Cone-beam breast computed tomography with a displaced flat panel detector array.

PURPOSE: In cone-beam computed tomography (CBCT), and in particular in cone-beam breast computed tomography (CBBCT), an important issue is the reduction of the image artifacts produced by photon scatter and the reduction of patient dose. In this work, the authors propose to apply the detector displacement technique (also known as asymmetric detector or "extended view" geometry) to approach this goal. Potentially, this type of geometry, and the accompanying use of a beam collimator to mask the unirradiated half-object in each projection, permits some reduction of radiation dose with respect to conventional CBBCT and a sizeable reduction of the overall amount of scatter in the object, for a fixed contrast-to-noise ratio (CNR). METHODS: The authors consider a scan configuration in which the projection data are acquired from an asymmetrically positioned detector that covers only one half of the scan field of view. Monte Carlo simulations and measurements, with their CBBCT laboratory scanner, were performed using PMMA phantoms of cylindrical (70-mm diameter) and hemiellipsoidal (140-mm diameter) shape simulating the average pendant breast, at 80 kVp. Image quality was evaluated in terms of contrast, noise, CNR, contrast-to-noise ratio per unit of dose (CNRD), and spatial resolution as width of line spread function for high contrast details. RESULTS: Reconstructed images with the asymmetric detector technique deviate less than 1% from reconstruction with a conventional symmetric detector (detector view) and indicate a reduction of the cupping artifact in CT slices. The maximum scatter-to-primary ratio at the center of the phantom decreases by about 50% for both small and large diameter phantoms (e.g., from 0.75 in detector view to 0.40 in extended view geometry at the central axis of the 140-mm diameter PMMA phantom). Less cupping produces an increase of the CT number accuracy and an improved image detail contrast, but the associated increase of noise observed may produce a decrease of detail CNR. By simulating the energy deposited inside the phantoms, the authors evaluated a maximum 50% reduction of the absorbed dose at the expense of a decrease of CNR, for the half beam irradiation of the object performed with the displaced detector technique with respect to full beam irradiation. The decrease in CNR, and in absorbed dose as well, translates into a detail CNRD showing values comparable to or higher than the ones obtained for a conventional symmetric detector technique, attributed to the effect of decreased scatter in particular at the axis of the irradiated object. An estimate is provided (about 12%) for the average dose reduction possible in CBBCT at constant CNR for the average uncompressed breast (14 cm diameter, 50% glandularity), in case of minimum image overlapping in extended view. CONCLUSIONS: Simulations and experiments show that CBCT reconstructions with the displaced detector technique and with a half beam collimator are less affected by scatter artifacts, which could lead to some decrease of the radiation dose to the irradiated object with respect to a conventional reconstruction. This dose reduction is associated with increase of noise, decrease of CNR, but equal or improved CNRD values. The use of a small area detector would allow also to reduce the apparatus cost and to improve the data transfer speed with a corresponding increment of frame rate.

Optical imaging of alpha emitters: simulations, phantom, and in vivo results.

There has been growing interest in investigating both the in vitro and in vivo detection of optical photons from a plethora of beta emitters using optical techniques. In this paper we have investigated an alpha particle induced fluorescence signal by using a commercial CCD-based small animal optical imaging system. The light emission of a (241)Am source was simulated using GEANT4 and tested in different experimental conditions including the imaging of in vivo tissue. We believe that the results presented in this work can be useful to describe a possible mechanism for the in vivo detection of alpha emitters used for therapeutic purposes.

Radiation emission dose from patients administered 90Y-labelled radiopharmaceuticals: comparison of experimental measurements versus Monte Carlo simulation.

AIM: To estimate the radiation dose delivered from patients injected with yttrium-90 (Y)-labelled tiuxetan (Zevalin) to parents and the general population, comparing different techniques. METHODS: The radiation dose delivered from a group of eight patients injected with Y-Zevalin to treat recurrent lymphoma was measured. The data obtained with the Monte Carlo simulation test were compared with the experimental measurements obtained with an ionization chamber detector and with a crystal NaI(Tl) detector. RESULTS: A good correlation was found between the Monte Carlo simulation test and the ionization chamber detector results: the air kerma dose rate was 4.2+/-0.1 and 4.4+/-0.8 microGy/h, respectively (r=0.9, P<0.01). Moreover, more than 99.7% of the air kerma dose rate measured with the ionization chamber detector was because of the contribution of electrons, whereas the contribution of photons was less than 0.3%. In contrast, the air kerma dose rate measured with the crystal NaI(Tl) detector was significantly lower (0.76+0.12 microGy/h) in comparison with the Monte Carlo simulation test. This underestimation was related to the limited crystal NaI(Tl) detector response to low energy rates at variance with the ionization chamber detector. The effective radiation dose released by patients treated with Y-labelled tiuxetan to parents and the general population was approximately 0.1 mSv per treatment cycle. CONCLUSION: Using the Monte Carlo model as a benchmark to compare the experimental measurements obtained by the two different detectors, we found that the ionizing chamber detector was more accurate than the crystal Na(Tl) detector for measuring the exposure radiation dose delivered from patients administered with Y-labelled radiopharmaceuticals. Moreover, the effective radiation dose released by these patients to their parents and the general population is significantly lower than the value recommended by international reports and regulations.