New semi-analytical method for fast transcranial ultrasonic field simulation.

Objective.To optimize and ensure the safety of ultrasound brain therapy, personalized transcranial ultrasound simulations are very useful. They allow to predict the pressure field, depending on the patient skull and probe position. Most transcranial ultrasound simulations are based on numerical methods which have a long computation time and a high memory usage. The goal of this study is to develop a new semi-analytical field computation method that combines realism and computation speed.Approach.Instead of the classic ray tracing, the ultrasonic paths are computed by time of flight minimization. Then the pressure field is computed using the pencil method. This method requires a smooth and homogeneous skull model. The simulation algorithm, so-called SplineBeam, was numerically validated, by comparison with existing solvers, and experimentally validated by comparison with hydrophone measured pressure fields through anex vivohuman skull.Main results.SplineBeam simulated pressure fields were close to the experimentally measured ones, with a focus position difference of the order of the positioning error and a maximum pressure difference lower than 6.02%. In addition, for those configurations, SplineBeam computation time was lower than another simulation software, k-Wave's, by two orders of magnitude, thanks to its capacity to compute the field only at the focal spot.Significance.These results show the potential of this new method to compute fast and realistic transcranial pressure fields. The combination of this two assets makes it a promising tool for real time transcranial pressure field prediction during ultrasound brain therapy interventions.

A hybrid scatter correction for 3D PET based on an estimation of the distribution of unscattered coincidences: implementation on the ECAT EXACT HR+.

We implemented a hybrid scatter-correction method for 3D PET that combines two scatter-correction methods in a complementary way. The implemented scheme uses a method based on the discrimination of the energy of events (the estimation of trues method (ETM)) and an auxiliary method (the single scatter simulation method (SSSI) or the convolution-subtraction method (CONV)) in an attempt to increase the accuracy of the correction over a wider range of acquisitions. The ETM takes into account the scatter from outside the field-of-view (FOV), which is not estimated with the auxiliary method. On the other hand, the auxiliary method accounts for events that have scattered with small angles, which have an energy that cannot be discriminated from that of unscattered events using the ETM. The ETM uses the data acquired in an upper energy window above the photopeak (550-650 keV) to obtain a noisy estimate of the unscattered events in the standard window (350-650 keV). Our implementation uses the auxiliary method to correct the residual scatter in the upper window. After appropriate scaling, the upper window data are subtracted from the total coincidences acquired in the standard window, resulting in the final scatter estimate, after smoothing. In this work we compare the hybrid method with the corrections used by default in the 2D and 3D modes of the ECAT EXACT HR+ using phantom measurements. Generally, the contrast was better with the hybrid method, although the relative errors of quantification were similar. We conclude that hybrid techniques such as the one implemented in this work can provide an accurate, general-purpose and practical way to correct the scatter in 3D PET, taking into account the scatter from outside the FOV.

Comparison of clinical data sets acquired on different tomographs using 6-18F-L-dopa.

Longitudinal positron emission tomography (PET) studies of 6-18F-L-dopa uptake in the striatum are used to assess the progression of Parkinson's disease or the survival of neuronal cells grafted in parkinsonian patients. These studies are performed over several years, and data analysis may suffer from the change from old tomographs to new machines with better sensitivity and spatial resolution. Furthermore, such studies on parkinsonian patients may be accomplished in either 2D or 3D acquisition mode. The aforementioned improvements offer great benefits for the study of neurodegenerative diseases, especially those affecting the striatum. However, direct comparison of data is not straightforward owing to variation in scanner characteristics. In this study, we assessed the feasibility of comparing the 6-18F-L-dopa striatal uptake values (Kc) measured in two groups of healthy subjects using two tomographs of different generations. We re-studied and compared acquisitions performed on 14 healthy subjects using 6-18F-L-dopa. Half of these studies had been performed in 2D acquisition mode using an ECAT 953B. The other half had been performed in 3D acquisition mode using an ECAT EXACT HR+. Different reconstruction protocols were used and the Kc values obtained were statistically compared. The results showed that lowering the transverse spatial resolution of images obtained with the scanner having the better spatial resolution, so that it more closely matched that of the other machine, allowed similar KC values to be obtained in healthy subjects. This study shows that quantitative results of 6-18F-L-dopa scans can be matched between different scanners with different intrinsic resolutions. This can be accomplished using adequate modifications of the reconstruction parameters. Such modifications can be used to help in the longitudinal monitoring of parkinsonian patients using different tomographs.

Comparison of fluorine-18 and bromine-76 imaging in positron emission tomography.

