Investigation of the relationship between linear attenuation coefficients and CT Hounsfield units using radionuclides for SPECT

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1 Applied Radiation and Isotopes 66 (2008) Investigation of the relationship between linear attenuation coefficients and CT Hounsfield units using radionuclides for SPECT Saxby Brown a,b,, Dale L. Bailey a,c, Kathy Willowson a,c, Clive Baldock a a Institute of Medical Physics, School of Physics, University of Sydney, NSW 2006, Australia b Northern Sydney Cancer Centre, Department of Radiation Oncology, Royal North Shore Hospital, Sydney, NSW 2065, Australia c Department of Nuclear Medicine, Royal North Shore Hospital, Sydney, NSW 2065, Australia Received 30 May 2007; received in revised form 3 December 2007; accepted 7 January 2008 Abstract This study has investigated the relationship between linear attenuation coefficients (m) and Hounsfield units (HUs) for six materials covering the range of values found clinically. Narrow-beam m values were measured by performing radionuclide transmission scans using 99m Tc, 123 I, 131 I, 201 Tl and 111 In. The m values were compared to published data. The relationships between m and HU were determined. These relationships can be used to convert computed tomography (CT) images to m-maps for single photon emission computed tomography (SPECT) attenuation correction. r 2008 Elsevier Ltd. All rights reserved. Keywords: Radionuclide; CT; SPECT; Attenuation correction 1. Introduction Single photon emission computed tomography (SPECT) is an imaging modality used to visualise the biological uptake and distribution of an injected radionuclide. Photons produced through radioactive decay interact with soft tissue and bone inside the patient before reaching an external detector. The probability that a photon will undergo an interaction while passing through a unit thickness of material is defined by the linear attenuation coefficient (m), which has units of cm 1. The linear attenuation coefficient is dependent on the composition of the attenuating material and the photon energy. For a mono-energetic narrow beam of photons, intensity I, the exponential attenuation law states I ¼ I 0 expð mxþ, (1) where I 0 is the initial intensity of the incident beam and x is the thickness of material which the beam passes through. Corresponding author at: Institute of Medical Physics, School of Physics, University of Sydney, NSW 2006, Australia. Tel.: address: sbrown@physics.usyd.edu.au (S. Brown). Fusing SPECT and computed tomography (CT) images has been shown to be a useful clinical method of spatially localising radionuclide uptake within organs and tumour volumes, giving both anatomical and functional information (Roach et al., 2006). X-ray CT data have previously been used to improve the accuracy of a SPECT image by creating a patient specific attenuation map for photon attenuation correction (Fleming, 1989). CT image data are dependent on the density of tissues and the beam energy, where each image pixel is assigned a Hounsfield unit (HU). HU is directly related to m at the effective energy (E CT )of the CT scanner: HU ¼ 1000 mðe CT Þ m w ðe CT Þ m w ðe CT Þ, (2) where m w is the linear attenuation coefficient of water. Linear attenuation coefficients are referred to as being narrow-beam or broad-beam depending on whether they contain scattered transmission photons or not. Narrowbeam linear attenuation coefficients should be used for SPECT attenuation correction when combined with an explicit correction for scattered photons (Bailey, 1998). A previous investigation (Bai et al., 2003) produced a simple mathematical model for calculating linear attenuation /$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi: /j.apradiso

2 S. Brown et al. / Applied Radiation and Isotopes 66 (2008) coefficients of a given material from CT numbers by introducing a material dependent conversion factor that categorised materials into either having an HU less than zero ( water air assumption ) or having an HU greater than zero ( water bone assumption ). This model was based on the piecewise bilinear fitting technique developed by Blankespoor et al. (1996), with a linear fit between the lowest density material (air with an HU of 1000) and water (HU of 0) and a second linear fit between water and the highest density material (cortical bone with an HU41000). In order to use this bilinear model proposed by Bai et al. (2003) for materials with a density greater than water, a sample of cortical bone equivalent material must be CT scanned at each required kvp for accurate conversion of the CT numbers to linear attenuation coefficients. This study aims to quantify the relationship between linear attenuation coefficients and CT HU by performing radionuclide transmission scans using a selection of clinically relevant radionuclides and a