior altitudinal visual field defect and signs consistent with acute anterior ischaemic optic neuropathy, confirmed on fluorescein angiography. He also had a left inferior nasal step, but no evidence of glaucomatous cupping. The disc drusen were documented clinically and on B scan ultrasound and computed tomography.

CONCLUSIONS

The diagnosis of acute anterior ischaemic optic neuropathy should be considered in patients with optic disc drusen who present with reduced visual acuity, particularly when the visual loss has been acute and non-progressive and is associated with altitudinal field loss and characteristic fluorescein angiography signs.

Modulation of major histocompatibility complex class II expression in retinas with age-related macular degeneration

PURPOSE

To investigate antigenic and morphologic features of microglial and vascular elements in the neural retina associated with age-related macular degeneration (ARMD) compared with those features in age-matched normal and young adult retinas.

METHODS

Adult eyes (n = 97) were classified histopathologically into normal and ARMD-associated groups. Peroxidase imunohistochemical examination of retinal flatmounts was used to visualize major histocompatibility complex class II (MHC-II) immunoreactivity; the intensity and distribution of labeling were quantified by image analysis. In histochemical investigation, reduced nicotinamide-adenine dinucleotide phosphate diaphorase and glial fibrillary acidic protein or MHC-II double labeling were used to detect vascular changes in some preparations.

RESULTS

An increase in the proportion of the retina (percentage of total area) expressing MHC-II immunoreactivity was observed in age-matched retinas compared with that seen in normal retinas. A significant increase (P < 0.05) in the percentage of area immunoreactive for MHC-II was observed, primarily on vascular elements, in retinas with incipient ARMD compared with the area affected in the age-matched group. Increased MHC-II immunoreactivity on vessels in the normal-aged group observed with confocal microscopy was associated with irregularities in the organization of astrocytes. Hypertrophy of retinal microglia was also apparent, although the intensity of microglial MHC-II immunoreactivity was not significantly different between groups.

CONCLUSIONS

The results indicate that an increase in MHC-II immunoreactivity on retinal vascular elements is associated with normal aging. A further increase in MHC-II immunoreactivity on vascular elements and morphologic changes in microglia was associated with incipient ARMD. Immunologic responses in neural retinal microglia and vascular elements appear to be related to early pathogenetic changes in retinal pigment epithelium pigmentation and drusen formation.

Retrieval of organs for transplantation: experience of the Australian National Liver Transplantation Unit

The Australian National Liver Transplantation Unit carried out 251 multiple organ-retrieval procedures between January 1986 and December 1994: 53% were in the Sydney metropolitan area, 22% in country New South Wales and the Australian Capital Territory, 21% interstate and 4% in New Zealand; mean total procurement times were, respectively, 4 hours 57 minutes, 6 hours 58 minutes, 9 hours 20 minutes and 12 hours 20 minutes. Donor operation times steadily decreased over the nine years. One-year patient and graft survival rates were 77.1% and 70.3%, respectively; primary non-function occurred in only one case. The importance of coordination of donor organ procurement and efficient organ retrieval to the success of transplantation programs is discussed.

Laparoscopic splenectomy

The innovations in laparoscopic technology have expanded the variety of general surgical procedures amenable to laparoscopic approach. An initial experience with eight cases of laparoscopic splenectomy is presented. The indications for splenectomy were immune thrombocytopenic purpura (ITP, n = 6), hereditary spherocytosis (n = 1) and enlarged spleen with filling defects (n = 1). There were six females and two males, aged 27 to 46 years. Seven patients had the spleen removed laparoscopically and one had laparoscopically assisted splenectomy. The operations took from 2 h 15 min to 3 h 30 min (mean 2 h 45 min). The spleens removed varied from 70 to 563 g (mean = 250). Blood loss ranged from insignificant to 1400 mL. Significant bleeding was encountered in three patients with moderate splenomegaly (240, 350, 563 g). Two patients received autologous blood transfusion. The average narcotic required was three doses. The patients were discharged after a mean postoperative stay of 3 days (range 2-4). One patient developed a below knee deep venous thrombosis. Laparoscopic splenectomy is possible and promises to provide the advantages associated with other laparoscopic procedures. Patients with an enlarged spleen can present a technical challenge, and there is a potential for significant blood loss. The current technology and laparoscopic expertise means that this procedure should probably be limited to patients whose spleens are not palpable clinically.

