Photoelectric-enhanced radiation therapy with quasi-monochromatic computed tomography.

Photoelectric-enhanced radiation therapy is a bimodal therapy, consisting of the administration of highly radiation-absorbing substances into the tumor area and localized regional irradiation with orthovoltage x-rays. Irradiation can be performed by a modified computed tomography (CT) unit equipped with an additional x-ray optical module which converts the polychromatic, fan-shaped CT beam into a monochromatized and focused beam for energy-tuned photoelectric-enhanced radiotherapy. A dedicated x-ray optical module designed for spatial collimation, focusing, and monochromatization was mounted at the exit of the x-ray tube of a clinical CT unit. Spectrally resolved measurements of the resulting beam were performed using an energy-dispersive detection system calibrated by synchrotron radiation. The spatial photon fluence was determined by film dosimetry. Depth-dose measurements were performed and compared to the polychromatic CT and a therapeutic 6 MV beam. The spatial dose distribution in phantoms using a rotating radiation source (quasimonochromatic CT and 6 MV, respectively) was investigated by gel dosimetry. The photoelectric dose enhancement for an iodine fraction of 1% in tissue was calculated and verified experimentally. The x-ray optical module selectively filters the energy of the tungsten Kalpha emission line with an FWHM of 5 keV. The relative photon fluence distribution demonstrates the focusing characteristic of the x-ray optical module. A beam width of about 3 mm was determined at the isocenter of the CT gantry. The depth-dose measurements resulted in a half-depth value of approximately 36 mm for the CT beams (quasi-monochromatic, polychromatic) compared to 154 mm for the 6 MV beam. The rotation of the radiation source leads to a steep dose gradient at the center of rotation; the gel dosimetry yields an entrance-to-peak dose ratio of 1:10.8 for the quasi-monochromatic CT and 1:37.3 for a 6 MV beam of the same size. The photoelectric dose enhancement factor increases from 2.2 to 2.4 by using quasi-monochromatic instead of polychromatic radiation. An additional increase in the radiation dose by a factor of 1.4 due to the focusing characteristic of the x-ray optical module was calculated. Photoelectric-enhanced radiation therapy based on a clinical CT unit combined with an x-ray optical module is a novel therapy option in radiation oncology. The optimized quasi-monochromatic radiation is strongly focused and ensures high photoelectric dose enhancement for iodine.

Monochromatic x-rays in digital mammography.

RATIONALE AND BACKGROUND: The emission spectrum of an x-ray tube is determined by the anode and filter materials as well as by the high voltage being used. For mammography, typical anode materials are molybdenum (Mo), rhodium (Rh), and tungsten (W); molybdenum, rhodium, and aluminum are favored for filters. Mammography is a soft tissue imaging modality demanding a high spatial resolution as well as a high detector sensitivity. Low-energy photons are only absorbed in tissue and have no contribution to the image; nevertheless, they increase the dose. High-energy photons mostly penetrate soft tissue and generate a background noise as a result of strong scattering that deteriorates the image quality. For mammography, the optimal energy window is in a range from 17 and 25 keV. From a theoretical perspective, one would favor monoenergetic x-rays (eg, the Mo-emission line at 17.5 keV). This article presents the realization of imaging with monochromatic x-rays using a diagnostic mammography unit. METHODS: Basically, a monochromatic module was added to a conventional mammographic system. The monochromatic module can be mounted at the end of the x-ray tube and it consists of a curved HOPG (highly oriented pyrolytic graphite) crystal and a slit collimator. For image generation, the object is moved through the fan-shaped monochromatic radiation field. In addition to the conventional polychromatic 2-dimensional case, the polychromatic irradiation was also able to be performed under similar conditions. For image acquisition, image plates or a linear array detector were used. Exposure doses were measured for both poly- and monochromatic radiation. The initial evaluation of the system performance was carried out by imaging a contrast-detail phantom and biologic specimens. RESULTS: The monochromatic x-ray beam has a size of approximately 35 mm x 200 mm in the object plane. The photon flux of the monochromatic x-rays is considerably lower than the photon flux of the polychromatic x-rays but adequate for initial studies of phantoms, biologic tissue, or small animals. The comparison of the results obtained with the monochromatic and polychromatic imaging modalities reveal a conspicuous increase of image contrast in the monochromatic case. CONCLUSION: The results suggest that the experimental setup for monochromatic excitation shows clear potentials for improvements of the image in comparison to the conventional polychromatic case.

