Comparison of image quality of different iodine isotopes (I-123, I-124 and I-131)

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1 Comparison of image quality of different iodine isotopes (I-123, I-124 and I-131) Erwann Rault, Stefaan Vandenberghe, Roel Van Holen, Jan De Beenhouwer, Steven Staelens, and Ignace Lemahieu MEDISIP, ELIS department, Ghent University, Sint-Pietersnieuwstraat 41, B-9000, Ghent, Belgium Abstract I-131 is a frequently used isotope for radionuclide therapy. This technique for cancer treatment requires a pretherapeutic dosimetric study. The latter is usually performed (for this radionuclide) by directly imaging the uptake of the therapeutic radionuclide in the body or by replacing it by one of its isotopes, more suitable for imaging. This study proposes to compare the image quality that can be achieved by three iodine isotopes: I-131 and I-123 for Single Photon Emission Computed Tomography (SPECT) imaging, and I-124 for Positron Emission Tomography (PET) imaging. The imaging characteristics of each isotope were investigated by simulated data. Their spectrums, point spread functions and contrast recovery curves were drawn and compared. I-131 was imaged with a HEAP (High Energy All Purpose) collimator while two collimators were compared for I-123: LEHR (Low Energy High Resolution) and ME (Medium Energy). No mechanical collimation was used for I-124. The influence of small high energy peaks (> 0.1%) on the main energy window contamination were evaluated. Furthermore, the effect of a scattering medium was investigated and the TEW (Triple Energy Window) correction was used for spectral-based scatter correction. Results show that I-123 gives best results with a LEHR collimator when the scatter correction is applied. Without correction, the ME collimator reduces the effects of high energy contamination. I-131 offers the worst results. This can be explained by the large amount of septal penetration from the photopeak and by the collimator which gives a low spatial resolution. I-124 gives the best imaging properties due to its electronic collimation (high sensitivity) and a short coincidence time window. 1. Introduction Radionuclide Therapy is a promising technique for cancer treatment. However, this technique requires an accurate pretherapeutic patient-specific dosimetry. The latter is usually performed by directly imaging the uptake of the tracer in the body. This can be done either by imaging the radioactive tracer used for therapy or by replacing it by one of its isotopes, more suitable for imaging. Due to its beta- emission (606 kev, 90%), I-131 is often used for therapy. It also emits some photons that can be used for SPECT imaging. Their main emission energy peak is about 364 kev and requires the use of a HEAP collimator. However, its spectrum of emission is complex and some emission peaks, above 364 kev, cause high energy contamination [1] and degrade the dosimetry. I-123 is more suitable for imaging. The energy of its main gamma emission peak is 159 kev which is close to the 140 kev from Tc-99m for which gamma-camera design has traditionally been optimized. I-123 can be imaged in a SPECT system with a LEHR (Low Energy High Resolution) collimator, optimized for the Tc-99m (140 kev), or with a ME (Medium Energy) collimator, optimized for energies up to 300 kev. However, this radionuclide also emits peaks of a higher energy which again can be responsible for high energy contamination. Due to its emission of positrons (23% of the decays), I-124 is used in PET imaging. PET systems are using an electronic collimation based on interaction time. The emission spectrum of I-124 is also very complex [2]. In 50% of the cases, the emission of a positron is followed by the emission of a gamma of 602 kev, which is likely to create a false coincidence (similar to scattered coincidences) [3]. The aim of this study was to evaluate the image quality and quantify the impact of high energy contamination for each isotope. High energy contamination [4, 5] ends up in the main energy window by one or by a combination of the following effects: scatter in the object, Compton effect in the crystal, penetration of the collimator, and backscatter from the camera end parts (light guides, PMTs, lead ending). 1

