Since the 1990s, stent graft implantation for aortic pathology has become an alternative to extensive surgical procedures in some patients. Indeed, many patients with such pathology are now treated ...endovascularly. Only limited data concerning the risk of a deterministic effect during aortic stent graft implantation are available Accordingly, 179 consecutive patients treated in our institute between October 2002 and July 2008 with endovascular aortic stent grafts were included in this study. Dosimetric data (kerma area product (KAP) and cumulative dose at the interventional reference point (CD(irp))) from radiograph reports were analysed for 172 patients. On a group of 19 patients, GAFCHROMIC XR type dosimetric films were also used to verify the automatic measurements. Readings from the integrated KAP meter were found to be too high and were therefore corrected - KAP to dose area product (DAP) and CD(irp) to entrance skin dose (ESD). Median DAP was 153 Gy cm² (35-700 Gy cm²) and median ESD was 0.44 Gy (0.12-2.73 Gy). Recorded dosimetric quantities were found to be good predictors of the skin dose and highlighted 4 patients (2.3%) who received skin doses that might cause possible deterministic effects. Endovascular stent graft implantation is less invasive than a surgical procedure and is widely used; mid-term results are encouraging. In a small number of patients, deterministic effects can occur even in departments with well-trained staff. Operators should inform the patients of possible skin injury after receiving high doses of ionising radiation and proper support must be available should that occur.
The RD48 (ROSE) collaboration has succeeded to develop radiation hard silicon detectors, capable to withstand the harsh hadron fluences in the tracking areas of LHC experiments. In order to reach ...this objective, a defect engineering technique was employed resulting in the development of Oxygen enriched FZ silicon (DOFZ), ensuring the necessary O-enrichment of about 2×10
17 O/cm
3 in the normal detector processing. Systematic investigations have been carried out on various standard and oxygenated silicon diodes with neutron, proton and pion irradiation up to a fluence of 5×10
14
cm
−2 (1
MeV neutron equivalent). Major focus is on the changes of the effective doping concentration (depletion voltage). Other aspects (reverse current, charge collection) are covered too and the appreciable benefits obtained with DOFZ silicon in radiation tolerance for charged hadrons are outlined. The results are reliably described by the “Hamburg model”: its application to LHC experimental conditions is shown, demonstrating the superiority of the defect engineered silicon. Microscopic aspects of damage effects are also discussed, including differences due to charged and neutral hadron irradiation.
Silicon detectors for combined MR–PET and MR–SPECT imaging Studen, A.; Brzezinski, K.; Chesi, E. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
02/2013, Letnik:
702
Journal Article
Recenzirano
Odprti dostop
Silicon based devices can extend PET–MR and SPECT–MR imaging to applications, where their advantages in performance outweigh benefits of high statistical counts.
Silicon is in many ways an excellent ...detector material with numerous advantages, among others: excellent energy and spatial resolution, mature processing technology, large signal to noise ratio, relatively low price, availability, versatility and malleability.
The signal in silicon is also immune to effects of magnetic field at the level normally used in MR devices. Tests in fields up to 7T were performed in a study to determine effects of magnetic field on positron range in a silicon PET device. The curvature of positron tracks in direction perpendicular to the field's orientation shortens the distance between emission and annihilation point of the positron. The effect can be fully appreciated for a rotation of the sample for a fixed field direction, compressing range in all dimensions. A popular Ga-68 source was used showing a factor of 2 improvement in image noise compared to zero field operation. There was also a little increase in noise as the reconstructed resolution varied between 2.5 and 1.5mm.
A speculative applications can be recognized in both emission modalities, SPECT and PET.
Compton camera is a subspecies of SPECT, where a silicon based scatter as a MR compatible part could inserted into the MR bore and the secondary detector could operate in less constrained environment away from the magnet. Introducing a Compton camera also relaxes requirements of the radiotracers used, extending the range of conceivable photon energies beyond 140.5keV of the Tc-99m.
