Is the synovial fluid cobalt-to-chromium ratio related to the serum partitioning of metal debris following metal-on-metal hip arthroplasty?

OBJECTIVES: We investigated the reliability of the cobalt-chromium (CoCr) synovial joint fluid ratio (JFR) in identifying the presence of a severe aseptic lymphocyte-dominated vasculitis-associated lesion (ALVAL) response and/or suboptimal taper performance (SOTP) following metal-on-metal (MoM) hip arthroplasty. We then examined the possibility that the CoCr JFR may influence the serum partitioning of Co and Cr. METHODS: For part A, we included all revision surgeries carried out at our unit with the relevant data, including volumetric wear analysis, joint fluid (JF) Co and Cr concentrations, and ALVAL grade (n = 315). Receiver operating characteristic curves were constructed to assess the reliability of the CoCr JFR in identifying severe ALVAL and/or SOTP. For part B, we included only patients with unilateral prostheses who had given matched serum and whole blood samples for Co and Cr analysis (n = 155). Multiple regression was used to examine the influence of JF concentrations on the serum partitioning of Co and Cr in the blood. RESULTS: A CoCr JFR > 1 showed a specificity of 83% (77% to 88%) and sensitivity of 63% (55% to 70%) for the detection of severe ALVAL and/or SOTP. In patients with CoCr JFRs > 1, the median blood Cr to serum Cr ratio was 0.99, compared with 0.71 in patients with CoCr JFRs < 1 (p < 0.001). Regression analysis demonstrated that the blood Cr to serum Cr value was positively associated with the JF Co concentration (p = 0.011) and inversely related to the JF Cr concentration (p < 0.001). CONCLUSION: Elevations in CoCr JFRs are associated with adverse biological (severe ALVAL) or tribocorrosive processes (SOTP). Comparison of serum Cr with blood Cr concentrations may be a useful additional clinical tool to help to identify these conditions.Cite this article: D. J. Langton, S. Natu, C. F. Harrington, J. G. Bowsher, A. V. F. Nargol. Is the synovial fluid cobalt-to-chromium ratio related to the serum partitioning of metal debris following metal-on-metal hip arthroplasty? Bone Joint Res 2019;8:146-155. DOI: 10.1302/2046-3758.83.BJR-2018-0049.R1.

Material loss at the femoral head taper: a comparison study of the Exeter metal-on-polyethylene and contemporary metal-on-metal total hip arthroplasty.

AIMS: There are limited published data detailing the volumetric material loss from tapers of conventional metal-on-polyethylene (MoP) total hip arthroplasties (THAs). Our aim was to address this by comparing the taper wear rates measured in an explanted cohort of the widely used Exeter THA with those measured in a group of metal-on-metal (MoM) THAs. PATIENTS AND METHODS: We examined an existing retrieval database to identify all Exeter V40 and Universal MoP THAs. Volumetric wear analysis of the taper surfaces was conducted using previously validated methodology. These values were compared with those obtained from a series of MoM THAs using non-parametric statistical methodology. A number of patient and device variables were accounted for using multiple regression modelling. RESULTS: A total of 95 Exeter MoP and 249 MoM THAs were examined. The median volumetric loss from the MoM cohort was over four times larger than that from the MoP cohort (1.01 mm3 vs 0.23 mm3, p < 0.001), despite a significantly shorter median period in vivo for the MoM group (48 months vs 90 months, p < 0.001). Multiple regression modelling indicated that the dominant variables leading to greater female taper material loss were bearing diameter (p < 0.001), larger female taper angles (p < 0.001), and male titanium stem tapers (p < 0.001). CONCLUSION: Consistent with the long-term clinical success of the device, the volumetric material loss from Exeter femoral head tapers was, in general, small compared with that from larger-diameter MoM head tapers. Cite this article: Bone Joint J 2018;100-B:1310-9.

Aseptic lymphocyte-dominated vasculitis-associated lesions are related to changes in metal ion handling in the joint capsules of metal-on-metal hip arthroplasties.

