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Background
Although X-ray imaging is the oldest imaging modality, high quality digital X-ray detectors have only recently enabled new applications not previously possible with conventional analog X-ray detectors. Digital detectors used in a real-time imaging mode possess a large dynamic range which allows accurate measurement of small changes in X-ray photon absorption. The ability to measure small X-ray absorption differences accurately and repeatably enables exploitation of the following well-known principles from basic science.
Illustration of Process
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Examples of Thermal Monitoring Under CT Guidance
The examples in this PowerPoint presentation have been acquired and processed at a later time using Abla-Tx prototype software: Examples of Thermal Monitoring Under CT Guidance

Thermal Treatment Planning
A Work In Progress to Plan the Thermal Ablation using Diagnostic Imaging

Objective

  • Current thermal ablative devices are used by experienced interventionalists to perform ablations of almost all solid tumors in patients
  • The goal of a thermal treatment plan is to provide a 3-D treatment plan prior to the procedure where the plan would attempt to predict the results of the ablation in the specific patient using specific devices

Example of a 3 phase diagnostic liver study

  • The following links to AVI files will demonstrate prototype segmentation software in a normal patient
  • The first segmentation defines the liver volume: Liver 3,427 KB .AVI
  • The vessels are next segmented where the right hepatic vein is blue, middle is white, left is orange and green is the portal vein: Liver Vessels 19,923 KB .AVI

Example of liver Segmentation

  • A tumor would be identified relative to the liver vasculature: Vessels 9,450 KB .AVI
  • A method of displaying the 3-D liver vasculature on a conventional 2-D CT display provides the interventionalist with information regarding the location of key heat sinks as well as the location of critical structures such as bile ducts: Vessel Slice 10,324 KB .AVI

Physics Principles - X-ray Attenuation
For temperatures above 4°C, the mass density of water (in grams/cc) decreases as temperature increases.1 This density variation phenomenon is commonly experienced when diving into a still pool or lake - warm, less dense, water is found at the surface, whereas cooler, denser, water is found at the lower levels. Similarly, the temperature of soft tissues alters the mass density of water present in the soft tissues.2 Although the density change is not large, sufficiently sensitive instruments and sufficiently sophisticated interpretative techniques can detect and interpret mass density changes in the water content of soft tissues. The ability to detect and interpret mass density changes offers a significant technical advance.

X-ray technology, and CT technology in particular, is capable of measuring soft tissue mass density changes.3 In the early to mid-1980's, hyperthermia treatments experienced some acceptance leading several sites to investigate the potential of using CT to monitor temperatures during hyperthermia.3,4,5 Typical results indicated that hyperthermia could be monitored with CT, with temperatures accurate to between 0.5 and 1.0 °C.6 Recent review of ablative management of liver tumors establishes critical temperatures for tissue hyperthermia, including the following thresholds: a 5°C increase above body temperature is the minimal effective temperature elevation for tumor death, while a 21°C elevation is required to begin protein denaturation. This strongly suggests CT temperature monitoring is more than adequate for thermotherapy treatment mapping applications.

A new method of thermal mapping has been described9 which should aid all thermal ablative techniques. The technique requires an x-ray imaging system with a digital detector. The tumor being treated is placed in the x-ray field and a "mask" image is acquired before heat is applied to the tumor. As local tissue temperatures approach 42°C the process of cell death begins. As the tissue temperature increases the time required to assure cell death decreases. At about 58°C proteins begin to denature and at higher temperatures tissue coagulates. At 100°C, a temperature which can occur at the tip of the ablative device, cell water vaporizes.7 The denaturation and coagulation of tissue contained within approximately the same volume, produces density changes.8 As heat is transported from the central coagulation zone and conducted to surrounding tissues, an expanding coagulation zone develops and increases as a function of thermal power delivered over time.7,8 As the lesion is heated, periodic x-rays are taken and digitally subtracted from the mask. The successive subtracted images show the propagation of the change in density indicative of coagulation. Binning of pixels will increase the signal to noise ratio of the digital image which helps to overcome overall system electronic and quantum noise. These resulting density variations can be monitored using digital X-ray and CT technology. Sophisticated software, which includes thermal conductivity modeling, allows measured density variations to form accurate thermal maps of tissue in and surrounding the ablation zone.9 With the availability of high detection quantum efficiency, low noise digital detectors with 12 bit analog-to-digital conversion to provide good contrast resolution, small changes in density can be accurately measured.10

1 Shortley G, Williams D: Elements of Physics (4th Ed.) Vol. 1, Prentice-Hall, Englewood Cliffs, NJ, 1965, pp. 302-304.

2 Hubble JH, Selzer SM: Tables of X-Ray Mass Attenuation Coefficients and Mass Energy-Absorption Coefficients 1 keV to 20 MeV for Elements Z = 1 to 92 and 48 Additional Substances of Dosimetric Interest, NISTIR 5632, National Institute of Standards and Technology, Gaithersburg, MD (1995). (now available at: http://physics.nist.gov/PhysRefData/XrayMassCoef/cover.html)

3 Bentzen SM, Overgaard J, and Jørgensen J: Isotherm Mapping in hyperthermia using subtraction x-ray computed tomography, Radiotherapy and Oncology, 2:255-260, 1984.

4 Fallone BG, Moran PR, and Podgorsak EB: Noninvasive thermometry with a clinical x-ray CT scanner, Med Phys 9(5):715-721, 1982.

5 Bydder GM and Kreel L: The temperature dependence of computed tomography attenuation values, J Comp Asst Tomog 3(4): 506-510, 1979.

6 Zamenhof, RG, Sternick ED, and Curran BM: Non-invasive temperature mapping by computed tomography, Int J Radiol Oncol Biol Phys 7: 1235, 1981.

7 Blaszkowsky L.S., Mueller P. , Managing Liver Lesions: Role of Adjunctive Radiofrequency Ablation, MedScape June, 2004.

8 Hayashi AH, Silver SF, van der Westhuizen NG, Donald JC, Parker C, Fraser S, Ross AC, Olivotto IA., Treatment of invasive breast carcinoma with ultrasound-guided radiofrequency ablation, Am J Surg. 2003 May;185(5):42

9 Salas N, Manns F, Milne PJ, Denham DB, Minhaj AM, Parel J-M, and Robinson DS: Thermal Analysis of Laser Interstitial Thermography in ex-vivo Fibro-Fatty Tissue using Exponential Functions, Phys Med Biol 49(9):1609-1624, 2004

10 Berns EA, Hendrick EA: Optimization of technique factors for a silicon diode array full-field digital mammography system and comparison to screen-film mammography with matched average glandular dose: Med Phys 30(3):334-340, 2003.