Continuous bed motion on clinical scanner: design, data correction, and reconstruction.

With the addition of variable bed speed capabilities, continuous bed motion (CBM) becomes an acquisition generalization for axial sensitivity modeling and therefore a flexible tool in axial local image quality adjustment. In this paper, we describe the principles behind CBM mode planning and its influence on image reconstruction. The data correction method underwent the most changes compared to the commonly used step and shoot (S&S) mode. The CBM normalization array accommodates for activity decay, dead time correction, and the fact that various detector pairs acquire the same data for different durations. The normalization array is computed by simulating the movement of an object through the scanner, assisted by the monitoring of basic scanner acquisition parameters such as the singles rate. The sensitivity, which is the number of counts acquired per image plane, is an intrinsic part of normalization computing. Basic equations to estimate acquisition time at matched sensitivity between single speed CBM and S&S mode are presented. The CBM feature was implemented on a Siemens clinical scanner and initial studies of phantoms and patients are presented. The equivalence of single speed CBM and S&S image quality is demonstrated.

Improving PET spatial resolution and detectability for prostate cancer imaging.

Prostate cancer, one of the most common forms of cancer among men, can benefit from recent improvements in positron emission tomography (PET) technology. In particular, better spatial resolution, lower noise and higher detectability of small lesions could be greatly beneficial for early diagnosis and could provide a strong support for guiding biopsy and surgery. In this article, the impact of improved PET instrumentation with superior spatial resolution and high sensitivity are discussed, together with the latest development in PET technology: resolution recovery and time-of-flight reconstruction. Using simulated cancer lesions, inserted in clinical PET images obtained with conventional protocols, we show that visual identification of the lesions and detectability via numerical observers can already be improved using state of the art PET reconstruction methods. This was achieved using both resolution recovery and time-of-flight reconstruction, and a high resolution image with 2 mm pixel size. Channelized Hotelling numerical observers showed an increase in the area under the LROC curve from 0.52 to 0.58. In addition, a relationship between the simulated input activity and the area under the LROC curve showed that the minimum detectable activity was reduced by more than 23%.

Simultaneous reconstruction of emission activity and attenuation coefficient distribution from TOF data, acquired with external transmission source.

The simultaneous PET data reconstruction of emission activity and attenuation coefficient distribution is presented, where the attenuation image is constrained by exploiting an external transmission source. Data are acquired in time-of-flight (TOF) mode, allowing in principle for separation of emission and transmission data. Nevertheless, here all data are reconstructed at once, eliminating the need to trace the position of the transmission source in sinogram space. Contamination of emission data by the transmission source and vice versa is naturally modeled. Attenuated emission activity data also provide additional information about object attenuation coefficient values. The algorithm alternates between attenuation and emission activity image updates. We also proposed a method of estimation of spatial scatter distribution from the transmission source by incorporating knowledge about the expected range of attenuation map values. The reconstruction of experimental data from the Siemens mCT scanner suggests that simultaneous reconstruction improves attenuation map image quality, as compared to when data are separated. In the presented example, the attenuation map image noise was reduced and non-uniformity artifacts that occurred due to scatter estimation were suppressed. On the other hand, the use of transmission data stabilizes attenuation coefficient distribution reconstruction from TOF emission data alone. The example of improving emission images by refining a CT-based patient attenuation map is presented, revealing potential benefits of simultaneous CT and PET data reconstruction.

Thromboelastography to monitor the intra-operative effects of low-molecular weight heparin following bridging anticoagulation in a child with normal renal function*.