In PET, one could exploit the compressed sub-millimeter range of positrons in the magnetic field. To exploit the advantage, detectors with spatial resolution commensurate to the effect must be used with silicon being an excellent candidate. Measurements performed outside of the MR achieving spatial resolution below 1mm are reported.
A silicon PET probe Studen, A.; Chesi, E.; Cindro, V. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
08/2011, Letnik:
648
Journal Article
Recenzirano
PET scanners with high spatial resolution offer a great potential in improving diagnosis, therapy monitoring and treatment validation for several severe diseases. One way to improve resolution of a ...PET scanner is to extend a conventional PET ring with a small probe with excellent spatial resolution. The probe is intended to be placed close to the area of interest. The coincidences of interactions within the probe and the external ring provide a subset of data which combined with data from external ring, greatly improve resolution in the area viewed by the probe.
Our collaboration is developing a prototype of a PET probe, composed of high-resolution silicon pad detectors. The detectors are 1
mm thick, measuring 40 by 26
mm
2, and several such sensors are envisaged to either compensate for low stopping power of silicon or increase the area covered by the probe. The sensors are segmented into 1
mm
3 cubic voxels, giving 1040 readout pads per sensor. A module is composed of two sensors placed in a back-to-back configuration, allowing for stacking fraction of up to 70% within a module. The pads are coupled to a set of 16 ASICs (VaTaGP7.1 by IDEAS) per module and read out through a custom designed data acquisition board, allowing for trigger and data interfacing with the external ring.
This paper presents an overview of probe requirements and expected performance parameters. It will focus on the characteristics of the silicon modules and their impact on overall probe performance, including spatial resolution, energy resolution and timing resolution. We will show that 1
mm
3 voxels will significantly extend the spatial resolution of conventional PET rings, and that broadening of timing resolution related to varying depth of photon interactions can be compensated to match the timing resolution of the external ring. The initial test results of the probe will also be presented.
1×1
cm
2 silicon pad p
+–n–n
+ detectors were irradiated with fast neutrons from the TRIGA research reactor in Ljubljana to fluences from 5×10
13 to 10
14n/cm
2. The observed time development of ...annealing of the full-depletion voltage (FDV) could be fitted by a constant and two exponentials. The characteristic time of the fast component is 4
h, independent of temperature in the interval 0–15°C. A comparison of MESA and planar pad detectors shows a 20–30% lower FDV for the MESA. A search for a flux dependence of the radiation damage was performed in the range from 2×10
8 to 5×10
15
n/cm
2s and no systematic differences were observed.
Simulation indicates that PET image could be improved by upgrading a conventional ring with a probe placed close to the imaged object. In this paper, timing issues related to a PET probe using ...high-resistivity silicon as a detector material are addressed. The final probe will consist of several (four to eight) 1-mm thick layers of silicon detectors, segmented into 1 × 1 mm2 pads, each pad equivalent to an independent p + nn+ diode. A proper matching of events in silicon with events of the external ring can be achieved with a good timing resolution. To estimate the timing performance, measurements were performed on a simplified model probe, consisting of a single 1-mm thick detector with 256 square pads (1.4 mm side), coupled with two VATAGP7s, application-specific integrated circuits. The detector material and electronics are the same that will be used for the final probe. The model was exposed to 511 keV annihilation photons from an 22Na source, and a scintillator (LYSO)–PMT assembly was used as a timing reference. Results were compared with the simulation, consisting of four parts: (i) GEANT4 implemented realistic tracking of electrons excited by annihilation photon interactions in silicon, (ii) calculation of propagation of secondary ionisation (electron–hole pairs) in the sensor, (iii) estimation of the shape of the current pulse induced on surface electrodes and (iv) simulation of the first electronics stage. A very good agreement between the simulation and the measurements were found. Both indicate reliable performance of the final probe at timing windows down to 20 ns.