OBJECTIVES: We have encountered patients who developed large joint fluid collections with massive elevations in chromium (Cr) and cobalt (Co) concentrations following metal-on-metal (MoM) hip arthroplasties. In some cases, retrieval analysis determined that these ion concentrations could not be explained simply by the wear rates of the components. We hypothesized that these effects may be associated with aseptic lymphocyte-dominated vasculitis-associated lesion (ALVAL). PATIENTS AND METHODS: We examined the influence of the ALVAL grade on synovial fluid Co and Cr concentrations following adjustment for patient and device variables, including volumetric wear rates. Initially restricting the analysis to include only patients with one MoM hip resurfacing device, we performed multiple regression analyses of prospectively collected data. We then repeated the same statistical approach using results from a larger cohort with different MoM designs, including total hip arthroplasties. RESULTS: In the resurfacing cohort (n = 76), the statistical modelling indicated that the presence of severe ALVAL and a large fluid collection were associated with greater joint fluid Co concentrations after adjustment for volumetric wear rates (p = 0.005). These findings were replicated in the mixed implant group (n = 178), where the presence of severe ALVAL and a large fluid collection were significantly associated with greater fluid Co concentrations (p < 0.001). CONCLUSION: The development of severe ALVAL is associated with elevations in metal ion concentrations far beyond those expected from the volumetric loss from the prosthetic surfaces. This finding may aid the understanding of the sequence of events leading to soft-tissue reactions following MoM hip arthroplasties.Cite this article: D. J. Langton, R. P. Sidaginamale, T. J. Joyce, J. G. Bowsher, J. P. Holland, D. Deehan, A. V. F. Nargol, S. Natu. Aseptic lymphocyte-dominated vasculitis-associated lesions are related to changes in metal ion handling in the joint capsules of metal-on-metal hip arthroplasties. Bone Joint Res 2018;7:388-396. DOI: 10.1302/2046-3758.76.BJR-2018-0037.

A comparison study of stem taper material loss at similar and mixed metal head-neck taper junctions.

AIMS: We sought to determine whether cobalt-chromium alloy (CoCr) femoral stem tapers (trunnions) wear more than titanium (Ti) alloy stem tapers (trunnions) when used in a large diameter (LD) metal-on-metal (MoM) hip arthroplasty system. PATIENTS AND METHODS: We performed explant analysis using validated methodology to determine the volumetric material loss at the taper surfaces of explanted LD CoCr MoM hip arthroplasties used with either a Ti alloy (n = 28) or CoCr femoral stem (n = 21). Only 12/14 taper constructs with a rough male taper surface and a nominal included angle close to 5.666° were included. Multiple regression modelling was undertaken using taper angle, taper roughness, bearing diameter (horizontal lever arm) as independent variables. Material loss was mapped using a coordinate measuring machine, profilometry and scanning electron microscopy. RESULTS: After adjustment for other factors, CoCr stem tapers were found to have significantly greater volumetric material loss than the equivalent Ti stem tapers. CONCLUSION: When taper junction damage is identified during revision of a LD MoM hip, it should be suspected that a male taper composed of a standard CoCr alloy has sustained significant changes to the taper cone geometry which are likely to be more extensive than those affecting a Ti alloy stem. Cite this article: Bone Joint J 2017;99-B:1304-12.

The clinical implications of metal debris release from the taper junctions and bearing surfaces of metal-on-metal hip arthroplasty: joint fluid and blood metal ion concentrations.