Early outcome and intermediate follow-up of vascular stents in the femoral and popliteal arteries without long-term anticoagulation

PURPOSE

The role of arterial stenting in the treatment of femoral and popliteal arterial disease is controversial and has been hampered by recommendations for patients to be given anticoagulants (oral warfarin) for several months or more. This study was undertaken to evaluate the immediate and midterm outcomes of vascular stents implanted percutaneously in the femoral and popliteal arteries, without long-term anticoagulation.

METHODS

Over a 3-year period, 32 patients admitted to a vascular surgery service had arterial stents implanted in the femoral (n = 22) or popliteal (n = 10) artery for the following indications: recurrent stenosis after angioplasty (n = 13), suboptimal result after angioplasty of occluded (n = 12) or calcified stenotic arteries (n = 2), percutaneous transluminal angioplasty-induced thrombosis or dissection (n = 5). Access to the artery was gained by percutaneous insertion of a hemostatic sheath into the ipsilateral common femoral artery. Systemic heparin was given at the time of stent insertion, and patients were prescribed daily low-dose aspirin.

RESULTS

Successful stent implantation was achieved in 31 of the 32 patients. Acute thrombosis (< 30 days) occurred in two patients. There was no incidence of false aneurysm formation, acute leg ischemia, or vessel perforation. All patients were monitored by Doppler scanning index and duplex scanning within 24 hours, and thereafter at 3- to 6-month intervals. The mean ankle-brachial systolic pressure index improved from 0.60 (before treatment) to 0.88 (3 to 6 months after stenting). Stent occlusion has occurred in six patients; two stents were successfully salvaged with urokinase infusion. In follow-up to date (range 3 to 33 months) the primary patency rate by life-table analysis was 75% at 18 months, whereas the secondary patency rate was 89% at the same interval. Restenosis (> 50% lumen diameter) was detected by duplex ultrasonography in seven of 25 patent stents (28%) at a mean interval of 9.5 months (range 4 to 15 months); of these, four patients remained clinically symptom-free despite the ultrasound findings.

CONCLUSIONS

We conclude that vascular stents can be implanted into the femoropopliteal arteries with few complications and with acceptable early and intermediate patency rates, without the need for long-term anticoagulation. Restenosis is not prevented by stents, and the main value of stenting at this site appears to be in salvaging acute complications of percutaneous transluminal angioplasty, or to correct suboptimal results after recanalization of occluded arteries.

Mesenteric cyst

Mesenteric cyst is one of the rarest abdominal tumours, with approximately 820 cases reported since 1507. The incidence varies from 1 per 100,000 to 250,000 admissions. The lack of characteristic clinical features and radiological signs may present great diagnostic difficulties. The cyst may present in one of three ways: (i) non-specific abdominal features; (ii) an incidental finding; or (iii) an acute abdomen. Abdominal pain is the major presenting symptom. Abdominal mass is found in more than 50% of cases and 40% of cases are discovered incidentally. More than one aetiological mechanism is probably involved in the development of mesenteric cysts. Mesenteric cysts have been reported from the duodenum to the rectal mesentery but are most commonly located in the ileal mesentery. Malignant cysts occur in less than 3% of cases. Enucleation of the cyst is the treatment of choice. Knowledge of these tumours is important due to the various complications associated with suboptimal surgical management. Two cases of mesenteric cysts are presented including a recurrent mesenteric cyst in a post-partum woman demonstrating the inferior technique of internal marsupialization. The association of mesenteric cyst with pregnancy is discussed.

Mesenteric cyst

Mesenteric cyst is one of the rarest abdominal tumours, with approximately 820 cases reported since 1507. The incidence varies from 1 per 100,000 to 250,000 admissions. The lack of characteristic clinical features and radiological signs may present great diagnostic difficulties. The cyst may present in one of three ways: (i) non-specific abdominal features; (ii) an incidental finding; or (iii) an acute abdomen. Abdominal pain is the major presenting symptom. Abdominal mass is found in more than 50% of cases and 40% of cases are discovered incidentally. More than one aetiological mechanism is probably involved in the development of mesenteric cysts. Mesenteric cysts have been reported from the duodenum to the rectal mesentery but are most commonly located in the ileal mesentery. Malignant cysts occur in less than 3% of cases. Enucleation of the cyst is the treatment of choice. Knowledge of these tumours is important due to the various complications associated with suboptimal surgical management. Two cases of mesenteric cysts are presented including a recurrent mesenteric cyst in a post-partum woman demonstrating the inferior technique of internal marsupialization. The association of mesenteric cyst with pregnancy is discussed.