Paediatric patients who require anticoagulation with therapeutic doses of low-molecular weight heparin are at risk of having a residual anticoagulant effect at the time of surgery, even if managed according to current peri-operative guidelines. Testing for residual effect is not currently recommended in such circumstances. A 15-year-old child with a mechanical aortic valve replacement requiring long-term warfarin treatment, as well as underlying coagulation defects, was administered low-molecular weight heparin for bridging anticoagulation before kyphoscoliosis surgery. Thromboelastography was used intra-operatively to diagnose residual heparinisation, which was demonstrated by a prolonged reaction (R) time of 16.0 min in the plain cup, compared with 9.2 min in the heparinase cup. Subsequently, thromboelastography was also used to monitor haemostatic therapy, which consisted of protamine 2 mg.kg(-1) and 500 IU cryoprecipitate. Thromboelastography was used intra-operatively to allow rapid testing of coagulation status and guide therapy, thereby minimising use of blood products and reducing complications.

Physical and clinical performance of the mCT time-of-flight PET/CT scanner.

Time-of-flight (TOF) measurement capability promises to improve PET image quality. We characterized the physical and clinical PET performance of the first Biograph mCT TOF PET/CT scanner (Siemens Medical Solutions USA, Inc.) in comparison with its predecessor, the Biograph TruePoint TrueV. In particular, we defined the improvements with TOF. The physical performance was evaluated according to the National Electrical Manufacturers Association (NEMA) NU 2-2007 standard with additional measurements to specifically address the TOF capability. Patient data were analyzed to obtain the clinical performance of the scanner. As expected for the same size crystal detectors, a similar spatial resolution was measured on the mCT as on the TruePoint TrueV. The mCT demonstrated modestly higher sensitivity (increase by 19.7 ± 2.8%) and peak noise equivalent count rate (NECR) (increase by 15.5 ± 5.7%) with similar scatter fractions. The energy, time and spatial resolutions for a varying single count rate of up to 55 Mcps resulted in 11.5 ± 0.2% (FWHM), 527.5 ± 4.9 ps (FWHM) and 4.1 ± 0.0 mm (FWHM), respectively. With the addition of TOF, the mCT also produced substantially higher image contrast recovery and signal-to-noise ratios in a clinically-relevant phantom geometry. The benefits of TOF were clearly demonstrated in representative patient images.

Pre-freeze bull sperm head morphometry related to post-thaw fertility.

Artificial insemination using cryopreserved semen is a common management tool of the contemporary livestock producer. The ability to determine post-thaw fertility from pre-freeze sperm function parameters would be of major benefit to producers. In this study computer-aided sperm head morphometry was used to determine whether there is any association between pre-freeze bull sperm head measurements and post-thaw non-return to oestrus rates. Sixteen commercial artificial insemination bulls were used for this study. Of the 16 bulls, eight had > or =69% non-return to oestrus rates while the remaining eight bulls had <69% non-return to oestrus rates, based on artificial insemination of cryopreserved semen from a minimum of 748 inseminations per bull. Microscope slides of extended or cryopreserved and thawed semen were prepared and stained by the MGZIN procedure. The morphometric dimensions for length, width, width/length, area and perimeter for a minimum of 200 sperm heads were analyzed from each slide by ASMA and the mean measurements and intra-analysis coefficients of variation (CV) recorded. The post-thaw non-return to oestrus rates for all bulls were correlated with pre-freeze measurements of width (r=0.53, P<0.05), and the change in width/length after cryopreservation (r=0.61, P<0.05). There were no significant differences in the mean pre-freeze or post-thaw sperm head measurements between the two groups. However, the mean post-thaw change in width/length was significantly (P=0.003) different between the groups. The mean post-thaw intra-analysis variability for width was significantly lower (P=0.03) in bulls with non-return to oestrus rates (NRR)> or =69%. Overall, the data suggests that pre-freeze and post-thaw bull sperm head morphometry measurements of individual bulls may have minimal predictive value of post-thaw fertility as determined by non-return to oestrus rates between groups of bulls with limited variance in non-return to oestrus rates. The clinical relationship of sperm head morphometry and fertility in bulls with a greater, clinically significant range of non-return to oestrus, remains to be studied.

Testicular and spermatozoan parameters in the pukeko (Porphyrio porphyrio melanotus).