Silicon as an unconventional detector in positron emission tomography Clinthorne, Neal; Brzezinski, Karol; Chesi, Enrico ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
2013-Jan-01, Letnik:
699, Številka:
21
Journal Article
Recenzirano
Odprti dostop
Positron emission tomography (PET) is a widely used technique in medical imaging and in studying small animal models of human disease. In the conventional approach, the 511keV annihilation photons ...emitted from a patient or small animal are detected by a ring of scintillators such as LYSO read out by arrays of photodetectors. Although this has been successful in achieving ∼5mm FWHM spatial resolution in human studies and ∼1mm resolution in dedicated small animal instruments, there is interest in significantly improving these figures. Silicon, although its stopping power is modest for 511keV photons, offers a number of potential advantages over more conventional approaches including the potential for high intrinsic spatial resolution in 3D. To evaluate silicon in a variety of PET “magnifying glass” configurations, an instrument was constructed that consists of an outer partial-ring of PET scintillation detectors into which various arrangements of silicon detectors are inserted to emulate dual-ring or imaging probe geometries. Measurements using the test instrument demonstrated the capability of clearly resolving point sources of 22Na having a 1.5mm center-to-center spacing as well as the 1.2mm rods of a 18F-filled resolution phantom. Although many challenges remain, silicon has potential to become the PET detector of choice when spatial resolution is the primary consideration.
► We examine the use of position-sensitive silicon detectors in magnifying PET geometries. ► A demonstrator using silicon detectors and BGO scintillation detectors was constructed. ► Both single-slice and volume PET configurations were tested. ► For a 4.5cm field-of-view, resolutions <1mm were achievable. ► Resolution will improve further with higher resolution silicon detectors.
Compton collimated imaging may improve the detection of gamma rays emitted by radioisotopes used in single photon emission computed tomography (SPECT). We present a crude prototype consisting of a ...single 500μm thick, 256 pad silicon detector with pad size of 1.4×1.4mm2, combined with a 15×15×1cm3 NaI scintillator crystal coupled to a set of 20 photo multipliers. Emphasis is placed on the performance of the silicon detector and the associated read-out electronics, which has so far proved to be the most challenging part of the set-up. Results were obtained using the VATAGP3, 128 channel low-noise self-triggering ASIC as the silicon detector's front-end. The noise distribution (σ) of the spectroscopic outputs gave an equivalent noise charge (ENC) with a mean value of 〈σ〉=137e with a spread of 10e, corresponding to an energy resolution of 1.15keV FWHM for the scattered electron energy. Threshold settings above 8.2keV were required for stable operation of the trigger. Coincident Compton scatter events in both modules were observed for photons emitted by 57Co source with principal gamma ray energies of 122 and 136keV.
In many of the recent
e
+
e
-
particle physics experiments, monitoring of the accumulated dose in silicon is essential to maximize the lifetime of silicon vertex detectors operating in severe ...radiation environments. Using radiation-sensing field-effect transistors (RadFET) as radiation monitoring devices, we studied their responses during irradiation and during subsequent annealing. The relation between the RadFET response and the dose was determined by irradiations with a
60Co source with known activity. The study of annealing at three different temperatures showed that RadFETs gradually anneal for up to 40%. Annealing can be fitted by a sum of two exponential functions with time constants of 3 and 85 days.
The ability to diagnose early prostate cancer has outpaced imaging methods for accurate localization and staging of the disease. A precise determination of the extent of local disease is difficult ...and, unfortunately, present imaging instrumentation employing radiotracer techniques (PET, SPECT, planar scintigraphy) is limited in its ability to provide detailed information when imaging the prostate due to modest spatial resolution, often poor counting efficiency, and photon attenuation. The innovation in the work described here is the use of a silicon prostate probe based on the principle of the Compton scatter camera. Simulation studies show that the endorectal prostate probe will enhance the spatial resolution by a factor 4–5 and, simultaneously, provide a gain in efficiency of a factor 16–40 over conventional nuclear medicine cameras. The ongoing tasks are focused on the design optimization of a Compton probe prototype.
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