Molecular imaging based on x-ray fluorescent high-Z tracers.

We propose a novel x-ray fluorescence imaging setup for the in vivo detection of high-Z tracer distributions. The main novel aspect is the use of an analyzer-based, energy-resolved detection method together with a radial, scatter reducing collimator. The aim of this work is to show the feasibility of this method by measuring the Bragg reflected K-fluorescence signal of an iodine solution sample in a proof of principle experiment and to estimate the potential of the complete imaging setup using a Monte Carlo simulation, including a quantification of the minimal detectable tracer concentration for in vivo imaging. The proof of principle experiment shows that even for a small detector area of approximately 7 mm(2), the collimated and Bragg reflected K-fluorescence signal of a sample containing an iodine solution with a concentration of 50 µg ml(-1) can be detected. The Monte Carlo simulation also shows that the proposed x-ray fluorescence imaging setup has the potential to image distributions of high-Z tracers in vivo at a radiation dose of a few mGy and at tracer concentrations down to 1 µg ml(-1) for iodine in small animals.

Spatial resolution of confocal XRF technique using capillary optics.

XRF (X-ray fluorescence) is a powerful technique for elemental analysis with a high sensitivity. The resolution is presently limited by the size of the primary excitation X-ray beam. A test-bed for confocal-type XRF has been developed to estimate the ultimate lateral resolution which could be reached in chemical mapping using this technique. A polycapillary lens is used to tightly focus the primary X-ray beam of a low power rhodium X-ray source, while the fluorescence signal is collected by a SDD detector through a cylindrical monocapillary. This system was used to characterize the geometry of the fluorescent zone. Capillary radii ranging from 50 µm down to 5 µm were used to investigate the fluorescence signal maximum level This study allows to estimate the ultimate resolution which could be reached in-lab or on a synchrotron beamline. A new tool combining local XRF and scanning probe microscopy is finally proposed.

Imaging-therapy computed tomography with quasi-monochromatic X-rays.

INTRODUCTION: Computed tomography (CT) is a widespread and highly precise technique working in the energy range around 50-100 keV. For radiotherapy, however, the MeV energy range enables a better dose distribution. This gap between diagnosis and therapy can be overcome by the use of a modified CT machine in combination with heavy elements targeted to the tumour and used as photoelectric radiation enhancer. MATERIALS AND METHODS: The experimental setup consists of an X-ray optical module mounted at the exit of the X-ray tube of a clinical CT. The module converts the standard fan-shaped beam into a high intensity, monochromatized and focused beam. The radiation was characterized using an energy-dispersive detection system calibrated by synchrotron radiation and gel dosimetry. The photoelectric radiation enhancement for different elements was calculated and experimentally verified. RESULTS: The X-ray optical module filters selectively the energy of the tungsten K alpha-emission line (59.3 keV) with a full width at half maximum (FWHM) of 5 keV and focused the radiation onto a focal spot which coincides with the isocentre of the gantry. This results in a steep dose gradient at the centre of rotation qualified for locoregional radiation therapy. The photon energy of the quasi-monochromatic radiation agrees with the energy range of maximal photoelectric dose enhancement for gadolinium and iodine. CONCLUSION: An additional X-ray optical module optimized for targeted therapy and photoelectric dose enhancement allows the combination of diagnosis and radiotherapy on a clinical CT.