Day-only admission for varicose vein surgery

Day-only admissions for surgery are strongly encouraged, in an effort to keep costs down. Varicose vein surgery has been considered too major for day-only management despite the fact that there have been studies from overseas showing that it can be done with a morbidity comparable to inpatient surgery. The morbidity of day-only surgery for varicose veins (both long and short saphenous procedures) was assessed and compared with the results of inpatient surgery. Patients were also asked whether they were satisfied with the surgery being done this way. There were 165 consecutive patients available for study, 64 day-only and 101 inpatient. All patients attending Shellharbour Hospital. Shellharbour, had surgery done as day-only (as it was a morning list and allowed adequate time for recovery). Patients attending Bulli Hospital, Bulli, had surgery done as an inpatient (afternoon list). All surgery was performed by one surgeon. There was no difference (Chi-squared) in the age distribution (mean 48 years for day-only, 51 years for inpatient) or sex proportion in either group (Chi-squared test of proportions with continuity correction). Assessment of the results was done by review of the surgeon's notes, as well as telephone interviews for day-only subjects. The complication rate in both groups was similar. Wound problems represented the main complication with an incidence of 10.5% in each group. There was one deep venous thrombosis (DVT) in each group (diagnosed by duplex scan). Response to the telephone interview suggested that most patients were happy to have the surgery done as a day-only procedure.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of MHC class II expression in the absence of lymphocytic infiltrates in Alzheimer's retinae

This study describes the expression of MHC class II antigens in retinal flat mounts from normal donors and patients with Alzheimer's disease (AD). We confirm previous observations of MHC class II immunoreactivity on microglia in normal retinae, while observing insignificant levels of reactivity on endothelial cells (EC). A significantly increased level of MHC class II expression was detected in AD retinae. This increased immunoreactivity was found to occur in the absence of lymphocytic infiltrates, suggesting that the pathogenesis of AD in the retina may be distinct from that reported to occur in some regions of the brain. MHC class II expression, measured using computerized optical densitometry, appeared to be increased principally as a result of induced MHC class II immunoreactivity on EC. Ramified microglia and perivascular macrophages, although hypertrophied, appeared to show unchanged levels of MHC class II expression. These findings are consistent with earlier suggestions that both aberrant MHC class II expression and suppressor activity of resident macrophages may restrict immune responses.

Complication of hard cervical collars in multi-trauma patients

All multi-trauma patients with suspected cervical spine injury should have their cervical spine protected while other life-threatening injuries are being managed. The application of a hard cervical collar is an acceptable method of temporarily immobilizing the cervical spine. Two cases of significant occipital pressure ulceration associated with the use of hard cervical collar are presented.

Noise propagation in SPECT images reconstructed using an iterative maximum-likelihood algorithm

The effects of photon noise in the emission projection data and uncertainty in the attenuation map on the image noise in attenuation-corrected SPECT images reconstructed using a maximum-likelihood expectation-maximization algorithm were investigated. Emission projection data of a physical Hoffman brain phantom and a thorax-like phantom were acquired from a prototype emission-transmission computed tomography (ETCT) scanner being developed at UCSF. Computer-simulated emission projection data from a head-like phantom and a thorax-like phantom were also obtained using a fan-beam geometry consistent with the ETCT system. The simulation assumed a 99Tcm source, included collimator blurring but ignored photon scatter. For each phantom, a region of interest (ROI) at the centre of the reconstructed image was chosen for the purpose of noise analysis. In all cases, the mean value (m) in the ROI approached a constant value after approximately 20 iterations. The standard deviation (sigma) generally increased with the number of iterations. The ratio (sigma/m) was found to be inversely proportional to the square root of the total detected counts and proportional to the relative uncertainty in the attenuation maps. These two noise components contributed independently towards the noise in the reconstructed image. In the ETCT system employing an x-ray tube for attenuation map acquisition, the uncertainty in the reconstructed radionuclide distribution is limited mainly by photon noise in the emission projection data. Our results are expected to be generally applicable to other emission-transmission systems, including those using external radionuclide sources for the acquisition of attenuation maps.