AIMS: We wished to investigate the influence of metal debris exposure on the subsequent immune response and resulting soft-tissue injury following metal-on-metal (MoM) hip arthroplasty. Some reports have suggested that debris generated from the head-neck taper junction is more destructive than equivalent doses from metal bearing surfaces. PATIENTS AND METHODS: We investigated the influence of the source and volume of metal debris on chromium (Cr) and cobalt (Co) concentrations in corresponding blood and hip synovial fluid samples and the observed agglomerated particle sizes in excised tissues using multiple regression analysis of prospectively collected data. A total of 199 explanted MoM hips (177 patients; 132 hips female) were analysed to determine rates of volumetric wear at the bearing surfaces and taper junctions. RESULTS: The statistical modelling suggested that a greater source contribution of metal debris from the taper junction was associated with smaller aggregated particle sizes in the local tissues and a relative reduction of Cr ion concentrations in the corresponding synovial fluid and blood samples. Metal debris generated from taper junctions appears to be of a different morphology, composition and therefore, potentially, immunogenicity to that generated from bearing surfaces. CONCLUSION: The differences in debris arising from the taper and the articulating surfaces may provide some understanding of the increased incidence of soft-tissue reactions reported in patients implanted with MoM total hip arthroplasties compared with patients with hip resurfacings. Cite this article: Bone Joint J 2016;98-B:925-33.

Selection on bristle length has the ability to drive the evolution of male abdominal appendages in the sepsid fly Themira biloba.

Many exaggerated and novel traits are strongly influenced by sexual selection. Although sexual selection is a powerful evolutionary force, underlying genetic interactions can constrain evolutionary outcomes. The relative strength of selection vs. constraint has been a matter of debate for the evolution of male abdominal appendages in sepsid flies. These abdominal appendages are involved in courtship and mating, but their function has not been directly tested. We performed mate choice experiments to determine whether sexual selection acts on abdominal appendages in the sepsid Themira biloba. We tested whether appendage bristle length influenced successful insemination by surgically trimming the bristles. Females paired with males that had shortened bristles laid only unfertilized eggs, indicating that long bristles are necessary for successful insemination. We also tested whether the evolution of bristle length was constrained by phenotypic correlations with other traits. Analyses of phenotypic covariation indicated that bristle length was highly correlated with other abdominal appendage traits, but was not correlated with abdominal sternite size. Thus, abdominal appendages are not exaggerated traits like many sexual ornaments, but vary independently from body size. At the same time, strong correlations between bristle length and appendage length suggest that selection on bristle length is likely to result in a correlated increase in appendage length. Bristle length is under sexual selection in T. biloba and has the potential to evolve independently from abdomen size.

Extended hypoxia in the alfalfa leafcutting bee, Megachile rotundata, increases survival but causes sub-lethal effects.

Many insects are tolerant of hypoxic conditions, but survival may come at a cost to long-term health. The alfalfa leaf-cutting bee, Megachile rotundata, develops in brood cells inside natural cavities, and may be exposed to hypoxic conditions for extended periods of time. Whether M. rotundata is tolerant of hypoxia, and whether exposure results in sub-lethal effects, has never been investigated. Overwintering M. rotundata prepupae were exposed to 10%, 13%, 17%, 21% and 24% O2 for 11 months. Once adults emerged, five indicators of quality - emergence weight, body size, feeding activity, flight performance, and adult longevity, - were measured to determine whether adult bees that survived past exposure to hypoxia were competent pollinators. M. rotundata prepupae are tolerant of hypoxic condition and have higher survival rates in hypoxia, than in normoxia. Under hypoxia, adult emergence rates did not decrease over the 11 months of the experiment. In contrast, bees reared in normoxia had decreased emergence rates by 8 months, and were dead by 11 months. M. rotundata prepupae exposed to extended hypoxic conditions had similar emergence weight, head width, and cross-thorax distance compared to bees reared in standard 21% oxygen. Despite no significant morphological differences, hypoxia-exposed bees had lower feeding rates and shorter adult lifespans. Hypoxia may play a role in post-diapause physiology of M. rotundata, with prepupae showing better survival under hypoxic conditions. Extended exposure to hypoxia, while not fatal, causes sub-lethal effects in feeding rates and longevity in the adults, indicating that hypoxia tolerance comes at a cost.

A simulator study of adverse wear with metal and cement debris contamination in metal-on-metal hip bearings.