State of the art positron emission tomography (PET) systems allow for scatter and attenuation correction. However, the size of the structure being studied and the region of interest (ROI) chosen also influence the accuracy of measurements of radioactive concentration. Furthermore, the limited spatial resolution of PET tomographs, which depends, among other factors, on the range of positrons in matter, can also contribute to a loss in quantitation accuracy. In this paper we address the influence of positron range, structure size and ROI size on the quantitation of radioactive concentration using PET. ECAT EXACT HR+ (HR+) and ECAT 953B/31 (ECAT 953B) PET systems were used in phantom acquisitions performed with two radioisotopes with different positron ranges. The 3D Hoffman phantom was scanned on both scanners with both radioisotopes, to visually analyse the image quality. A resolution phantom having six spheres of different diameters in a Plexiglas cylinder was used to calculate the values of the contrast recovery coefficient or hot spot recovery coefficient and of the spill-over or cold spot recovery coefficient under different imaging conditions used in clinical routine at our institution. Activity ratios were varied between 2 and 30 or between 0.4 and 200 by filling the spheres with fluorine-18 or bromine-76 respectively and the cylinder with 11C. Dynamic scans were performed on each scanner. Data were reconstructed using the same parameters as are used in clinical protocols. The variations in sphere and cylinder activities with time were fitted using the function M(t)=k1. A(t)+k2.B(t), where M(t) is the radioactivity concentration measured in an ROI placed on each sphere and A(t) and B(t) represent the true radioactivity concentrations present at time t in the spheres and in the cylinder respectively. k1 and k2 are factors representing the contrast recovery coefficient and the spill-over from surrounding activity on measurements respectively. The visual analysis of images obtained using a 3D Hoffman phantom showed that image resolution and image contrast between different regions are radioisotope dependent and clearly better when using 18F. Linear profiles taken on these images confirmed the visual assessment. For a given scanner, the k1 values obtained with 18F were systematically higher than those measured using 76Br in the same machine (especially for the smaller spheres) when using the same ROI. For a sphere of a particular diameter, the use of a wider ROI resulted in lower quantitative accuracy when using the same isotope and the same camera. Lower quantitative accuracy was found for smaller spheres for all ROI sizes used in image analysis. For the same scanner and for a similar imaging situation (same sphere and same ROI), it was found that k1 and k2 values depend on the radioisotope used. For the same isotope and tomograph, the k1 values obtained decreased with the size of the structures imaged, as well as with the increase in ROI size. The use of a tomograph with better spatial resolution (HR+, rather than ECAT 953B) greatly increased the k1 values for 18F while only a mild improvement in these values was observed for 76Br. The use of 76Br led to k2 values that were slightly higher than those measured using 18F. These differences may have been due to the difference in the range of the positrons emitted by the radioisotopes used in this study. The measurements performed in this study show that the comparison of studies obtained on the same camera depends on the radioisotope used and may require the adaptation of ROI size between examinations. Marked differences are visible if the positron ranges of such radioisotopes are very different. Therefore, when employing commercially available tomographs and imaging protocols used in clinical routine, the effects of differences in positron range on image quality and quantitation are noticeable and correction for these effects may be of importance. (ABSTRACT TRUNCATED)

Dosimetry of transmission measurements in nuclear medicine: a study using anthropomorphic phantoms and thermoluminescent dosimeters.

Quantification in positron emission tomography (PET) and single photon emission tomographic (SPET) relies on attenuation correction which is generally obtained with an additional transmission measurement. Therefore, the evaluation of the radiation doses received by patients needs to include the contribution of transmission procedures in SPET (SPET-TM) and PET (PET-TM). In this work we have measured these doses for both PET-TM and SPET-TM. PET-TM was performed on an ECAT EXACT HR+ (CTI/Siemens) equipped with three rod sources of germanium-68 (380 MBq total) and extended septa. SPET-TM was performed on a DST (SMV) equipped with two collimated line sources of gadolinium-153 (4 GBq total). Two anthropomorphic phantoms representing a human head and a human torso, were used to estimate the doses absorbed in typical cardiac and brain transmission studies. Measurements were made with thermoluminescent dosimeters (TLDs, consisting of lithium fluoride) having characteristics suitable for dosimetry investigations in nuclear medicine. Sets of TLDs were placed inside small plastic bags and then attached to different organs of the phantoms (at least two TLDs were assigned to a given organ). Before and after irradiation the TLDs were placed in a 2.5-cm-thick lead container to prevent exposure from occasional sources. Ambient radiation was monitored and taken into account in calculations. Transmission scans were performed for more than 12 h in each case to decrease statistical noise fluctuations. The doses absorbed by each organ were calculated by averaging the values obtained for each corresponding TLD. These values were used to evaluate the effective dose (ED) following guidelines described in ICRP report number 60. The estimated ED values for cardiac acquisitions were 7.7 x 10(-4) +/- 0.4 x 10(-4) mSv/MBq.h and 1.9 x 10(-6) +/- 0.4 x 10(-6) mSv/MBq.h for PET-TM and SPET-TM, respectively. For brain scans, the values of ED were calculated as 2.7 x 10(-4) +/- 0.2 x 10(-4) mSv/MBq.h for PET-TM and 5.2 x 10(-7) +/- 2.3 x 10(-7) mSv/MBq.h for SPET-TM. In our institution, PET-TM is usually performed for 15 min prior to emission. SPET-TM is performed simultaneously with emission and usually lasts 30 and 15 min for brain and cardiac acquisitions respectively. Under these conditions ED values, estimated for typical source activities at delivery time (22,000 MBq in SPET and 555 MBq for PET), were 1.1 x 10(-1) +/- 0.1 x 10(-1) mSv and 1.1 x 10(-2) +/- 0.2 x 10(-2) mSv for cardiac PET-TM and SPET-TM respectively. For brain acquisitions, the ED values obtained under the same conditions were 3.7 x 10(-2) +/- 0.3 x 10(-2) mSv and 5.8 x 10(-3) +/- 2.6 x 10(-3) mSv for PET-TM and SPET-TM respectively. These measurements show that the dose received by a patient during a transmission scan adds little to the typical dose received in a routine nuclear medicine procedure. Radiation dose, therefore, does not represent a limit to the generalised use of transmission measurements in clinical SPET or PET.