Suppression of high-density artefacts in x-ray CT images using temporal digital subtraction with application to cryotherapy

Image guidance in cryotherapy is usually performed using ultrasound. Althou gh not currently in routine clinical use, x-ray CT imaging is an alternativ e means of guidance that can display the full 3D structure of the iceball, including frozen and unfrozen regions. However, the quality of x-ray CT ima ges is compromised by the presence of high-density streak artefacts. To sup press these artefacts we applied temporal digital subtraction (TDS). This T DS method has the added advantage of improving the grey-scare contrast betw een frozen and unfrozen tissue in the CT images. Two sets of CT images were taken of a phantom material, cryoprobes and a urethral warmer (UW) before and during the cryoprobe freeze cycle. The high-density artefacts persisted in bath image sets. TDS was performed on these two image sets using the co rresponding mask image of unfrozen material and the same geometrical config uration of the cryoprobes and the UW. The resultant difference image had a significantly reduced artefact content. Thus TDS can be used to significant ly suppress or eliminate high-density CT streak artefacts without reducing the metallic content of the cryoprobes. In vivo study needs to be conducted to establish the utility of this TDS procedure for CT assisted prostate or liver cryotherapy. Applying TDS in x-ray CT guided cryotherapy will facili tate estimation of the number and location of ail frozen and unfrozen regio ns, potentially making cryotherapy safer and less operator dependent.