OBJECTIVES: Third-body wear is believed to be one trigger for adverse results with metal-on-metal (MOM) bearings. Impingement and subluxation may release metal particles from MOM replacements. We therefore challenged MOM bearings with relevant debris types of cobalt-chrome alloy (CoCr), titanium alloy (Ti6Al4V) and polymethylmethacrylate bone cement (PMMA). METHODS: Cement flakes (PMMA), CoCr and Ti6Al4V particles (size range 5 µm to 400 µm) were run in a MOM wear simulation. Debris allotments (5 mg) were inserted at ten intervals during the five million cycle (5 Mc) test. RESULTS: In a clean test phase (0 Mc to 0.8 Mc), lubricants retained their yellow colour. Addition of metal particles at 0.8 Mc turned lubricants black within the first hour of the test and remained so for the duration, while PMMA particles did not change the colour of the lubricant. Rates of wear with PMMA, CoCr and Ti6Al4V debris averaged 0.3 mm(3)/Mc, 4.1 mm(3)/Mc and 6.4 mm(3)/Mc, respectively. CONCLUSIONS: Metal particles turned simulator lubricants black with rates of wear of MOM bearings an order of magnitude higher than with control PMMA particles. This appeared to model the findings of black, periarticular joint tissues and high CoCr wear in failed MOM replacements. The amount of wear debris produced during a 500 000-cycle interval of gait was 30 to 50 times greater than the weight of triggering particle allotment, indicating that MOM bearings were extremely sensitive to third-body wear. Cite this article: Bone Joint Res 2015;4:29-37.

SU-E-T-104: Commissioning and Dosimetric Characteristics of TrueBeam System: Composite Data of Three TrueBeam Machines.

INTRODUCTION: A TrueBeam linear accelerator (TB-LINAC) is designed to deliver standard flattened and flattening-filter-free (FFF) beams. In our institute, three TB-LINAC units are installed. In this work, composite data of the three units and multi-unit comparison are presented. METHODS: Each TB-LINAC can deliver photon beams from 4MV to 15MV, electron beams from 6MeV to 22MeV, and 6MV-FFF and 10MV-FFF. Dosimetric characteristics are systematically measured for commissioning including percent depth dose (PDD), beam profile, relative scatter factor, dynamic leaf shift, output factor and MLC leakage. Critic considerations of Pion of FFF photon beams and dosimetric penumbra are investigated. RESULTS: All measured PDDs and profiles of photon and electron matched well across the three machines. Beam data were quantitatively compared and combined through average to yield composite beam data. The discrepancies among the machines were quantified using standard deviation (SD). For example, the mean SD of the PDDs among the three units is 0.12%, and the mean SD of the profiles is 0.40% for 10MV-FFF open fields. The variations of Pion of the chamber CC13 is 1.2±0.1% under 6MV-FFF and 2.0±0.5% from dmax to the 18cm-off-axis point at 35cm depth under 40×40cm2 . The measured relative output factors range from 0.866 to 1.141 with the mean discrepancy of 0.06±0.04% among the three units. The measured wedge factors range from 0.863 to 1.254 with the mean overall discrepancy of 0.04±0.04%. The mean MLC transmission and dynamic leaf shift were measured from 1.0% to 1.5% and from 0.77mm to 0.96 mm from 4MV to 15MV. The mean penumbra of various photon beams are measured from 5.88±0.09mm to 5.99±0.13mm from 4MV to 15MV at 10cm depth of 10×10 cm2 . CONCLUSIONS: Dosimetric data demonstrated that the three units could and had been matched well. The systematically measured data might be useful for future reference.

What is a "normal" wear pattern for metal-on-metal hip bearings?