The pukeko (Porphyrio porphyrio melanotus) is widespread in New Zealand, and is the closest living relative to the endangered takahe (Porphyrio mantelli), which has a relatively high rate of infertility. In this study, sperm collected from a number of pukeko was analysed in order to model the reproductive physiology of the male takahe. In addition, testicular parameters were measured. To ascertain the best method of sperm collection five techniques for harvesting sperm were used on two occasions during the breeding season. All five techniques resulted in the successful recovery of sperm. However, the float-out technique produced the best quality samples. Following collection, the morphometry of unstained sperm was assessed. Our findings suggest that pukeko sperm is non-motile in the male reproductive tract. We found the mean sperm head length in the pukeko is 16.9mum, but sperm head length varied significantly between birds. Testicular weight and length was significantly correlated with bird weight (P<0.05). Within each bird, testes weights were asymmetric. However, testes length was significantly correlated (P<0.05). There was a significant difference (P<0.05) in testes length between birds. The methodologies presented for obtaining and analysing pukeko sperm morphometry can be used to assist opportunistic studies of the reproductive biology of other New Zealand native birds.

Physical characteristics of the ECAT EXACT3D positron tomograph.

The 'EXACT3D' positron tomograph, which is now in routine clinical research use, was developed with the aim of achieving unprecedented sensitivity, high spatial and temporal resolution and simplicity of design using proven detector technology. It consists of six rings of standard detector blocks (CTI/Siemens EXACT HR+) with 4.39 mm x 4.05 mm x 30 mm elements, giving an axial field of view (FOV) of 23.4 cm. This extended FOV and the absence of interplane septa and retractable transmission rod sources has allowed greatly simplified gantry and detector cassette design. Operation in exclusive 3D mode requires an alternative to the conventional coincidence method for transmission scanning, and a single photon approach using a hydraulically driven 137Cs point source has been implemented. The tomograph has no other moving parts. A single time frame of data without any compression is very large (> 300 Mbyte) and two approaches are employed to overcome this difficulty: (a) adjacent sinograms can be summed automatically into different combinations and (b) listmode (event-by-event) acquisition has been instituted, which is both storage efficient (particularly for acquisition of sparse data sets) and maximizes temporal resolution. The high-speed I/O and computing hardware can maintain a sustained acquisition rate of about 4 million coincidence events per second. A disadvantage of the large axial FOV in 3D is the increased sensitivity to activity outside the coincidence FOV. However, this can be minimized by additional side shielding. The mean spatial resolution is 4.8 +/- 0.2 mm FWHM (transaxial, 1 cm off-axis) and 5.6 +/- 0.5 mm (axial, on-axis). Its absolute efficiency is 5.8% for a line source in air (just spanning the axial FOV) and 10% for a central point source (with thresholds of 350-650 keV). For a uniform 20 cm diameter cylinder, the efficiency is 69 kcps kBq(-1) ml(-1) (after subtraction of a scatter fraction of 42%). Sensitivity relative to the EXACT HR+ (with four rings of blocks) is 2.5 (3D) and 12 (2D) times respectively. The rate of random events in blood flow studies in the brain and body, using 15O-labelled water, can be controlled by limiting the administered dose and inserting additional side shielding.

Optimizing rod window width in positron emission tomography.

A technique determines the optimal window width for orbiting rod transmission studies in positron emission tomography (PET). Windowing reduces noise in orbiting rod transmission studies. Lines-of-response (LOR) which intersect the rods generate primarily true coincidence events. LOR which pass far from the rods generate random and scatter events. Since the angular position of the orbiting rods is known in real-time, LOR which produce mostly noise are gated off. When optimally determined, the rod window width maximizes the noise equivalent counts (NEC) collected in the transmission study. Transaxial rod projection profiles of trues, randoms, and scatter produce NEC versus window width plots. For the ECAT EXACT line of PET systems and a 20-cm water cylinder, optimal is five LOR wide.

Physical performance of a positron tomograph for brain imaging with retractable septa.