selection of materials with densities that span the entire Hounsfield scale (Table 1). These mathematical relationships could be used to accurately convert CT numbers to linear attenuation coefficients for SPECT attenuation correction without having to scan a sample of cortical bone at the required CT kvp. 2. Method 2.1. Material selection Six materials were selected with CT numbers that would span a wide range of densities over the range encountered clinically. Bone equivalent rectangular slabs (Gammex RMI, Middleton, Wis., USA), typically used in radiation oncology dosimetry, were used to provide a high-density bone-equivalent material. Water, defined by the CT number as zero, was included due to its soft tissue equivalence. Lowdensity sawdust, obtained from oak wood, was used to approximate lung tissue. Vegetable oil was used to approximate adipose tissue. Solid perspex and oak wood blocks were included to provide intermediate densities Material properties The water, oil and sawdust were all placed in sealable polypropylene containers (Techno-Plas, SA, Australia), of dimensions 8 cm diameter and 15 cm height and wall thickness 1.5 mm, where the volume of the material could be easily increased for successive transmission measurements. The Perspex and oak wood blocks were cut into five cm sheets with an uncertainty of less than 1 mm in all dimensions. Two bone equivalent slabs obtained were of dimensions cm and another two of dimensions and cm. The elemental composition and mass density (r) of the bone equivalent block was obtained from Gammex RMI (Gammex RMI, Middleton, Wis., USA) and the composition and density of the other materials were estimated from published data (Lide, 1996). A summary of the mass densities and elemental compositions of each of the materials is given in Table Transmission measurements Radionuclide lead containers were modified to produce a narrow beam of gamma-rays that could be directed towards the gamma-camera (Fig. 1). This setup was based Table 1 Summary of the radionuclides used and the corresponding gamma-ray emission energies, relative abundance and initial activity SPECT radionuclide Half-life g-emission energy (kev), abundance Initial activity (MBq) Technetium-99 m ( 99m Tc) 6.01 h 140 (89%) 396 Iodine-123 ( 123 I) 13.2 h 159 (83%) 60 Iodine-131 ( 131 I) 8.02 d 364 (81%); with additional b decay 408 Thallium-201 ( 201 Tl) 3.04 d a (95%), 167 (11%) 102 Indium-111 ( 111 In) 2.80 d 171 (90%), 245 (94%) 269 a201 Tl decays by electron capture to mercury-201 ( 201 Hg) with gamma emission energies of 135 kev (3%) and 167 kev (11%). Due to the low abundance of photons, the unresolved 201 Hg characteristic X-rays are used for SPECT imaging. Table 2 Physical density and weighted elemental composition of the materials Material r (g cm 3 ) w H w O w C w N w Cl w Ca Bone equivalent Perspex Water Vegetable Oil Wood Sawdust

3 1208 ARTICLE IN PRESS S. Brown et al. / Applied Radiation and Isotopes 66 (2008) Fig. 1. Illustration of the experimental setup used to obtain the linear attenuation coefficients for each of the materials. The radionuclides were placed inside the container device and transmission measurements were performed using a gamma-camera. The total counts recorded at each material thickness were obtained and plotted against the thickness in centimetres. on a modified narrow-beam shielding design used previously (Trapp et al., 2002; Brindha et al., 2004). The radionuclide lead container was built by placing two lead pots together and drilling a 5 mm diameter hole through the top of both pots. A glass vial containing the radionuclide could then be placed inside the bottom pot using a removable lead stopper in the base of the container. The initial activity of the five radionuclides (Table 1) was measured before each series of acquisitions. Transmission scans were performed on a Philips SkyLight (Philips Medical Systems, Milpitas, CA, USA) gamma-camera using a VXGP collimator with hexagonal holes of 1.78 mm width, mm septa and 42 mm length. The gamma-camera head was positioned with the

4 S. Brown et al. / Applied Radiation and Isotopes 66 (2008) Fig. 2. Plot of the natural log of the Skylight gamma-camera count rate from a 99m Tc source positioned at a distance of approximately 1.3 m using an increasing number of 1 mm copper absorber plates. Fig. 3. Skylight gamma-camera count rate curve showing the ideal detector response and observed dead-time fall off. collimator surface facing towards the floor (Fig. 1) and the radionuclide lead container was placed on the floor and positioned under the centre of the collimator. The collimator surface was positioned at a distance of one metre from the top of the lead container. A low attenuating polystyrene platform of dimensions cm was held 1 cm above the lead container by laboratory retort stands. This platform was used to keep the