In addition to classical run-in and steady-state wear phases, metal-on-metal (MOM) hip bearings have encountered "runaway wear" (RAW) trends in simulator studies. This puzzling behavior has resulted in 2- to 19-fold wear increases compared with other apparently "identical" bearings. MOM bearings have shown five identifiable RAW wear patterns in joint simulators; therefore, additional descriptive terms were used here to indicate various observed patterns, for example, "breakaway wear" (BAW), which was defined as a higher wear trend that recovers to steady-state wear. As these trends commonly occur for MOM, this raises the question of what can be considered "normal" behavior or "abnormal"? In an effort to identify possible causes for this behavior, the current study investigated six Co-Cr bearings, which closely matched with respect to geometrical tolerances. Despite close control of design and test variables, BAW occurred in 30% of the MOM bearings, producing a threefold wear increase above otherwise identical MOM bearings within the same group. The majority of the BAW (85%) occurred on the cup side and was validated by growth of wear scars and concentrations of metal ions. One bearing that showed continuing BAW at 5 Mc revealed a cup that was 50% smoother than other cups whereas its mating head was 50% rougher, thus signifying that highly polished areas were sites of the highest MOM wear. The two BAW bearings with high wear showed the greatest conformity at 5 Mc, in apparent contradistinction to classical lubrication theory.

Improving the quantitative accuracy of optical-emission computed tomography by incorporating an attenuation correction: application to HIF1 imaging.

Optical computed tomography (optical-CT) and optical-emission computed tomography (optical-ECT) are new techniques for imaging the 3D structure and function (including gene expression) of whole unsectioned tissue samples. This work presents a method of improving the quantitative accuracy of optical-ECT by correcting for the 'self'-attenuation of photons emitted within the sample. The correction is analogous to a method commonly applied in single-photon-emission computed tomography reconstruction. The performance of the correction method was investigated by application to a transparent cylindrical gelatin phantom, containing a known distribution of attenuation (a central ink-doped gelatine core) and a known distribution of fluorescing fibres. Attenuation corrected and uncorrected optical-ECT images were reconstructed on the phantom to enable an evaluation of the effectiveness of the correction. Significant attenuation artefacts were observed in the uncorrected images where the central fibre appeared approximately 24% less intense due to greater attenuation from the surrounding ink-doped gelatin. This artefact was almost completely removed in the attenuation-corrected image, where the central fibre was within approximately 4% of the others. The successful phantom test enabled application of attenuation correction to optical-ECT images of an unsectioned human breast xenograft tumour grown subcutaneously on the hind leg of a nude mouse. This tumour cell line had been genetically labelled (pre-implantation) with fluorescent reporter genes such that all viable tumour cells expressed constitutive red fluorescent protein and hypoxia-inducible factor 1 transcription-produced green fluorescent protein. In addition to the fluorescent reporter labelling of gene expression, the tumour microvasculature was labelled by a light-absorbing vasculature contrast agent delivered in vivo by tail-vein injection. Optical-CT transmission images yielded high-resolution 3D images of the absorbing contrast agent, and revealed highly inhomogeneous vasculature perfusion within the tumour. Optical-ECT emission images yielded high-resolution 3D images of the fluorescent protein distribution in the tumour. Attenuation-uncorrected optical-ECT images showed clear loss of signal in regions of high attenuation, including regions of high perfusion, where attenuation is increased by increased vascular ink stain. Application of attenuation correction showed significant changes in an apparent expression of fluorescent proteins, confirming the importance of the attenuation correction. In conclusion, this work presents the first development and application of an attenuation correction for optical-ECT imaging. The results suggest that successful attenuation correction for optical-ECT is feasible and is essential for quantitatively accurate optical-ECT imaging.

"Severe" wear challenge to 36 mm mechanically enhanced highly crosslinked polyethylene hip liners.