2 2. Materials and Methods 2.1. Simulations and Reconstruction These studies were performed on two simulated systems, the Philips Axis (SPECT) [6] for I-123 and I-131 and the Philips Allegro (PET) [7] for I-124. Two types of collimators were compared for I-123, as it is mainly imaged with a LEHR (Low Energy High Resolution) collimator or a ME (Medium Energy) collimator. I-131 was studied with a HEAP (High Energy All Purpose) collimator. The GATE (GEANT4 Application for Tomographic Emission) [8] package was used for the simulations. It is a Monte Carlo simulation tool well validated, and its use of the GEANT4 libraries offers a high accuracy. Gate models of the aforementioned scanners were extensively validated [6, 9]. The simulated data were reconstructed with the MLEM algorithm [10, 11]. The number of iterations was chosen to allow the better contrast with a reasonable level of noise for each of the isotopes (around 100 iterations in each case). The images were reconstructed in 2 dimensions and their matrix size was chosen to 256*256 pixels. (a) I-123 (b) I Spectrum analysis This study aims to point out the proportion of contamination for each of the three isotopes. Those characteristics will help explaining the imaging possibilities of each isotope. The detected spectrum of each isotope (without collimator) is shown on figure 1. All the peaks representing more than 0.1% of the emission and that can be detected in the main energy window, have been investigated. To do so, a point source has been placed at 15 cm from the collimator. In order to better differentiate the history of detected photons, the sources have been simulated by their discrete energy peaks. For I-123, six peaks have been simulated (159 kev: 83%, 346 kev: 0.13%, 440 kev: 0.42%, 505 kev: 0.32%, 529 kev: 1.39%, 539 kev: 0.38%). Six peaks have also been simulated for I-131 (364 kev: 82%, 326 kev: 0.27%, 503 kev: 0.36%, 637 kev: 7.18%, 643 kev: 0.22%, 723 kev: 1.77%). For I-124, the source can not be decomposed in discrete energy peaks without changing the results because of the time coincidence between emission of positrons and gammas. This relation is important as it can create false coincidences. For this isotope, proportion of scatter and high energy contamination was quantified as a whole. For each of those isotopes, the efficiency of detection was calculated. Then the proportion of high energy contamination was determined and finally, the peaks responsible for that contamination were pointed out for the SPECT studies. The effect of a scattering medium (cylinder of water of 10cm radius and 18cm height centred in the source) was (c) I-131 Figure 1: Various spectras of photons (singles) on the detector without the use of a collimator. The energies shown are the energies of the main emission peaks of the isotopes. The energy of the main peak used for the imaging is in a red box. additionally investigated Resolution study The PSF (Point Spread Functions) was studied for each isotope. A point source of 0.1 mm diameter, at the centre of the field of view, was acquired with the different systems and the sinograms were constructed. The radius of rotation of the camera was set to 15 cm. As the point source was placed in the centre of the scanner, the PSF was defined by the sum of all the angles of the sinograms. In order to quantify the resolution, the FWHM (Full Width at Half Maximum) and the FWTM (Full Width at Tenth Maximum) have been computed on the PSF. Those two parameters have been compared with or without a scattering 2

3 medium (cfr. section 2.2). The influence of the TEW correction [12] was also investigated. This is a scatter correction method based on the estimation of the scatter in the main energy window by interpolating it from two adjacent energy windows. In this study, the main energy window was centered on the main energy peak of the isotope with a width of 20% of this energy (I-123: kev, and I-131: kev). The two other windows were adjacent to the main window with a width of 10% of the main energy (I-123: kev and kev, and I-131: keV and kev). I-131 and 155 times than the one with I-123 on a ME collimator. Figure 3 points out the influence of each high energy peak on the photopeak contamination. I-123 imaged with a LEHR collimator suffers from 49.1% of high energy contamination, compared to 7.8% with a ME collimator if no scattering medium is present. Those results are higher (LEHR: 63.9% and ME: 19.9%) with a scattering medium. The high energy contamination represents 35.9% for I-124 and 27.4% for I-131 with a scattering medium Contrast Recovery Curves A 2D source, composed of a circular background of 17 cm diameter and eight hotspots of diameters from 8 mm to 20 mm, was designed to study the contrast evolution with the size of the objects. The contrast between the hotspots and the background was set to 4:1. The effect of a scattering medium (cfr: section 2.2) was investigated. The contrast is defined as the ratio between the mean value in the object and the mean value in the background. The calculated contrast recovery factor is then the ratio between the contrast that has been found on the images and the contrast of the source. This parameter gives an estimation of the accuracy of quantification that can be achieved with the studied isotope without applying any kind of partial volume correction. 3. Results 3.1. Spectrum analysis (a) I-123 LEHR (b) I-123 ME (c) I-131 HEAP Figure 2: Efficiency of detection achieved with the isotopes. The grey part of the plots corresponds to the scatter and penetration photons. The efficiency of detection for all of these isotopes is shown in figure 2. When only the geometric photons are considered, the ME collimator offers a better efficiency (1.4 times the efficiency of the LEHR collimator) for I-123. Due to its electronic collimation, the efficiency that can be achieved with the I-124 is 96 times higher than the one achieved with Figure 3: History of the photons reaching the detector for I-123 with a LEHR collimator (a), I-123 with a ME collimator (b) and I-131 with a HEAP collimator (c). Penetration photons are the photons which penetrate the collimator and get detected in the crystal (back scatter is not considered as scatter). Scatter photons are the ones which scatter in the collimator. Finally the geometric ones are the photons detected on the detector without any scatter nor penetration. Results for I-123 show that peaks of 0.4% of the emission can be responsible for up to 7.9% (539 kev) of contamination with a LEHR and without a scattering medium. With 3

