The impinging position of a 511 keV photon onto a continuous scintillator can be obtained from the light distribution measured by a pixelated photodetector such as avalanche photodiode (APD) arrays. This information is extracted using neural networks trained for events with a particular incidence angle. Using a 20/spl times/10/spl times/10 mm block of lutetium oxyorthosilicate mounted onto a S8550 Hamamatsu APD matrix we achieved an intrinsic resolution of 1.9 mm full-width at half-maximum (FWHM) for perpendicular incident photons and 2.6 mm FWHM at a 40/spl deg/ incidence angle. A possible implementation for tomographic imaging is presented.

Neural network-based position estimators for PET detectors using monolithic LSO blocks

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Mapping the distribution of gamma-ray-emitting isotopes with the scintillation camera

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.

https://iopscience.iop.org/article/10.1088/0031-9155/48/15/302/pdf

Washout measurement of radioisotope implanted by radioactive beams in the rabbit

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 β+ activity can be used for imaging and the rest was washed out of the target area.

https://iopscience.iop.org/article/10.1088/0031-9155/48/7/305/pdf

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

A treatment planning of heavy ion cancer therapy is performed with an aid of X-ray CT, which has uncertainties in an estimation of ion range. In order to obtain a dose distribution with high accuracy, irradiation and range measurement systems are developed at HIMAC using positron emitter beams. Exact stopping points of the ions can be measured by detecting annihilation γ-rays from positron emitters themselves. A new irradiation system has been installed, for creating an irradiation field with 3-dimensionally desired shape. A positron camera, which is similar to an Anger-type γ camera, is used to determine a range distribution of the heavy ions in a patient’s body. An existing PET system can be also used to measure an image of the 3dimensional irradiation-field.

/pdf/medical-application-of-the-positron-emitter-beam-at-himac-2bmhe1t51u.pdf

Medical application of the positron emitter beam at himac

Scintillation cameras of thick NaI(Tl) crystals without collimators have been used in the field of gamma ray astronomy, positron camera and PET. In such cases, gamma rays enter into a crystal at various angles, so that the depth of interaction causes position errors in two-dimensional position encoding. In principle, the light distribution inside the crystal depends on the depth of the light source, from which depth information may be derived. The authors introduced a position arithmetic based on maximum likelihood estimation and investigated the effect of light reflection of the surface opposite to PMT array. It turned out that depth encoding is impossible with diffusive reflection, while depth information can be obtained with mirror reflection, semi-opaque and opaque reflection at a cost of spatial resolution. The compromise must be made between the spatial resolution and the depth resolution, which may depend on the specific applications.

Depth encoding of point-of-interaction in thick scintillation cameras

A new beam course, a projectile fragment separator to provide radioactive beams, is under construction at HIMAC. A primary purpose of the new beam course is to study medical applications of radioactive beams. The specifications and status of the beam course and a preliminary result using carbon beams are summarized.

New secondary beam course for medical use in HIMAC

The positron camera system was designed and developed at the secondary beam course of HIMAC to measure heavy-ion ranges in bodies of patients. By using the pencil-like beam of positron emitters, such as 11 C, the range is checked directly and precisely by detecting pairs of annihilation gamma rays emitted from the stopping point of the beam. The positron camera consists of a pair of Anger-type scintillation cameras. In order to achieve high detection efficiency, each detector is equipped with a large NaI(Tl) crystal having a thickness of 30 mm and a diameter of 600 mm. The test results reveal that the range can be measured with accuracy of less than 1 mm under the limitation that the irradiation dose has to be less than a few percent of the therapeutic one.

/pdf/positron-camera-system-for-heavy-ion-radiotherapy-at-himac-p4bdwt6zig.pdf

Positron camera system for heavy-ion radiotherapy at himac

We have constructed the spot scanning irradiation system at the secondary beam course in HIMAC. This system has scanning magnet and range sifter. The response time of the range shifter is less than 300 milliseconds. Beam position is measured by a multi-strip parallel plate ionization chamber with strip pitch of 1mm. The calculation of beam center is completed within 1 milliseconds in the VME, which is placed independently for horizontal and vertical directions.

Spot scanning system with ri beam at himac

Since 1994 clinical trials have been performed successfully with carbon beam. To improve the clinical result further, new irradiation systems are under development such as a 3D-irradiation system and a verification system of range with positron emitter. There are also improvements on the accelerator performances. One is the wide range of ion species; the others are concerned with the machine devices and new beam monitors to get good machine operation. In this report we present current status of HIMAC.

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Y. Futami

Papers

Depth encoding of point-of-interaction in thick scintillation cameras

New secondary beam course for medical use in HIMAC

Positron camera system for heavy-ion radiotherapy at himac

Spot scanning system with ri beam at himac

Present status of HIMAC at NIRS