Human retinal microglia express phenotypic characteristics in common with dendritic antigen-presenting cells

Neural tissue has been considered to be immunologically privileged and major histocompatibility complex (MHC) class II antigens not expressed in normal human brain grey matter and retina. In the present study we compare phenotypic characteristics of human retinal microglia and dendritic Langerhans cells, including their morphologies and distribution, MHC class II and CD45 antigen expression and nucleotidase reactivity. Levels of class II expression were measured using optical densitometry in combination with standard immunohistochemical techniques applied to retinal flatmounts. The results indicate that ramified retinal microglia have features in common with dendritic antigen presenting cells of cornea and conjunctivum, including the constitutive expression of MHC class II antigens.

Description of a prototype emission-transmission computed tomography imaging system

We have developed a prototype imaging system that can perform simultaneous x-ray transmission CT and SPECT phantom studies. This system employs a 23-element high-purity-germanium detector array. The detector array is coupled to a collimator with septa angled toward the focal spot of an x-ray tube. During image acquisition, the x-ray fan beam and the detector array move synchronously along an arc pivoted at the x-ray source. Multiple projections are obtained by rotating the object, which is mounted at the center of rotation of the system. The detector array and electronics can count up to 10(6) cps/element with sufficient energy-resolution to discriminate between x-rays at 100-120 kVp and gamma rays from 99mTc. We have used this device to acquire x-ray CT and SPECT images of a three-dimensional Hoffman brain phantom. The emission and transmission images may be superimposed in order to localize the emission image on the transmission map.

Noise, resolution, and sensitivity considerations in the design of a single-slice emission-transmission computed tomographic system

A prototype Emission-Transmission Computed Tomography (ETCT) system is being developed that will acquire single-slice x-ray transmission CT images simultaneously with single photon emission computed tomography (SPECT) images. This system will permit the correlation of anatomical information from x-ray CT with functional information from SPECT images. The patient-specific attenuation map derived from the x-ray CT images can be used to perform attenuation correction of the SPECT images, so that accurate quantitative information can be obtained. The fan-beam scanning geometry and the use of a segmented HPGe detector array impose special constraints on the design of the collimator for the system. Based on a signal detection model, an efficiency-resolution figure of merit (ERFM) as a function of the collimator geometric efficiency, system resolution width, and object diameter is defined. The ERFM is proportional to the square of the detection signal-to-noise ratio. The collimator design parameters can then be optimized by optimizing the ERFM for an anticipated object diameter. The collimator point-spread function, geometric efficiency, and resolution are calculated. The collimator optimized for the detection of a 1-cm object will have a single-slice point source efficiency of 1.2 X 10(-4), and a FWHM of 6.5 mm at the center of the reconstruction circle, at 12 cm from the collimator face. The minimum object contrast which will give a detection SNR of 5 is 74%, for a total accumulated count per slice of 2 X 10(6).

Anisotropic elasticity of bovine pericardial tissues

Artificial heart valves fashioned from bovine pericardial tissues appear to perform favourably when compared to those fashioned from other biological materials. Tissue mechanical properties have been implicated as a possible factor in determining the success or failure of the materials used for constructing tissue valve substitutes. In this study, strips of bovine pericardial tissues, cut in three different directions (along the base-apex axis of the heart, perpendicular to this axis and at an angle of 45 degrees to this axis), were subjected to uniaxial tensile tests using an Instron machine, at strain rates of 666.7, 166.7, 66.7, and 6.7% X min-1. The pericardial tissues were found to have non-linear stress-strain curves. Anisotropy was also observed, in that the tissue was most extensible when stretched along the vertical direction and least extensible when stained along the horizontal direction. The tissues also exhibited viscoelastic properties, as the extensibility was found to decrease with the increasing strain-rate. The strain at rupture for these tissues, however, were found to be independent of the strain rates and the direction in which they were stretched. On the other hand the stress at rupture was largest when stretched in the horizontal direction and least when stretched in the vertical direction. Stresses at rupture also increase with the strain rate. The implications of the observed tissue properties, when these are used in the fashioning of tissue valve substitutes, will be discussed.