a scattering medium, this proportion of contamination goes up to 10.4%. The main peak responsible for contamination for I-123 is the 529 kev peak which represents 1.

Figure 3 also shows for each emission peak, the proportion of penetration (photons which penetrates the collimator and that did not scatter in the collimator nor in the object), scatter (photons that

4 a scattering medium, this proportion of contamination goes up to 10.4%. The main peak responsible for contamination for I-123 is the 529 kev peak which represents 1.39% of the emission (LEHR: 28% and ME:4.8% without a scattering medium). For I-131, the main peak responsible for high energy contamination is the peak of 637 kev (12.5%). Figure 3 also shows for each emission peak, the proportion of penetration (photons which penetrates the collimator and that did not scatter in the collimator nor in the object), scatter (photons that scatter in the collimator or in the object) and geometric photons detected in the crystal. Although the amount of detected photons from the main energy peak is high for I-131, it is important to consider that a large amount from this peak is detected after septal penetration. This fraction is very important compared to the two other isotopes: 52% of the photons from the main energy peak are penetration photons compared to 18% for I-123 with a LEHR collimator and 3% for I-123 with a ME collimator Resolution study Figure 5: Comparison of the FWHM and the FWTM for each of those isotopes (with or without attenuation matter and with or without TEW correction). FWHM and FWTM are respectively written (in mm) under the name of the isotopes Figure 6: Images of the contrast study sources for all of the isotopes. Figure 4: Point Spread Functions (in mm, without a scattering medium). The PSF obtained for all of the isotopes, without a scattering medium, are shown in figure 4. I-124 offers a better resolution than the two other isotopes. The curve of I-123 with the ME collimator shows the effects of the collimator (septas) which lowers the resolution. The use of a LEHR collimator is therefore better for good spatial resolution. For I-131, the septas of the collimator are clearly visible on the PSF and the resolution obtained is very poor. Results for both FWHM and FWTM are shown on figure 5. It shows that, for each isotope, the scattering medium is increasing the FWTM while the FWHM remains roughly the same. The TEW correction helps lowering the first effect Contrast Recovery Curves Reconstructed images used for the contrast study are displayed on figure 6. As shown on figure 7, the best contrast recovery has been found for I-124. It reaches for objects of 20mm diameter. I-123 with a ME collimator reaches better results than with a LEHR collimator when no TEW correction is applied (ME: and LEHR: for 20mm). However, the results are the contrary when the TEW correction is applied (ME: and LEHR: for 20mm). I-131 imaged with HEAP collimator shows very low results (0.363 for 20mm). The scattering medium lowers those results for I-123 (ME: to 0.559) and I-124 (0.935 to 0.746). I-131 does not suffer from the effects of the scattering medium (0.363 to 4

