title: Feasibility of kilovoltage x-ray energy modulation by gaseous media and its application in contrast-enhanced radiotherapy.
abstract: OBJECTIVE
  To present a method to modulate the energy contents of a kilovoltage x-ray beam that makes use of a gas as the modulating medium. The method is capable of producing arbitrary x-ray spectra by varying the pressure of the modulating gas and the peak kilovoltage (kVp) of the x-ray beams whose energy is being modulated.
  
  
  METHODS
  An aluminum chamber was machined with a 0.5 cm wall thickness, designed to withstand pressures of more than 80 atm. A pressure sensor and electrovalves were used to monitor and regulate the gas pressure. Argon was used as the modulating gas. A CdTe spectrometer was used to measure x-ray spectra for different combinations of kVp and gas pressure, thus obtaining a set of basis x-ray functions. An arbitrary x-ray spectrum can then be formed by the linear combination of such basis functions. In order to show one possible application of the modulation method, a contrast-enhanced radiotherapy prostate treatment was optimized with respect to the x-ray beam energy, without restrictions on the possible shape of the resultant x-ray spectra.
  
  
  RESULTS
  The x-ray spectra basis functions obtained display a smooth and gradual variation of their average energy as a function of the gas pressure for a given kVp, sometimes in the order of 1 or 2 keV. This gradual variation would be difficult to obtain with a conventional aluminum or copper filters, as the change in thickness necessary to reproduce the data presented would be in the order of micrometers, making necessary the use of a large number of such filters. Using the modulation method presented here, the authors were able to reconstruct the optimized x-ray spectra from the measured basis functions, for different optimization objectives.
  
  
  CONCLUSIONS
  A method has been developed that allows for the controlled modulation of the energy contents of kilovoltage x-ray spectra. The method has been shown to be able to reproduce spectra of arbitrary shape, such as those obtained from the optimization of contrast-enhanced radiotherapy. The method may have other applications as well, such as in the precise matching of diagnostic x-ray catalog spectra.
authors.affiliation: Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Unidad Monterrey, Vía del Conocimiento 201 Parque PIIT, Apodaca, Nuevo León 66600, Mexico.
authors.forename: E L
authors.initials: EL
authors.lastname: Facundo-Flores
authors.forename: H M
authors.initials: HM
authors.lastname: Garnica-Garza
mesh_terms: D003287:Contrast Media; D005240:Feasibility Studies; D005740:Gases; D006801:Humans; D008297:Male; D011471:Prostatic Neoplasms / Q000175:diagnosis / Q000532:radiotherapy; D061089:Radiotherapy, Image-Guided / Q000295:instrumentation / Q000379:methods*
chemical_list: D003287:Contrast Media; D005740:Gases