David P. McElroy

Performance evaluation of A-SPECT: a high resolution desktop pinhole SPECT system for imaging small animals

Pinhole collimation of gamma rays to image distributions of radiolabeled tracers is considered promising for use in small animal imaging. The recent availability of transgenic mice, coupled with the development of /sup 125/I and /sup 99m/Tc labeled tracers, has allowed the study of a range of human disease models while creating demand for ultrahigh resolution imaging devices. We have developed a compact gamma camera that, in combination with pinhole collimation, allows for accessible, ultrahigh resolution in vivo single photon emission computed tomography (SPECT) imaging of small animals. The system is based on a pixilated array of NaI(Tl) crystals coupled to an array of position sensitive photomultiplier tubes. Interchangeable tungsten pinholes with diameters ranging from 0.5 to 3 mm are available, allowing the camera to be optimized for a variety of imaging situations. We use a three dimensional maximum likelihood expectation maximization algorithm to reconstruct the images. Our evaluation indicates that high quality, submillimeter spatial resolution images can be achieved in living mice. Reconstructed axial spatial resolution was measured to be 0.53, 0.74, and 0.96 mm full width at half maximum (FWHM) for rotation radii of 1, 2, and 3 cm, respectively, using the 0.5-mm pinhole. In this configuration, sensitivity is comparable to that of a high-resolution parallel hole collimator. SPECT images of hot- and cold-rod phantoms and a highly structured monkey brain phantom illustrate that high quality images can be obtained with the system. Images of living mice demonstrate the ability of the system to obtain high-resolution images in vivo. The effect of object size on the quantitative assessment of isotope distributions in an image was also studied.

Evaluation of breast tumor detectability with two dedicated, compact scintillation cameras

As part of the development of dedicated scintillation cameras for breast imaging, we compared the performances of two dedicated cameras with a standard clinical camera (Siemens Orbiter). One dedicated camera was based on a single position-sensitive photomultiplier tube (PSPMT) coupled to a 6 cm, by 6 cm by 6 mm NaI(Tl) crystal. The other (LumaGem) was based on multiple-PSPMTs coupled to a 2 mm /spl times/ 2 mm by 6 mm matrix of NaI(Tl) crystals. Spatial resolution was measured with all cameras as a function of depth. The ability of the cameras to image small superficial tumors was tested with a phantom consisting of six hot cylindrical tumors of height 3 mm and varying diameters against a warm background. The tumors were imaged at various depths within the background using tumor-to-background activity concentration ratios of 10:1 and 5:1. Tumor contrast and signal-to-noise ratio were measured in all cases for all cameras. The dedicated cameras show improved shallow depth of field spatial resolution, contrast, and signal-to-noise ratio relative to the clinical camera when imaging the breast tumor phantom., This, with the additional advantage of being able to position the cameras close to the region of interest, suggests that dedicated cameras will play a strong role in scintimammography and other shallow depth of field applications.

High-resolution hand-held gamma camera

A high resolution, hand-held scintillation camera has been designed and built for specific Nuclear Medicine applications. Primary intended applications are pre-surgical and intra-operative lymphoscintigraphy. The detector head is highly compact with a 1-inch by 1-inch physical field of view. A variety of easily interchangeable collimators including parallel hole, diverging hole, and pinhole allow several choices of image parameters including variable spatial resolution, sensitivity and field of view. The camera can be operated in imaging mode or as a probe in a non-imaging mode. Surgeons performing sentinel node surgeries have the option of using the device asa standard audio-guided counting probe or as an imaging device to improve surgical management. The 20 mm FOV camera has 1 mm intrinsic spatial resolution. System FWHM in air is 2.1 mm and 2.6 mm at 0 cm from a high-resolution parallel hole collimator, respectively. FWHM of 3.8 mm is measured 2 cm from a 3 mm pinhole. Pinhole sensitivity is 600 cps/MBq above a 125 cps/MBq background for a 1 cm lesion 1 cm below a water surface. Nodes are identified in images even when overall count rate is not above the background from a nearby injection site.