materials horizontal and equidistant from the top of the lead container. Each material was placed on the platform over the centre of the narrow beam. A series of scans were then acquired for 60 s at each thickness for each of the materials. The intrinsic camera dead-time was experimentally analysed to estimate the accuracy of the measured count rates. Following the method suggested by Geldenhuys et al. (1988), an experiment was performed utilising a 99m Tc source positioned at a distance of approximately 1.3 m from the centre of the open scintillation camera. Copper plates of thickness 1 mm were then placed between the source and detector to attenuate the count rate. A plot was made of the natural log of the observed count rate against the number of absorber plates (Fig. 2). A line fitted to the low count rate data was back-extrapolated to provide a value for no loss at higher count rates. The region where count rate losses start occurring due to dead-time was approximately 200 kcps. The data were also used to produce a count rate curve (Fig. 3) which depicts the falloff of the observed count rate when the true count rate is of the order of 200 kcps or greater. The observed count rate for a 20% count loss is approximately 300 kcps. The count rates used throughout this research were of the order of approximately 8 kcps, and consequently suffered no losses due to camera dead-time. The attenuated count rate was measured by acquiring events when no attenuating material was in the beam. The counts recorded at each thickness were corrected for radioactive decay (d) during the measurement process. The narrow-beam linear attenuation coefficient of each material was measured by plotting the corrected counts recorded by the gamma-camera against the thickness of attenuating material and determining the exponential slope (Eq (1)). The linear attenuation coefficients were compared to the published X-ray attenuation and scattering data provided by the National Institute of Standards and Technology (NIST), USA (Chantler et al., 2005) CT material scans Each material was scanned in a General Electric RT LightSpeed (GE Medical Systems Milwaukee, Wis., USA) diagnostic X-ray CT scanner to estimate an average HU value over a number of slices using a peak X-ray energy of 120 kvp and a current-time product of 250 mas. Spiral slices were obtained at 5 mm intervals for each of the materials in air. The water, oil and sawdust materials were scanned in the plastic containers (8 cm diameter), with each container filled to a height of 10 cm. A slice thickness of 5 mm was used for these three materials. A single cm block of wood was positioned upright and centred in the scanner with the shortest dimension parallel to the CT X-ray ring. A scan was performed for a single block of Perspex of the same dimensions positioned upright in the scanner. A slice thickness of 2.5 mm was used for the wood and Perspex. The bone equivalent material slab of dimensions cm was placed flat on the CT couch and a slice thickness of 10 mm used. A 1 cm 2 region of interest (ROI) was drawn on each image and an HU value determined within each ROI.

5 1210 ARTICLE IN PRESS S. Brown et al. / Applied Radiation and Isotopes 66 (2008) Results and discussion 3.1. Narrow-beam linear attenuation coefficients The narrow-beam linear attenuation coefficients (Table 3) were plotted as a function of the radionuclide emission energies for each of the materials studied (Fig. 4). The Table 3 Measured narrow-beam linear attenuation coefficients (m) Energy (kev) Water Oil Perspex Wood Sawdust Bone Mean m error: decrease in linear attenuation coefficients values was found to be the greatest for high-density materials, such as bone, and less for the low-density materials with increasing gamma-ray energy. The percentage difference between the measured linear attenuation coefficients and those documented by NIST are given in Table 4. The m values for all the materials were on average 2% less than the NIST values for 140 kev, 4% less for 159 and 167 kev, 3% less for 171 kev, 1% less for 245 kev and 5% less for 364 kev. These underestimations are most likely due to the energy resolution of the inorganic scintillator crystal in the gamma-camera (10% FWHM at 140 kev) and the partial energy deposition of gammarays creating background noise. In order to obtain truly quantitative SPECT reconstructions it may be that a separate (i.e. non-summed) energy window should be assigned to each gamma-ray emission for radionuclides with multiple emission energies. The values for gamma-ray energies kev (from the characteristic X-rays of 201 Hg produced from the decay of 201 Tl) were on average 19% lower than the documented NIST values. The difference at Fig. 4. The measured narrow-beam linear attenuation coefficients plotted against Hounsfield units for five of the radionuclide emission energies.