Performance characteristics of a new design of positron tomograph with automatically retractable septa for brain imaging have been studied. The device, consisting of block BGO detectors (8 x 8 elements per block), has a ring diameter of 76 cm and an axial FOV of 106.5 mm. The in-plane resolution is on average 5.8 mm and 5.0 mm (FWHM) for stationary and wobble sampling, respectively, over the central 18 cm of the transaxial FOV. Its unique feature is the capability of data acquisition both in the 'conventional' 2D mode (with septa) or 3D mode (septa retracted) where coincidences between any of the 16 detector rings are acquired. When scattered events are subtracted, the efficiency for a 20 cm diameter uniform cylinder increases overall by a factor of 4.8 between 2D (septa extended) and 3D modes. For a 20 cm phantom the trues/singles ratio is higher for 3D than for 2D but for a given unscattered trues rate, the randoms rate in 3D is higher. At 380 keV the scatter fraction within a 20 cm cylinder is 10% (septa extended) and 36% (retracted). In spite of the increase in scatter when septa are retracted, the increased efficiency in the 3D mode of acquisition yields distinct advantages, particularly in the many studies where tracer concentration is low and consequently where dead time and random rates are less important.

Performance standards in positron emission tomography.

A standard set of performance measurements is proposed for use with positron emission tomographs. This set of measurements has been developed jointly by the Computer and Instrumentation Council of the Society of Nuclear Medicine and the National Electrical Manufacturers Association. The measurements include tests of spatial resolution, scatter fraction, sensitivity, count rate losses and randoms, uniformity, scatter correction, attenuation correction, and count rate linearity correction.

Site-directed metastasis: a model to study site variations in treatment response.

A model for the site-directed metastasis of a transplantable rat mammary carcinoma is described. The R3230AC tumor is a moderately well differentiated, phenotypically stable tumor which arose in a Fischer 344 rat and resembles human breast carcinoma in being hormonally sensitive, poorly immunogenic, and sensitive to radiation and chemotherapeutic agents. Metastases with this tumor can be consistently produced at specific sites (lung, liver, or subcutaneous) in a reproducible manner by the methods described in this paper. This model appears ideal for the evaluation of site-specific variation in response to treatment modalities.

Quantitation in positron emission computed tomography: 7. A technique to reduce noise in accidental coincidence measurements and coincidence efficiency calibration.

A method of processing accidental coincidence events (AC) and detector efficiency (DE) calibration data, which reduces the statistical noise in these measurements, and, consequently, reduces the noise in positron emission tomographic images using the technique, is described. The technique uses the fact that, in these measurements with N detectors in coincidence with N other detectors, N2 values of ACs or DEs are measured. However, these values are composed of only 2N components, which are either singles rates or individual DEs. The full set of data is used to implicitly solve for these values and the individual ACs or DEs recalculated with an improvement in statistical error equivalent to an N2/(2N + 1) increase in accumulated events for the case of a uniform distribution. This result was verified experimentally.

The selection of critical subsets for signal, image, and scene matching.

A basic matching problem consists of locating a reference subset in a larger set of data subject to a given criterion. Variations of the problem include template matching for object recognition, matched filtering for signal detection, image registration, change detection, cartography feature location, correlation guidance, and scene matching. In the general case the data sets may be made by completely different sensors at different geometrical orientations. A data set in N dimensions can be considered as a function in N-space. The critical subset selection problem arises when one is given a function and must select some subset of the function to match with the original. In some cases uniqueness is a key feature of the best subset. Uniqueness may be measured by the number and relative magnitudes of the peaks in the cross correlation function of the original and subset functions. For a unique subset, another desirable characteristic is lack of ambiguity. This characteristic may be measured using the correlation length or 50 percent width of the main correlation peak. The smaller the correlation length, the greater the certainty one has in detecting the correct match position. In this paper a method is presented for selecting the ``best'' subset of a scene where best is defined in terms of the minimum correlation length. The best solution is shown to be a function of the entire scene, i.e., an improper subset.