Evaluation of XRI-UNO CdTe detector for nuclear medical imaging

Over the last two decades advances in semiconductor detector technology have reached the point where they are sufficiently sensitive to become an alternative to scintillators for high energy gamma ray detection for application in fields such as medical imaging. This paper assessed the Cadmium-Telluride (CdTe) XRI-UNO semiconductor detector produced by X-RAY Imatek for photon energies of interest in nuclear imaging. The XRI-UNO detector was found to have an intrinsic spatial resolution of <0.5mm and a high incident count rate capability up to at least 1680cps. The system spatial resolution, uniformity and sensitivity characteristics are also reported.


Introduction
In the field of medical imaging, small field of view (SFOV) gamma cameras have been developed to give high photon detection efficiency together with high spatial resolution. SFOV cameras have the potential to be used as dedicated small-organ imaging systems, such as for cardiac and breast imaging, intraoperative sentinel lymph node mapping, thyroid or parathyroid imaging and for radiopharmaceutical guided surgery [1]. Scintillation detectors and semiconductor detectors are the most commonly used radiation detectors in nuclear measurement systems to convert radiation energy into an electronic signal.
Although the use of CdTe detectors in medical imaging systems has been limited, there have been some applications of these detectors in diagnostic X-ray mammography and (SFOV) gamma imaging [2,3]. These CdTe devices have pixel detector arrays to convert the incident photons directly into charge signals therefore offering new potential as imaging systems [4].
A fundamental phase in the management of patients with breast tumours is the assessment of the sentinel lymph nodes (SLNs) to determine if there has been tumour spread form the primary site. During breast cancer surgery the sentinel lymph nodes (SLNs) are identified using lymphoscintigraphy. The preferred radiopharmaceutical for this procedure is 99m Tc labelled nanocolloid. During the operation a gamma probe is used for measuring the radioactive uptake in SLNs, however the introduction of imaging probes based on semiconductor detectors has been developed [5].
-1 -Examples of CdTe semiconductor detection for X-ray and gamma-ray imaging indicate that the newly developed semiconductor detectors have the potential to be competitive with the existing imaging devices based on scintillators [4,5].
We present an evaluation of the XRI-UNO CdTe-based detector for medical imaging with 99m Tc and other radioactive sources [6]. The measured performance characteristics include: spatial resolution (both intrinsic and system), intrinsic spatial uniformity (integral and differential), intrinsic sensitivity, count rate capability and contrast to noise ratio.

Properties of CdTe
Cadmium Telluride (CdTe) is an example of a semiconductor that can be used for high energy photon detection due to its high absorption properties.
The absorption of photons by a material can be calculated by: where, I 0 is the initial intensity of photon flux, t the thickness of the attenuator and µ the linear attenuation coefficient [7]. As the photon interaction mechanisms are energy dependent, therefore the linear attenuation coefficient depends mainly on energy. The attenuation is usually expressed using the mass attenuation coefficient µ/ρ where ρ is the density of the medium (gcm −3 ) [8]. For CdTe the mass attenuation coefficient is 0.6071 cm 2 g −1 at a photon energy of 150 keV and its density is 6.20 gcm −3 , therefore the linear attenuation coefficient µ is 3.67 cm −1 at 150 keV.
The absorption curves for a range of CdTe thicknesses of 600, 1200 and 1400 µm are shown in figure 1 as a function of the photon energy (keV). The mass attenuation coefficients µ/ρ have been derived from the NIST database [9].

XRI-UNO system
The XRI-UNO system is an imaging device which uses the Medipix2 chip. The Medipix2 chip is a high spatial resolution, high contrast, complementary metal-oxide semiconductor (CMOS) pixel readout chip. It can be combined with different semiconductor sensors which convert the X-ray directly into detectable electric signals. The Medipix2 readout chip is bump bonded to the 1 mm thick CdTe detector. The Medipix2 measures 16.12 mm × 14.11 mm with an active area of 19.82 mm 2 . It consists of 256 × 256 identical elements, each working in single photon counting mode for positive or negative input charge signals, each pixel occupies a total area of 55 µm × 55 µm. This detector normally operates at a temperature above 32 • [6]. Figure 2 (top) shows a photograph of the detector with a white square indicating the active area. A schematic is also shown in figure 2 (bottom), which details the layout of the CdTe sensor within the detector casing. Table 1 shows the physical specifications of the detector as provided by the manufacturer.