The use of retro-reflective tape for improving spatial resolution of scintillation detectors

We are investigating the use of a retro-reflective tape (3M Industries), which reflects light along a line parallel to the incident light, as a substitute for Teflon tape or other reflective material used with scintillation crystals. We expect this retro-reflective property to improve the spatial resolution in large continuous crystals relative to that seen with Teflon, or other reflective coatings, when the reflective tape is coupled to the front (radiation incident) side of the crystal. Three CsI(TI) crystals of dimensions 5.8 cm /spl times/ 5.6 cm /spl times/ 5 mm, 6 cm /spl times/ 6 cm /spl times/ 12 mm, and 6 cm /spl times/ 6 cm /spl times/ 20 mm were evaluated by stepping a collimated /sup 99m/Tc source across the crystal face. Images were taken with the crystal coupled to a position-sensitive photomultiplier tube. The energy resolution and signal amplitude were similar for both the reflector and Teflon. The average spatial resolution in the central region of the 5-mm-thick crystal with black sides was measured to be 3.6 /spl plusmn/ 0.3 mm and 5.0 /spl plusmn/ 0.3 mm for retro-reflective tape and Teflon tape, respectively.

Evaluation of performance of dedicated, compact scintillation cameras

As part of the development of dedicated scintillation cameras, the authors compared the performances of 2 dedicated cameras with a standard clinical camera (Siemens Orbiter). One dedicated camera was based on a single Position Sensitive Photomultiplier (PSPMT) coupled to a 6 cm by 6 cm by 6 mm NaI(Tl) crystal and the other was based on multiple-PSPMTs coupled to a 2 mm by 2 mm by 6 mm matrix of NaI(Tl) crystals. Spatial resolution was measured with all cameras as a function of depth. The ability of the cameras to measure small superficial tumors was tested with a phantom consisting of 6 hot cylindrical tumors of height 3 mm and varying diameters against a warm background. The tumors were imaged at various depths within the background using tumor to background activity concentration ratios of 10:1 and 5:1. The dedicated cameras show improved performance in imaging the breast tumor phantom, suggesting that these devices will have a role in scintimammography and assisting in O.R, procedures such as sentinel node dissection and other shallow depth of field applications.

Hand-held gamma-ray imaging device for localization of sentinel lymph nodes

A hand-held scintillation camera has been developed for localizing Sentinel Lymph Nodes (SLN) during or prior to surgery. Use of gamma proves has increased in recent years to identify SLNs as these are increasingly implicated in the spread of cancerous cells. The functionality of this new imaging probe will go beyond that of current non-imaging devices by displaying activity distributions in addition to counting rates. Using different collimators changes the spatial resolution, sensitivity and field of view (FOV) of the camera. In addition, the camera produces audible tones proportional to the count rate. The tone is modulated in real time to indicate the presence of complex distributions of activity within the FOV. The modulated tone alerts the user to localized concentrations of activity that may be nodes. The 22 mm FOV camera has 1 mm intrinsic spatial resolution. System FWHM in air is 2.1 mm and 2.6 mm at 0 cm and 1 cm from a high-resolution parallel hole collimator, respectively. FWHM of 3.8 mm is measured 2 cm from a 3 mm pinhole. Pinhole sensitivity is 600 cps/MBq above a 125 cps/MBq background for a 1 cm lesion 1 cm below a water surface. Nodes are identified in images even when overall count rate is not above background.

Effects of Varying Exit Exposure and Energy Spectrum on the Conspicuity of Calcifications and Masses in Digitally Acquired Mammograms of Cadaveric Breasts

Screen film mammography is currently accepted as being the best method available for the early detection of breast cancer. Digital mammography promises to replace screen film mammography as the primary screening tool of the future. The primary advantage it offers is the ability to perform image processing for optimisation of image contrast, which may be especially useful for lesion detection in patients with dense breasts [1] and may, despite its lower spatial resolution, make it more effective than conventional mammography at detecting lesions. Screen film systems are limited because there is no possibility for image contrast enhancement and hence when imaging thick or dense breasts, which require a higher energy spectrum for penetration and dose reduction, there is a loss of subject contrast resulting in a loss of calcification detection [2].