Discussion 1 Spatial resolution

Spatial resolution is defined as the size of the smallest possible feature that can be detected by an imaging system. The spatial resolution properties of an imaging system may be described by its MTF. MTF describes the variation of apparent contrast that is recorded by the imaging

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system as a function of spatial frequency. As for clinical CT, the spatial resolution of the micro-CT system is affected by various physical parameters such as the focal spot size, the detector element size, the magnification ratio, the reconstruction algorithm and detector resolving power (Holdsworth et al., 1993).

The spatial resolution of the MARS-CT3 system is 110 µm at the magnification ratio of 1.44 is sufficient for pre-clinical examinations such as scans of tissue samples, organs or small animals that are used as models to evaluate human diseases. In atherosclerotic plaque study, this configuration system may not be able to resolve the iron deposits or fine micro- calcification with a single energy measurement (Langheinrich et al., 2009). The detection of iron deposits in the plaque can be differentiated from their morphological features at voxel sizes up to (200 µm)3 with multi-energy measurements (Wang et al., 2010). In this case, the detection of iron deposits in the plaque is not limited to the spatial resolution of the system. However, better spatial resolution might be obtained from the MARS-CT3 system by using a higher magnification ratio or an x-ray tube with a smaller focal spot size.

9.4.2 Image uniformity

The CT number profile through the water phantom is essentially uniform. Ideally, the same attenuation coefficient should be recorded for a homogenous material regardless of the measurement position within a reconstructed slice. However, some systematic effects such as beam hardening that occurs with polychromatic sources cause cupping artefacts in the reconstructed slices and require specific corrections to the data prior to reconstruction (Herman, 1979). The polynomial beam-hardening correction reduces the cupping artifacts and improves the uniformity of CT image.

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9.4.3 Pixel noise

Image noise is the random fluctuations of pixel counts in a region that receives the same radiation exposure as another. The statistical fluctuation of CT numbers within a homogenous region of interest contributes to the measurement uncertainties of derived quantities such as linear attenuation and material composition. Statistical noise particularly affects the ability to detect low contrast materials (Lin et al., 1993). The amount of random quantum noise present in the reconstructed image depends on scan parameters such as mAs and ideally other sources of system error should be relatively insignificant (Kalender, 2001).

As expected image pixel noise increases with decreasing tube current. The tube kilovoltage and current affect both image contrast and dose delivered to the object. Therefore, these technical parameters must be selected carefully to provide adequate CNR at minimal dose. A tube voltage of 50 kVp was chosen for this study because the focus is on soft tissue imaging and other studies also used the same tube voltage for this application (Zhu et al., 2009; Zhang et al., 2010). The use of a higher tube voltage would reduce contrast resolution for soft tissues whereas the x-ray flux obtained with lower voltages would be insufficiently penetrating and beam hardening effects would become significant.

The general form of the relation between pixel noise and exposure obtained from our results (Figure 9.5) concur with the established result that image noise is inversely related to the square root of exposure Q (Judy et al., 1977; Lin et al., 1993; McNitt-Gray, 2006). Total measured pixel noise can be written as the quadrature summation of random quantum noise and system noise (Du et al., 2007; Zhu et al., 2009). The systematic component of the noise is given by the intercept (12.6 HU) of Figure 9.5. For low exposure measurements, the contribution from random quantum effects will dominate the total noise of the system whereas

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the systematic contribution will limit the performance of the MARS-CT3 system for higher exposures.

9.4.4 Linearity

Linearity is important for quantitative applications such as the analysis of blood clots and calcium deposits in atherosclerotic plaque (Langheinrich et al., 2007; Hyafil et al., 2007), fat or iron in the metabolic syndrome (Luu et al., 2009) and improved detection of

microcalcification in breast cancer (Lemacks et al., 2002). These specific tasks require accurate CT numbers to be able to distinguish different types of tissues. The linear range of

the MARS-CT3 system operated at 50 kVp extends to approximately 2000 HU which is adequate for most soft-tissue materials. However, the CT numbers of more highly attenuating materials may not be accurately measured by the system in its current configuration.

Optimisation of scanning parameters such as peak voltage and selection of energy bins will be required for acceptable performance in some applications. As well as the beam hardening mentioned above, it can cause an artificial reduction in the measured attenuation values that appear as dark bands or streaks between dense objects. The magnitude of the effect depends on the thickness, density and atomic number of tissues in the ray path (Judy et al., 1977). The Source-Ray x-ray tube used for this work includes filtering equivalent to 1.8 mm aluminium that is designed to remove the low energy portion of the spectrum and thereby reduce beam hardening effects. Nevertheless, beam hardening streaks are evident in the measurements of the calcium chloride linearity phantom (Figure 9.6).

9.4.5 Spectroscopic calibration

Spectral CT imaging is becoming an important pre-clinical technique for determining material types (Schlomka et al., 2008; Anderson et al., 2010). Spectral CT is able to discriminate and

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quantify materials according to differences in the energy dependence of their attenuation coefficients. The x-ray attenuation properties are energy and material dependent. The spectroscopic characteristics of CT number measured with the MARS-CT3 system are consistent with the energy dependent attenuation coefficients of the test materials (Ca, Fe, I, oil and water).

At low energies the CT number response for the higher-Z materials is much greater than that of water due to the strong influence of the photoelectric effect at these energies. With increasing energy the CT numbers for these materials decrease as the relative contribution of the photoelectric effect reduces and Compton scattering becomes more significant. The CT numbers of the lower-Z materials increase with energy. The CT number for iodine increases with energy due to the influence of the K-edge at 33.2 keV. The reference materials within the phantom are relevant to pre-clinical spectral CT research. It is therefore expected that a basis for quantification can be established for the analysis of tissue specimens and small animal models (Zainon et al., 2011).

9.4.6 Dose measurement

The issue of radiation exposure is generally not a problem for in vitro measurements of samples such as surgical specimens or dead specimens. The dose rate measurement at the isocentre of the scanner for specimen and phantom imaging in this study is 1 mGy.s-1. This dose rate is acceptable for in vitro imaging of specimens but must be taken into account when imaging live biological systems such as intact specimens and live animals.

The radiation dose depends in tube current (amperage), scan time and tube peak kilovoltage. Findings of our measurements are in good agreement with the expectation that a 50% reduction in mA produces a 50% reduction in dose and it also shows that image noise increases by 40% with this condition. Another aspect that affects radiation dose is the tube

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voltage. Increasing the tube voltage increases the intensity and mean energy of the x-ray output thereby increasing the radiation dose rate at the sample. Furthermore, higher energy x- rays are more penetrating and there will be a consequent increase in the intensity of the x-ray flux that passes through the object to reach the detector.