Imaging procedure
All images, unless otherwise specified, were subject to the same image processing; a hot pixel mask was created by taking several background images and then the internal masking program was used to mask hot pixels. Background images were taken each day and subtracted from each image. Masks were re-created about every 3 hours. A number of radioactive sources were used: 109 Cd, 57 Co and 99m Tc (see table 2).

Intrinsic spatial resolution
In medical imaging, spatial resolution is one of the most important factors when considering a new device, as it is the accuracy at which the detector is able to distinguish a discrete source.
To measure the intrinsic spatial resolution, a 5 mm thick lead block with a slit of 2 mm width and 20 mm length was placed on top of the detector case and a 109 Cd source (7.92 mm in diameter) [10] was placed at a distance of 107 mm above the surface of the lead block in order to produce a uniform flux of X-rays onto the surface to approximate parallel incident photons. Ten images were then taken, each with a total of 999 frames (with each frame having an acquisition time of 100 ms), these were combined and fitted together using ImageJ software [11]. A typical image is shown in figure 3A.

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A line profile was then found by placing a line segment horizontally across the image. The resulting values were analysed using Origin software [12] to find the Edge Response Function (ERF) figure 3B. The derivative of the ERF was taken to produce the Line Spread Function (LSF) figure 3C and the modulus of the LSF was taken to produce figure 3D. The Full Width at Half Maximum (FWHM) and the Full Width at Tenth Maximum (FWTM) were then calculated for each of the LSF peaks, and the average value of the two measurements is shown in table 3. Table 3 also shows an estimation of errors in the FWHM of both peaks.

System spatial resolution
A more clinically relevant experiment was performed to investigate the spatial resolution of the XRI-UNO. This was achieved by imaging the source with a collimator and suitable scattering media in place. A pinhole collimator was used for imaging as it increases the field of view. Mejia et al. [13] state that by reducing the pinhole's diameter, the spatial resolution is improved. The collimator was a 0.5 mm diameter tungsten pinhole collimator, which measured 6 mm thick and 45 mm in diameter with an acceptance angle of 60˚ [14]. The collimator was placed on top of the detector and masking tape was used to keep it correctly positioned (making sure to only mask the areas around the pinhole). A layer of PMMA poly methyl methacrylate (Perspex) with 9.5 mm thickness was then placed on top of the collimator, with the 109 Cd source also positioned on the Perspex and taped down. A 999 frame images was taken having a ∼10 s acquisition time. This process was repeated with more layers of Perspex (increasing up to 47.5 mm in 5 steps of 9.5 mm); increasing the source to detector distance. The experimental setup is illustrated in figure 4, where the pinhole collimator to detector distance is fixed t= 10 mm. the distance between the pinhole collimator to the source being imaged is varied depending on the experiment and this distance determines the demagnification on the detector. Five images of the 109 Cd radioisotope source were obtained with different Perspex thicknesses. Figure 5 shows examples of these images. In figure 5A the 109 Cd source is 17 mm away from the collimator with 9.5 mm Perspex. And in figure 5B the 109 Cd source is 57 mm away from the collimator with 47.5 mm Perspex. All images were processed using ImageJ in order to obtain their line profiles, which were then used to find the spatial resolution. Figure 6 shows the relationship of spatial resolution, both FWHM and FWTM as a function of increasing Perspex thicknesses. The magnification factor was calculated [15] for each source distance and then used to calculate the actual FWHM and FWTM values. A linear fit was found (Adj. R 2 = 0.950) for the FWHM data and (Adj. R 2 = 0.892) for the FWTM data. Error bars represent the accuracy of the Gaussian distribution fitting.
-6 -  Commonly, the resolution of a LFOV cameras is given as a single value, measured at the collimator face. The FWHM of the XRI-UNO detector can be calculated for any known distance using slope-intercept formula from the line fit, equation (2.2): where d is the distance from the detector to the source. When d = 10 mm, the distance between the detector and the collimator face, the source is said to be at the non-magnifying position. Thus, the system spatial resolution FWHM value at the non-magnifying position was found to be 1.61 mm.