Our purpose was to compare the wear performance of mechanically enhanced 5Mrad highly crosslinked polyethylene (MEP, ArComXL) hip liners to (control) 3Mrad UHMWPE liners (ArCom) in 36 mm head size. As a more severe synergy of clinically relevant test models, we contrasted wear with custom roughened Co-Cr surfaces (Ra 500 nm) to the standard pristine Co-Cr heads (Ra < 20 nm) using a severe microseparation test mode in our hip simulator. We adopted a previously published model to estimate potential biological activity. On new Co-Cr heads, the MEP liners showed a 47% reduction in volumetric wear a 13% reduction in wear particle size and a 27% reduction in Functional Biological Activity (FBA) compared to our control. On rough Co-Cr heads, the MEP liners showed little advantage in terms of volumetric wear compared with the control. However, the MEP liners overall showed a 38% reduction in FBA compared to the control owing to a larger volume fraction of larger particles. Thus overall the MEP liners appeared to offer advantages in terms of reduced FBA indices.

Performance of dedicated emission mammotomography for various breast shapes and sizes.

We evaluate the effect of breast shape and size and lesion location on a dedicated emission mammotomography system developed in our lab. The hemispherical positioning gantry allows ample flexibility in sampling a pendant, uncompressed breast. Realistic anthropomorphic torso (which includes the upper portion of the arm) and breast phantoms draw attention to the necessity of using unique camera trajectories (orbits) rather than simple circular camera trajectories. We have implemented several novel three-dimensional (3D) orbits with fully contoured radius-of-rotation capability for compensating for the positioning demands that emerge from different breast shapes and sizes. While a general orbit design may remain the same between two different breasts, the absolute polar tilt range and radius-of-rotation range may vary. We have demonstrated that using 3D orbits with increased polar camera tilt, lesions near the chest wall can be visualized for both large and small sized breasts (325 ml to 1,060 ml), for a range of intrinsic contrasts (three to ten times higher activity concentration in the lesion than breast background). Overall, nearly complete 3D acquisition schemes yield image data with relatively high lesion SNRs and contrasts and with minimal distortion of the uncompressed breast shape.

'Severe' wear challenge to 'as-cast' and 'double heat-treated' large-diameter metal-on-metal hip bearings.

The wear generation of double-heat-treated and as-cast large-diameter metal-on-metal (MOM) hip bearings was investigated using standard- and 'severe'-gait simulations. The test hypothesis was that double heat treatment would change MOM hip wear compared with the as-cast condition. Two groups of high-carbon MOM bearings of 40 mm diameter were manufactured and subjected to either hot isostatic pressing (HIP) and solution annealing (SA) or no heat treatment (as cast). The results showed no statistical difference between the two groups under both running-in and steady state conditions. Even under the most 'severe'-gait simulation published to date, the mean volumetric wear rates were 2.9 and 3.9 mm3 per 10(6) cycles for the HIP-SA and as-cast bearings respectively, showing a ten-fold increase in wear compared with walking. These differences were not statistically different; therefore our hypothesis was negated. Changes in alloy microstructure do not appear to influence the wear behaviour of high-carbon cast MOM articulations with similar chemical compositions. This is in sharp contrast with the published significance of bearing diameter and radial clearance on the wear of MOM hip bearings.

Metal-on-metal hip simulator study of increased wear particle surface area due to 'severe' patient activity.

This study investigated changes in metal-on-metal (MOM) hip wear and wear particle characteristics arising from a more aggressive patient activity level compared with normal walking. The test hypothesis was that 'severe'-gait conditions will change wear, wear particle sizes, and morphology owing to a decline in joint lubrication. Four carbon MOM hip bearings 40 mm high were subjected to normal-walking and fast-jogging simulations in an orbital hip joint simulator with 25 per cent alpha-calf serum as a lubricant. Co-Cr-Mo wear particles were extracted using an enzymatic method, and prolate ellipsoid equations were used to estimate particle volume and surface area. Fast-jogging simulations generated a sevenfold increase in volumetric wear, a 33 per cent increase in mean wear particle size, and a threefold increase in the number of larger (needle) particles compared with walking. This resulted in a twentyfold increase in total wear particle surface area per 10(6) cycles compared with walking, thereby confirming our hypothesis. The clinical significance of this result suggests that highly active MOM patients may exhibit greater ion release than less active patients.

