Silicon detectors for combined MR-PET and MR-SPECT imaging.

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 7 T 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 1.5 mm.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 140.5 keV 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 1 mm are reported.

Silicon as an Unconventional Detector in Positron Emission Tomography.

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 511 keV 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 a 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 511 keV photons, offers a number of potential advantages over more conventional approaches. Foremost is its high spatial resolution in 3D: our past studies show that there is little diffculty in localizing 511 keV photon interactions to ~0.3mm. Since spatial resolution and reconstructed image noise trade off in a highly non-linear manner that depends on the PET instrument response, if high spatial resolution is the goal, silicon may outperform standard PET detectors even though it has lower sensitivity to 511 keV photons. To evaluate silicon in a variety of PET "magnifying glass" configurations, an instrument has been constructed that consists of an outer partial-ring of PET scintillation detectors into which various arrangements of silicon detectors can be inserted to emulate dual-ring or imaging probe geometries. Recent results have demonstrated 0.7 mm FWHM resolution using pad detectors having 16×32 arrays of 1.4mm square pads and setups have shown promising results in both small animal and PET imaging probe configurations. Although many challenges remain, silicon has potential to become the PET detector of choice when spatial resolution is the primary consideration.

Risk of deterministic effects during endovascular aortic stent graft implantation.

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.

Timing performance of the silicon PET insert probe.

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 x 1 mm(2) 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 (22)Na 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.

Patient peak skin doses from cardiac interventional procedures.

Interventional cardiac procedures not only lead to significant effective doses for the patient but also can potentially cause deterministic effects on the patient's skin. Information about the peak (maximal) skin doses (PSD) received by patients during percutaneous transluminal coronary angioplasty procedures were collected from three cardiac catheter rooms. Cumulative dose at the interventional reference point (CD(IRP)) was collected for 161 patients and for 16 patients PSD was determined using Gafchromic dosimetry films. The comparison showed that CD(IRP) readings give a useful but conservative estimation of patient peak skin dose as it can lead to a significant overestimation. The median and third quartile values of CD(IRP) were 0.64 and 0.92 Gy, respectively. The 2 Gy threshold for deterministic effects was exceeded in nine patients. A good correlation was found between CD(IRP) and kerma area product measurements while the correlation with fluorography time was very weak.

Reticulocyte analysis using light microscopy and two different flow cytometric procedures.

The reticulocyte count is a clinically important indirect indicator of erythropoietic activity of the bone marrow. Reticulocyte enumeration by light microscopy is rather inaccurate and has poor reproducibility. Automation of the reticulocyte count by means of flow cytometry has considerably improved the quality of this investigation. In our study, we compared three methods of establishing the blood reticulocyte number: the microscopic brilliant cresyl blue method and two flow cytometric procedures using thiazole orange (TO), namely FACSort (Becton-Dickinson) and EPICS Profile (Coulter). The aims of the study were (1) to select the most suitable TO concentration to be used with the EPICS Profile cytometer, (2) to determine the correlation between the microscopic method and the two flow cytometric procedures, and (3) to appraise the suitability of flow cytometry for reticulocyte analysis in routine clinical work. According to our results, the most appropriate TO concentration for the EPICS Profile counter is 0.1 mg/L. We observed a good correlation between the three methods tested; the correlation coefficients ranged from 0.82 to 0.87. The mean intra-assay coefficients of variation for the microscopic method and the EPICS Profile and FACSort procedures were 27.5%, 8.4% and 6.3%, respectively.

Monitoring the remission induction therapy of acute nonlymphoblastic leukemia by bone marrow cell culture criteria.

In 31 cases of acute nonlymphoblastic leukemia, bone marrow cells were serially cultured in semi-solid agar during the remission induction therapy. A normal in vitro cell growth pattern returned in 15 out of 22 patients up to 77 days before a complete remission was established by clinical and hematological criteria. In 6 cases the return of normal colonies coincided with clinical and hematological evidence of a complete remission. Nine patients failed to attain a remission and died from complications of bone marrow aplasia. Only one had a normal number of colonies and a normal cluster/colony ratio in cultures prepared 11 days after the completion of the first course of chemotherapy. At this time, his platelet count increased to normal level, possibly indicating a developing remission. Bone marrow cell culture criteria are useful in monitoring the remission induction therapy in patients with acute nonlymphoblastic leukemia. An early return of normal in vitro cell growth pattern suggests an approaching remission, which may be achieved several weeks later.