Takehiro Tomitani

Image Reconstruction and Noise Evaluation in Photon Time-of-Flight Assisted Positron Emission Tomography

In positron CT, the path difference of annhilation pair gamma rays can be measured by time-of-flight (TOF) difference of pair gamma rays. This TOF information gives us rough position information along a projection line and will reduce noise propagation in the reconstruction process. A reconstruction algorithm for TOF-positron computed tomography (PCT) based on the back-projection with 1-dimensional weight and 2-dimensional filtering is presented. Also a formula to evaluate the variance of the reconstructed image and the optimal back-projection function are presented. The advantage of TOF-PCT over conventional PCT was investigated in view of noise figure. An example of such noise figure evaluations for CsF and liquid Xenon scintillators is given.

Design study of a raster scanning system for moving target irradiation in heavy-ion radiotherapy

A project to construct a new treatment facility as an extension of the existing heavy-ion medical accelerator in chiba (HIMAC) facility has been initiated for further development of carbon-ion therapy. The greatest challenge of this project is to realize treatment of a moving target by scanning irradiation. For this purpose, we decided to combine the rescanning technique and the gated irradiation method. To determine how to avoid hot and/or cold spots by the relatively large number of rescannings within an acceptable irradiation time, we have studied the scanning strategy, scanning magnets and their control, and beam intensity dynamic control. We have designed a raster scanning system and carried out a simulation of irradiating moving targets. The result shows the possibility of practical realization of moving target irradiation with pencil beam scanning. We describe the present status of our design study of the raster scanning system for the HIMAC new treatment facility.

Washout measurement of radioisotope implanted by radioactive beams in the rabbit.

Washout of 10C and 11C implanted by radioactive beams in brain and thigh muscle of rabbits was studied. The biological washout effect in a living body is important in the range verification system or three-dimensional volume imaging in heavy ion therapy. Positron emitter beams were implanted in the rabbit and the annihilation gamma-rays were measured by an in situ positron camera which consisted of a pair of scintillation cameras set on either side of the target. The ROI (region of interest) was set as a two-dimensional position distribution and the time-activity curve of the ROI was measured. Experiments were done under two conditions: live and dead. By comparing the two sets of measurement data, it was deduced that there are at least three components in the washout process. Time-activity curves of both brain and thigh muscle were clearly explained by the three-component model analysis. The three components ratios (and washout half-lives) were 35% (2.0 s), 30% (140 s) and 35% (10 191 s) for brain and 30% (10 s), 19% (195 s) and 52% (3175 s) for thigh muscle. The washout effect must be taken into account for the verification of treatment plans by means of positron camera measurements.

Washout studies of 11C in rabbit thigh muscle implanted by secondary beams of HIMAC

Heavy ion therapy has two definite advantages: good dose localization and higher biological effect. Range calculation of the heavy ions is an important factor in treatment planning. X-ray CT numbers are used to estimate the heavy ion range by looking up values in a conversion table which relates empirically photon attenuation in tissues to particle stopping power; this is one source of uncertainty in the treatment planning. Use of positron emitting radioactive beams along with a positron emission tomograph or a positron camera gives range information and may be used as a means of checking in heavy ion treatment planning. However, the metabolism of the implanted positron emitters in a living object is unpredictable because the chemical forms of these emitters are unknown and the metabolism is dependent on the organ species and may be influenced by many factors such as blood flow rate and fluid components present. In this paper, the washout rate of 11C activity implanted by injecting energetic 11C beams into thigh muscle of a rear leg of a rabbit is presented. The washout was found to consist of two components, the shorter one was about 4.2 +/- 1.1 min and the longer one ranged from 91 to 124 min. About one third of the implanted beta+ activity can be used for imaging and the rest was washed out of the target area.

Spot Scanning Using Radioactive 11C Beams for Heavy-Ion Radiotherapy

A scheme for spot scanning using 11C beams has been developed in order to form and verify a three-dimensionally conformal irradiation field for cancer radiotherapy. By selecting the momentum spread of a 11C beam, we could considerably decrease the distal falloff of the irradiation field, thus conserving the beam quality. To estimate and optimize the dose distribution in the irradiation field, it is essential to evaluate the precise dose distribution of spot beams. The coupling of the lateral dose and depth-dose distributions originating from a wide momentum spread should be taken into account to calculate the dose distribution of 11C beams. The reconstructed dose distribution of the irradiation field was in good agreement with the experimental results, i.e., within ±0.2%. An irradiation field of 35×35×43 mm3 was optimized and spot scanning using 11C beams was carried out. The flatness was within ±2.3% with an error of 1% in the detector resolution.

