Spatial Resolution Values Research Articles

New generation position-sensitive PMT for nuclear medicine imaging

Recently, Hamamatsu has developed a new dynode structure in a position-sensitive photomultiplier tube (PSPMT) based on metal channel technology, producing a very focused charge distribution. Hamamatsu R5900-C8 is the first metal channel dynode PSPMT with a crossed wire anode. The outline dimension is 28 × 28 × 20 mm 3 with an active area of 21 × 22 mm 2 and four wire anodes (4 X + 4 Y). A weighted summing device was realised to calculate the charge distribution centroid where each anode was directly connected to a preamplifier and amplifier. Tests of position linearity, energy response and spatial resolution were carried out coupling the PSPMT to an NaI(Tl) planar crystal and to a YAP:Ce scintillation array. To test the influence of charge spread on position linearity, the photocathode was irradiated by a light spot with 1 mm of aperture size. Position non-linearity resulted when a scintillating array with 1 × 1 mm 2 pixel size was coupled to PSPMT. On the contrary, increasing the spread of charge distribution by a planar scintillation crystal, a good linearity response was obtained. Impressive spatial resolution values were obtained from the YAP:Ce scintillating array. They ranged between 0.5 and 0.2 mm irradiating the crystals by 57Co and 137Cs source, respectively.

YAP multi-crystal gamma camera prototype

The Anger camera principle has shown a practical limit of a few millimeters spatial resolution. To overcome this limit, a new gamma camera prototype has been developed, based on a position-sensitive photomultiplier tube (PSPMT) coupled with a new scintillation crystal. The Hamamatsu R2486 PSPMT is a 76-mm diameter photomultiplier tube in which the electrons produced in the conventional bi-alkali photocathode are multiplied by proximity mesh dynodes and form a charge cloud around the original coordinates of the light photon striking the photocathode. A crossed wire anode array collects the charge and detects the original position. The intrinsic spatial resolution of PSPMT is better than 0.3 mm. The scintillation crystal consists of yttrium aluminum perovskit (YAP:Ce or YAlO/sub 3/:Ce). This crystal has a light efficiency of about 38% relative to NaI, no hygroscopicity and a good gamma radiation absorption. To match the characteristics of the PSPMT, a special crystal assembly was produced by the Preciosa Company, consisting of a bundle of YAP:Ce pillars where single crystals have 0.6/spl times/0.6 mm/sup 2/ cross section and 3 mm to 18 mm length. Preliminary results from such gamma camera prototypes show spatial resolution values ranging between 0.7 mm and 1 mm with an intrinsic detection efficiency of 37/spl divide/65% for 140 keV gamma energy.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Collimator optimization for lesion detection incorporating prior information about lesion size

A Bayesian estimator has been developed as a paradigm for human observer performance in detecting lesions of unknown size in a uniform noisy background. The Bayesian observer used knowledge of the range of possible lesion sizes as a prior; its predictions agreed well with the results of a six-observer perceptual study. The average human response to changes in collimator resolution, as measured by the detectability index, dA, was tracked by the Bayesian detector's signal-to-noise ratio (SNR) somewhat better than by two other estimation models based, respectively, on lesser and greater degrees of lesion size uncertainty. As the range of possible lesion sizes increased, the Bayesian detector's SNR decreased and the optimal collimator resolution shifted towards better resolution. An analytic approximation for the variance of lesion activity estimates (which included the same prior) was shown to predict the variance of the Bayesian estimator over a wide range of collimator resolution values. Because the bias of the Bayesian estimator was small (< 1%), the analytic variance estimate permitted a rapid and convenient prediction of the Bayesian detection SNR. This calculation was then used to optimize the geometric parameters of a two-layer tungsten collimator being constructed from crossed grids for a new imaging detector. A Monte Carlo program was first run to estimate all contributions to the radial point-spread function for collimators of differing tungsten contents and spatial resolution values, imaging 140-keV photons emitted from the center of a 15-cm-diameter, water-filled attenuator. The optimal collimator design for detecting lesions with unknown diameters in the range 2.5-7.5 mm yielded a system resolution of approximately 8.5-mm FWHM, a geometric collimator efficiency of 1.21 x 10(-4), and a single-septum penetration probability of 1%.

Toward a nuclear medicine with sub-millimiter spatial resolution

The HIRESPET Collaboration is developing a new concept of a gamma camera with sub-millimiter spatial resolution. The first prototype consists of a small field size gamma camera based on a Position Sensitive Photo-Multiplier Tube (PSPMT) coupled to a novel scintillation crystal. The intrinsic spatial resolution of the PSPMT is better than 0.3 mm. The scintillation crystal consists of yttrium aluminium perovskit (YAP:Ce). It has a light efficiency of about 40% relative to NaI, a good gamma radiation absorption (Z = 39) and a high density (5.37 g/cm3). It is inert and not hygroscopic. To match the PSPMT characteristics, a special crystal assembly has been made consisting of a bundle of YAP pillars, where a single crystal has the transversal dimension of 0.6 × 0.6 mm2 and a thickness ranging between 1 mm and 28 mm. Each scintillation pillar is optically separated from the other by a reflective layer of 5 μm thick. The preliminary results obtained from the gamma camera prototype (YAP camera) show spatial resolution values ranging between 0.6 mm and 1 mm and an intrinsic detection efficiency comparable with a standard Anger camera.

MR imaging of avascular necrosis and transient marrow edema of the femoral head.

The value of high spatial resolution and contrast material-enhanced magnetic resonance (MR) imaging was assessed in 69 patients with either femoral head avascular necrosis (AVN) or transient bone marrow edema lesions. An AVN lesion was typically a well-demarcated epiphyseal area of variable signal intensity. A transient bone marrow edema lesion appeared as an ill-delimited low-signal-intensity epiphyseal area on T1-weighted images that converted to a high-signal-intensity area on T2-weighted images. T1- and T2-weighted spin-echo images helped distinguish from AVN lesions some transient lesions apt to simulate the segmental pattern of AVN lesions. T2-weighted images also helped detect necrotic tissue in some unusual AVN lesions that mostly showed ill-delimited edemalike marrow changes. In some cases, contrast-enhanced MR images may increase diagnostic confidence by showing homogeneous hypervascularization in bone marrow edema lesions and by depicting hypovascular marrow areas in AVN lesions. The authors believe sequential MR imaging is valuable in the assessment of equivocal femoral head lesions.

Charge division in a small proportional chamber constructed with aluminized mylar tubes

A tracking detector composed of aluminized mylar drift tubes is under development for the Fermilab experiment 760. A prototype chamber has been constructed. Results on the longitudinal coordinate determined by charge division are given. Spatial resolution values below 2 mm (rms) were found, corresponding to <1% of the chamber length. Results on chamber ageing are also discussed.

Characteristics of a home-made flash X-ray system

A 45 kV flash radiography unit which is distinguished by simplicity, easy maintenance and repair of the electrode system, and good radiographic qualities was built in the authors' laboratory. The radiographic characteristics such as line spread function or modulation transfer function (MTF), focal spot size and shape, radiation intensity profile, effective energy and beam coverage were measured. A comparison with values of focal spot size and spatial resolution (or MTF) obtained for commercial flash units and for conventional X-ray systems shows that the performance of the home-made unit is not inferior.