Intrinsic spatial uniformity
Several quantitative measurements can be made of the spatial uniformity of a detector; integral uniformity and differential uniformity. For this investigation the standard measurements suggested in the IPEM report 86 [16] were used. The integral uniformity (IU) (i.e. across the entire detector) was calculated using: where, C max and C min are the maximum and minimum counts detected per pixel by the detector. Equation (2.3) gives the overall uniformity of the detector, to look at the local variations the differential uniformity (DU) can be calculated using equation (2.3) for a small localised group of pixels. IPEM suggest calculating the DU 10 times for each pixel by using the closest five pixels in a row and a column across an entire image, and then calculating the mean of these values. The image used to measure the spatial uniformity was produced by taking a flood image of a 57 Co source (6 mm in diameter) [10] placed at a distance of 107 mm away from the detector, a distance at which it would produce a uniform illumination on to the detector. Figure 7 shows a typical image produced. It should be noted that in the right upper corner there is an area of non-uniformity.
The IU for any detector is 100% if there is a minimum count value of 0, which was the case for these images. Therefore, IPEM report 86 [16] suggests a more useful measure is the coefficient of variation (COV) which is the standard deviation of counts per pixel divided by the mean counts. The COV of the XRI-UNO detector is 38%. The DU for this detector was found to be 5.174%. Figure 7. A raw flood image of a 57 Co source placed at 107 mm away from the detector to measure the spatial uniformity.

Intrinsic sensitivity
Intrinsic sensitivity is a quantitative measure of the detector's ability to measure incident photon counts. To measure the intrinsic sensitivity, a 109 Cd source was placed at a fixed distance of 357 mm away from the surface of the detector with increasing layers of Perspex (ranging from 0-65 mm) on top of the detector. The sensitivity was calculated by dividing the total recorded counts by the incident counts on the detector. The incident counts were calculated using the solid angle, the -8 -

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known activity of the source, the distance to the detector and the detector size; equation (2.4): Where, (Ω) is the solid angle, (h) source to detector distance and (ab) is the dimension of the detector (i.e. height and width). Each of the images were analysed and the sensitivity calculated, figure 8. An exponential curve was fitted (Adj. R 2 = 0.988) to the data shown in Figure 8. The experimental sensitivity was found to be 99% for the 109 Cd source with (22 keV energy), this is in close agreement with the theoretical sensitivity of 99% calculated with the known detector parameters; thickness, mass attenuation and density. The sensitivity for the 99m Tc source (140 keV) was calculated at 31.3%. A comparison of theoretical and experimental values is shown in figure 9.

Count rate capability
To measure the count rate capability of the detector -its ability to linearly measure counts -a container of 99m Tc source (10MBq) was placed directly on top of the detector to allow for maximum saturation. In addition to 99m Tc being the main radionuclide used in nuclear medicine this source was chosen due to its short half-life (6 hours). Images of 999 frames were taken over a period of 3 days. Incident counts were plotted against measured counts to produce a count rate capability curve (see figure 10). A linear fit is calculated for the linear proportional section of the curve (Adj. R 2 = 0.999) which indicates that the XRI-UNO detector behaves linearly until at least 1680 counts per second (cps) incident on the detector.