6 S. Brown et al. / Applied Radiation and Isotopes 66 (2008) Table 4 Percentage differences of measured linear attenuation coefficients to NIST Energy (kev) Water (%) Oil (%) Perspex (%) Wood (%) Sawdust (%) Bone (%) Mean % difference Table 5 Hounsfield unit value for each material Material Hounsfield value (HU) Table 6 Calculated bilinear relationships between the narrow-beam linear attenuation coefficients (m) and Hounsfield unit values (H) at each radionuclide emission energy Bone equivalent Perspex Water 2714 Vegetable oil Wood Sawdust the low energies may be attributed, in part, to an X-ray fluorescent component from the lead collimator HU estimates The measured HU value for each of the six materials are summarised in Table 5. A poly-chromatic CT X-ray beam is hardened by highdensity materials due to the preferential absorption of lower energy components (Zaidi et al., 2003) such that the total number of transmitted X-rays do not obey the exponential attenuation law (Eq. (1)). It has been reported (LaCroix et al., 1994) that CT images accurately estimate the attenuation coefficients for muscle and soft tissue but produce errors in bone of 21 42% for spinal bone and 34 58% for rib bone. The bone equivalent sheets caused notable artefacts in the produced CT image contributing to the uncertainty in the HU value, as shown by LaCroix et al. (1994). The standard deviation in the HU estimate over the ROI was found to be as high as 25% over a number of slices so a small ROI was defined in a homogenous region in the bone block to minimise the error in the HU value Relating linear attenuation coefficients and HUs The seven relationships between linear attenuation coefficients and HUs are summarised in Table 6. Five of the bilinear relationships have been plotted in Fig. 4 to show the trend of the bilinear curves with increasing gamma-ray energy. It has been shown for high-density materials, such as bone, that linear scaling produces inaccurate m values due to photoelectric contributions which dominate at lower CT Energy (kev) energies (LaCroix et al., 1994). Different scaling factors are required for materials with a density higher than that of water because of the increased Compton interactions resulting in more attenuation for lower density materials. A bilinear fit was applied using a narrow-beam of photons with a discontinuity at 0 HU. In a study by Burger et al. (2002), CT numbers were converted into positron emission tomography (PET) 511 kev attenuation coefficients using a combined PET/CT scanner and a bilinear fit. The bilinear equations from this PET/CT study have been included in Table 6. Fig. 2 shows that the relationship between the attenuation coefficients and HU values is linear at low radionuclide emission energies (75 80 kev). The photoelectric effect dominates at low energies (Johns et al., 1983) and the occurrence of photoelectric interactions is proportional to the atomic number (Z) of the material by Z 3 for high-z materials and approximately Z 4 for low-z materials. The linearity observed in Fig. 2 for kev is a result of the photon interactions during the CT scan being similar in nature to the interactions of a transmission scan using a low-energy radionuclide. At higher radionuclide emission energies ( kev) the gamma-rays are more likely to undergo Compton scatter in the attenuating materials before reaching the gamma-camera. 4. Conclusion Bilinear relationship for Ho0 Bilinear relationship for HX m ¼ 0.16+( ) 10 4 H m ¼ 0.16+( ) 10 4 H 140 m ¼ 0.15+( ) 10 4 H m ¼ 0.15+( ) 10 4 H 159 m ¼ 0.14+( ) 10 4 H m ¼ 0.14+( ) 10 4 H 167 m ¼ 0.13+( ) 10 4 H m ¼ 0.13+( ) 10 4 H 171 m ¼ 0.14+( ) 10 4 H m ¼ 0.14+( ) 10 4 H 245 m ¼ 0.12+( ) 10 4 H m ¼ 0.12+( ) 10 5 H 364 m ¼ 0.09+( ) 10 5 H m ¼ 0.09+( ) 10 5 H 511 a m ¼ H m ¼ H a PET/CT data from Burger et al. (2002). Defining the relationship between the linear attenuation coefficient and HU is essential when using X-ray CT data