Analytic determination of the pinhole collimator's point-spread function and RMS resolution with penetration.

Pinhole collimators are widely used to image small organs and animals. The pinhole response function (PRF) of knife-edge pinhole collimators has been estimated previously using geometric constructions without considering penetration and using "roll-off" models that employ an exponential model for the flux. An analytic expression for the PRF on the imaging plane that includes the effect of aperture penetration is derived in this paper by calculating the flux for photons passing through the aperture and those passing through the attenuating material. The PRF is then used to approximate the angular-dependent root-mean-square resolution in the directions parallel and perpendicular to the tilt of the point source. The corresponding aspect ratio is then obtained. The formulas are then compared with experimental data.

Modeling the axial extension of a transmission line source within iterative reconstruction via multiple transmission sources.

Reconstruction algorithms for transmission tomography have generally assumed that the photons reaching a particular detector bin at a particular angle originate from a single point source. In this paper, we highlight several cases of extended transmission sources, in which it may be useful to approach the estimation of attenuation coefficients as a problem involving multiple transmission point sources. Examined in detail is the case of a fixed transmission line source with a fan-beam collimator. This geometry can result in attenuation images that have significant axial blur. Herein it is also shown, empirically, that extended transmission sources can result in biased estimates of the average attenuation, and an explanation is proposed. The finite axial resolution of the transmission line source configuration is modeled within iterative reconstruction using an expectation-maximization algorithm that was previously derived for estimating attenuation coefficients from single photon emission computed tomography (SPECT) emission data. The same algorithm is applicable to both problems because both can be thought of as involving multiple transmission sources. It is shown that modeling axial blur within reconstruction removes the bias in the average estimated attenuation and substantially improves the axial resolution of attenuation images.

Analytic determination of pinhole collimator sensitivity with penetration.

Pinhole collimators are widely used to image small organs and animals. The sensitivity of knife-edge pinhole collimators has been previously estimated using an "effective diameter" formulation and experimentally described using a sin(x) theta fit, where theta is the angle between the line segment from the center of the aperture to the photon source and its projection onto the plane of the aperture. An analytic form of the sensitivity of the pinhole collimator is derived in this paper. A numerical formula for predicting the sin(x) theta form of the sensitivity is calculated from the analytic form. Experimental data are compared with the theoretical estimate and the sin(x) theta prediction. The agreement is excellent.

An EM algorithm for estimating SPECT emission and transmission parameters from emissions data only.

A maximum-likelihood (ML) expectation-maximization (EM) algorithm (called EM-IntraSPECT) is presented for simultaneously estimating single photon emission computed tomography (SPECT) emission and attenuation parameters from emission data alone. The algorithm uses the activity within the patient as transmission tomography sources, with which attenuation coefficients can be estimated. For this initial study, EM-IntraSPECT was tested on computer-simulated attenuation and emission maps representing a simplified human thorax as well as on SPECT data obtained from a physical phantom. Two evaluations were performed. First, to corroborate the idea of reconstructing attenuation parameters from emission data, attenuation parameters (mu) were estimated with the emission intensities (lambda) fixed at their true values. Accurate reconstructions of attenuation parameters were obtained. Second, emission parameters lambda and attenuation parameters mu were simultaneously estimated from the emission data alone. In this case there was crosstalk between estimates of lambda and mu and final estimates of lambda and mu depended on initial values. Estimates degraded significantly as the support extended out farther from the body, and an explanation for this is proposed. In the EM-IntraSPECT reconstructed attenuation images, the lungs, spine, and soft tissue were readily distinguished and had approximately correct shapes and sizes. As compared with standard EM reconstruction assuming a fix uniform attenuation map, EM-IntraSPECT provided more uniform estimates of cardiac activity in the physical phantom study and in the simulation study with tight support, but less uniform estimates with a broad support. The new EM algorithm derived here has additional applications, including reconstructing emission and transmission projection data under a unified statistical model.