Contrast to noise ratio
Image noise is a very important parameter in medical imaging. There are two types of image noise; random and structured noise. Random noise is the main cause of concern in nuclear medicine, and can be present everywhere in the image. Random noise can reduce the detectability of the system especially if the source being imaged has low contrast. The critical factor for detectability -10 -is the contrast to noise ratio (CNR) of the source in the image. Detectability of the source not only depends on the contrast of the source, but also on the size of the source imaged and on the background noise [17]. The CNR can be calculated using the following equations [18]: Where C is the contrast that is measured with consideration of a background region. N l is the average number of counts per pixel measured in the region of interest (ROI). N bg is the average number of counts of the background, COV is the coefficient of variation, σ bg is the standard deviation of the background. To measure the CNR of the detector, a 109 Cd source was placed at fixed distance 100 mm from the collimator while the collimator to detector distance was varied from 10 mm to 40 mm. The collimator used was a 0.5 mm diameter pinhole. Images were acquired with 999 frames and each frame had an acquisition time of 100 ms. A 'ROI' region was defined based on visual inspection of the image of the source and a 'noise' region of identical size was also chosen. The CNR was calculated for each image and the results are shown in figure 11. An exponential fit was calculated (Adj. R 2 = 0.995) for the CNR data and also is plotted on figure 11. Figure 11. Contrast to noise ratio (CNR) of a 109 Cd source placed at 100 mm away from the 0.5 mm pinhole collimator. Distance from collimator to detector was varied from 10 mm to 40 mm. Table 4 contains the measured performance characteristics for the intrinsic spatial resolution, intrinsic spatial uniformity, intrinsic sensitivity and count rate capability of the XRI-UNO detector. System spatial resolution results are also presented, using a 0.5 mm pinhole collimator.

Results and discussion
-11 - The measured intrinsic spatial resolution of the XRI-UNO detector shows that the XRI-UNO operates with a small spatial resolution. This is expected as the XRI-UNO is a direct photon counting device. However a detector's intrinsic resolution is usually much smaller than that of the entire system and the system spatial resolution is a more clinically relevant value. In this investigation a 0.5 mm diameter pinhole collimator was used to assess the spatial resolution of the XRI-UNO detector. As stated previously by Mejia et al. [13] the pinhole's diameter can be reduced to improve the spatial resolution but this would be at the cost of the sensitivity of the detector. Also, single pinhole collimators are better than multi-hole collimators in improving the spatial resolution because of the associated intrinsic magnification factor. The magnification factor is the ratio between the collimator to source distance and the collimator to detector distance. So with this in mind the system spatial resolution of the detector was determined and was found to have a value of 1.61 mm at the non-magnifying point using a 0.5 mm diameter pinhole collimator.
From the measurements of spatial uniformity, it is interesting to note that in the upper right corner figure 7, there is an area of non-uniformity. The exact reason is currently unknown but may be the result of radiation degradation [19] , minor mechanical or electrical damage to the detector. As the integral uniformity for the XRI-UNO may provide inappropriate values, the COV was measured instead. The COV for the XRI-UNO is 38%. The mean differential uniformity of the XRI-UNO was found to be 5.17%.
After investigating the intrinsic sensitivity, it was found that using the 109 Cd source with an energy of 22 keV, the sensitivity was approximately 99%. This is remarkably close to the 99% theoretical efficiency calculated for the 109 Cd source at 22 keV energy.
The detector has a linear count rate up to at least 1680counts per second (cps) incident.
For contrast to noise ratio there was a declining trend as the collimator to detector distance increases, which is thought by Strauss [20] to be mainly due to the decrease in contrast caused by increasing the distance, as fewer photons are detected by the detector. In addition, according to Dickerscheid [18] a loss in resolution will reduce the CNR due to scatter and attenuation. The -12 -

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Rose criterion states that "if the CNR of a hotspot becomes smaller than 3 to 5, it becomes very difficult to observe the hotspot" [21]. In the XRI-UNO detector the worst ratio detected at furthest distance measured (i.e. 40 mm collimator to detector distance) is 12. This value suggests that the detector has the ability to detect the source used as it was above the threshold value.

Conclusion
The performance of the XRI-UNO has been investigated indicating that a further characterisation of the full clinical performance, including energy resolution, should be carried out. The results presented in this paper show that the XRI-UNO has promising performance for use with lower energies less than 30 keV, but due to relatively thin CdTe layer would not be suitable for imaging with higher energy radionuclides such as 99m Tc, which is most commonly used in clinical nuclear imaging.