7 1212 ARTICLE IN PRESS S. Brown et al. / Applied Radiation and Isotopes 66 (2008) for SPECT attenuation correction. This study has determined a series of such relationships for six materials that span the density range encountered clinically. Narrow-beam linear attenuation coefficients were measured for each of the materials using the five SPECT radionuclides. The coefficients were compared to values published by NIST and were found to be on average less than 5% lower for the energy range kev. This difference was attributed to the energy resolution of the crystal and the partial energy deposition of the gamma-rays on the detector. The linear attenuation coefficients corresponding to the low kev gamma-rays were found to be on average 19% lower than the published data from NIST. This underestimation was attributed to the X-ray florescent component from the lead collimator. An average HU value was determined for each of the materials using a 120 kvp X-ray CT scanner. The relationship between narrow-beam linear attenuation coefficient and HU was determined for each radionuclide emission energy using the data obtained for all six materials. This study found that the relationships are linear at low energies and can be defined through bilinear fitting over the energy range kev. References Bai, C., Shao, L., Da Silva, A., Zhao, Z., A generalized model for the conversion from CT numbers to linear attenuation coefficients. IEEE Trans. Nucl. Sci. 50 (5), Bailey, DL., Transmission scanning in emission tomography. Eur. J. Nucl. Med. 25, Blankespoor, S., Wu, X., Kalki, K., Brown, J., Cann, C., Hasegawa, B., Attenuation correction of SPECT using X-ray CT on an emission transmission CT system: myocardial perfusion assessment. IEEE. Trans. Nucl. Sci. 43, Brindha, S., Venning, A.J., Hill, B., Baldock, C., Experimental study of attenuation properties of normoxic polymer gel dosimeters. Phys. Med. Biol. 49, Burger, C., Goerres, G., Schoenes, S., Buck, A., Lonn, A.H., Von Schulthess, G.K., PET attenuation coefficients from CT images: experimental evaluation of the transformation of CT into PET 511- kev attenuation coefficients. Eur. J. Nucl. Med. Mol. Imaging 29 (7), Chantler, C.T., Olsen, K., Dragoset, R.A., Chang, J., Kishore, A.R., Kotochigova, S.A., Zucker, D.S., X-ray Form Factor, Attenuation and Scattering Tables (version 2.1). National Institute of Standards and Technology, Gaithersburg, MD (Available at: / physics.nist.gov/ffasts, accessed 2006, June 18). Fleming, J.S., A technique for using CT images in attenuation correction and quantification in SPECT. Nucl. Med. Commun. 10, Geldenhuys, E.M., Lotter, M.G., Minnaar, P.C., A new approach to NEMA scintillation camera count rate curve determination. J. Nucl. Med. 29, Johns, H.E., Cunningham, J.R., The Physics of Radiology. Thomas Books, Springfield, IL, p LaCroix, K.J., Tsui, B.M.W., Hasegawa, B.H., Brown, J.K., Investigation of the use of X-ray CT images for attenuation compensation in SPECT. Nucl. Sci. 41 (6), Lide, D.R., CRC Handbook of Chemistry and Physics, 77th ed. CRC Press, Boca Raton, FL. Roach, P., Schembri, G., Ho Shon, I., Bailey, E., Bailey, D., SPECT/CT imaging using a spiral CT scanner for anatomical localization: impact on diagnostic accuracy and reporter confidence in clinical practice. Nucl. Med. Commun. 27, Trapp, J.V., Michael, G., de Deene, Y., Baldock, C., Attenuation of diagnostic energy photons by polymer gel dosimeters. Phys. Med. Biol. 47, Zaidi, H., Hasegawa, B., Determination of the attenuation map in emission tomography. J. Nucl. Med. 44 (2),

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