Application of an RI-beam for cancer therapy: In-vivo verification of the ion-beam range by means of positron imaging

Abstract In cancer treatment with heavy ions, verification of the ion range in the patients body is important. For this purpose, a positron emitter beam provides the possibility of range verification. To use the positron emitter beam, we have constructed a secondary beam course and its irradiation system. In this paper the constructed system is presented together with some results of beam experiments.

Image reconstruction from limited angle Compton camera data

The Compton camera is used for imaging the distributions of gamma ray direction in a gamma ray telescope for astrophysics and for imaging radioisotope distributions in nuclear medicine without the need for collimators. The integration of gamma rays on a cone is measured with the camera, so that some sort of inversion method is needed. Parra found an analytical inversion algorithm based on spherical harmonics expansion of projection data. His algorithm is applicable to the full set of projection data. In this paper, six possible reconstruction algorithms that allow image reconstruction from projections with a finite range of scattering angles are investigated. Four algorithms have instability problems and two others are practical. However, the variance of the reconstructed image diverges in these two cases, so that window functions are introduced with which the variance becomes finite at a cost of spatial resolution. These two algorithms are compared in terms of variance. The algorithm based on the inversion of the summed back-projection is superior to the algorithm based on the inversion of the summed projection.

Analytical study of the performance of a multilayer positron computed tomography scanner.

The image-forming performance of multilayer positron tomographs for extended sources is evaluated analytically. The analysis is simplified by “rotation transform,” by which three-dimensional photon detection problems are solved by two-dimensional treatment. Event rates of singles, unscattered true coincidence, and single- and double-scattered coincidence are formulated for a uniform cylinder phantom as functions of various design parameters. Angle factors for Compton scattering and other parameters used in the evaluation are presented. Scatter components in projections and their effect on the reconstructed images are also evaluated. The scatter component in the reconstructed image depends critically on the detector ring radius, phantom radius, method of attenuation correction, etc. When the detector ring radius is relatively small (40 ± 45 cm in diameter), the scatter/true ratio at the image center of a 20 cm diameter phantom may be larger than the scatter/true ratio in the event rates. Comparison with experimental data obtained with a head positron tomograph, positologica, showed reasonable agreement both in the total coincidence rates and in the scatter components in the images for a cylindrical phantom of 20 cm in diameter.

An analytical image reconstruction algorithm to compensate for scattering angle broadening in Compton cameras

Compton cameras have been developed for use in gamma-ray astronomy and nuclear medicine. Their defining merit is that they do not need collimators; however, on the demerit side, they need inversion procedures for image reconstruction, since a measured datum is proportional to the integration of incident gamma rays along a cone surface with the same Compton scattering angle. First, an iteration method was adopted for this task. Later, analytical methods were found under restricted conditions. Parra (2000 IEEE Trans. Nucl. Sci. 47 1543-50) deduced a purely analytical reconstruction algorithm for a complete set of scattering-projection data that include data at all the scattering angles. Tomitani and Hirasawa (2002 Phys. Med. Biol. 47 2129-45) found that by making a slight modification, Parras algorithm could be extended to the scattering-projection data in limited scattering angles. However, their algorithm neglected the effects of practical problems that cause the degradation of spatial resolution. Sources of degradation were identified as noise in the energy signal of their front detector and the Doppler effect in the scattering process. In this paper, we first analyse the effects of these sources on the angular resolution of the scattering-projection data and then present a revised reconstruction algorithm in which these two factors are incorporated. Simulation studies on digital phantoms reveal that the algorithm can reconstruct images even when these two factors are included.

Range verification system using positron emitting beams for heavy-ion radiotherapy

It is desirable to reduce range ambiguities in treatment planning for making full use of the major advantage of heavy-ion radiotherapy, that is, good dose localization. A range verification system using positron emitting beams has been developed to verify the ranges in patients directly. The performance of the system was evaluated in beam experiments to confirm the designed properties. It was shown that a 10C beam could be used as a probing beam for range verification when measuring beam properties. Parametric measurements indicated the beam size and the momentum acceptance and the target volume did not influence range verification significantly. It was found that the range could be measured within an analysis uncertainty of +/-0.3 mm under the condition of 2.7 x 10(5) particle irradiation, corresponding to a peak dose of 96 mGyE (gray-equivalent dose), in a 150 mm diameter spherical polymethyl methacrylate phantom which simulated a human head.