5 emission peak. This result and the one obtained for its spatial resolution explained the poor results for this isotope. I-124 offers the best image quality due to its electronic collimation which gives a better efficiency and resolution. The short coincidence window also limits influence of high energy peaks. Given those results, I-124 seems to be the more promising isotope for a pretherapeutic dosimetry study. Such a methodology has already been developed by Sgouros for I-131 cancer therapy [13]. Figure 7: Contrast Recovery obtained for all isotopes without a scattering medium as a function of the hotspot size (in mm) ). 4. Discussion This study was performed with an accurate Monte Carlo simulator allowing us to store history of the detected events which is not possible with measured data. The number of simulated photons varied from detected photons in the photopeak window for the spectrum analysis to for the imaging characteristics definitions. Those values were sufficient to obtain good statistics over the distribution of the energies and over the spatial distribution. However, to compare the imaging characteristics of the isotopes (spatial resolution and contrast recovery), the same source activity and equal acquisition time have been used. This aims to give comparable results between the isotopes. This study shows that, for SPECT, even small peaks of emission (> 0,1%) in the spectrum of those isotopes can have a high influence and degrade significantly the image quality. High energy contamination is mainly due to septal penetration in the collimator followed by Compton detection in the crystal and backscatter from the endparts. For those peaks, of a higher energy than the photopeak, scatter in the collimator or in the scattering medium has a lower influence. By looking at the spectrums of the I-123, one can clearly see that using a ME collimator helps lowering the influence of high energy contamination. However, contrast is also directly related to resolution. The results obtained for the contrast recovery can then be explained by those found for the resolution. I-123 is best imaged with a LEHR collimator when the TEW correction is applied while the use of a ME collimator is required when no scatter correction is made. Those results are a good approximation of what can be achieve for quantification by these isotopes. Even if I-131 does not suffer a lot from high energy contamination, it suffers from a lot of penetration from the main 5. Conclusion For imaging I-123, the use of a LEHR collimator is important when a good resolution is required. However, a ME collimator reduces the influence of the high energy peaks. For I-123 and for contrast, it is better to use a ME collimator when no scatter correction is applied. With TEW correction, it is then better to use a LEHR collimator because of the better spatial resolution. Due to its electronic collimation which gives a better efficiency and resolution than in SPECT, I-124 offers the best image quality. The short coincidence window also limits influence of high energy peaks. Due to its relatively high energy imaging peak, the use of a HEAP collimator is required to image I-131. The image quality that can be achieved with this collimator is much lower than the one that has been achieved with the I-123 or the I-124. This result can be explained by the important amount of septal penetration from the main energy peak. References [1] Y. K. Dewaraja, M. Ljungberg, and K. F. Koral. Accuracy of 131I tumor quantification in radioimmunotherapy using SPECT imaging with an ultrahigh-energy collimator: Monte Carlo study. J Nucl Med, 41(10): , Oct non lu. [2] Allan C. G. Mitchell, Jose O. Juliano, Charles B. Creager, and C. W. Kocher. Disintegration of I124 and I123. Phys. Rev., 113: , [3] Stefaan Vandenberghe. Three-dimensional positron emission tomography imaging with 124i and 86y. Nucl Med Commun, 27(3): , Mar [4] Steven Staelens, Tim de Wit, Ignace Lemahieu, and Freek Beekman. Significant septal penetration effects caused by impurities in cyclotron-produced thallium J. Nucl. Med., 47:381, [5] A. Cot, E. Jane, J. Sempau, C. Falcon, S. Bullich, J. Pavia, F. Calvino, and D. Ros. Modeling of high- 5

6 energy contamination in SPECT imaging using Monte Carlo simulation. IEEE TRANSACTIONS ON NU- CLEAR SCIENCE, 53(11): , FEB [6] Steven Staelens, Daniel Strul, Giovanni Santin, Stefaan Vandenberghe, Michel Koole, Yves D Asseler, Ignace Lemahieu, and Rik Van de Walle. Monte carlo simulations of a scintillation camera using gate: validation and application modelling. Phys Med Biol, 48(18): , Sep [7] Suleman Surti and Joel S Karp. Imaging characteristics of a 3-dimensional gso whole-body pet camera. J Nucl Med, 45(6): , Jun [8] S. Jan, G. Santin, D. Strul, and S. Staelens et al. GATE: a simulation toolkit for PET and SPECT. Phys Med Biol, 49(19): , Oct [9] F. Lamare, A. Turzo, Y. Bizais, C. Cheze Le Rest, and D. Visvikis. Validation of a monte carlo simulation of the philips allegro/gemini pet systems using gate. Phys Med Biol, 51(4): , Feb [10] L. A. Shepp and Y. Vardi. Maximum likelihood reconstruction for emission tomography. MI-1(2): , [11] K. Lange and R. Carson. EM reconstruction algorithms for emission and transmission tomography. J. Comput. Assist. Tomogr., 8(2): , [12] K. Ogawa, Y. Harata, and T. Ichihara. A practical method for position dependent scatter correction in single photon emission ct. IEEE Trans. Med. Imag, 10(3): , [13] George Sgouros, Katherine S Kolbert, Arif Sheikh, Keith S Pentlow, Edward F Mun, Axel Barth, Richard J Robbins, and Steven M Larson. Patientspecific dosimetry for 131I thyroid cancer therapy using 124I PET and 3-dimensional-internal dosimetry (3D-ID) software. J Nucl Med, 45(8): , Aug

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