A comparison of two methods for the determination of freein-air geometric efficiency in MDCT

Transcription

1 A comparison of two methods for the determination of freein-air geometric efficiency in MDCT Theocharis Berris *1, Kostas Perisinakis 1,, Antonios E. Papadakis and John Damilakis 1, 1 Department of Medical Physics, Faculty of Medicine, University of Crete, P.O. Box 08, Heraklion, Crete, Greece Department of Medical Physics, University Hospital of Heraklion, P.O. Box 135, Heraklion, Crete, Greece * Presenting author: theocharisberris@yahoo.com

2 Introduction Single slice CT systems Multi detector CT systems Active detector width along z-axis incorporates the beam penumbra No radiation is wasted for non-imaging purposes Penumbra must fall outside the active detector array to avoid calibration problems [1] A portion of the radiation is wasted for nonimaging purposes Overbeaming []

3 Introduction Free-in-air geometric efficiency (GE) is an important dosimetric feature of MDCT scanners and comprises an index of radiation utilization efficiency along z-axis [3] GE = Area under curve within nominal active detector width Area under total dose profile Eq. (1) [4]

4 Purpose To compare two methods used for the assessment of free-in-air geometric efficiency on MDCT scanners, namely: Radiographic film Solid state CT dose profiler Materials and methods A. The CT scanner Siemens SOMATOM Sensation 16-slice CT scanner (Siemens AG, Forchheim, Germany) Modes of operation Head (80, 10, 140 kvp) Body (80, 10, 140 kvp) Available focal spot sizes mm mm Available collimations = = = 1 5 = = 9 1 = 0.6 = 1.* *N/A at head mode 140 kvp *N/A for the large focal spot

5 Materials and methods B. Determination of dose profiles using radiographic films B1. Film calibration Film type: Fuji HR U-30, 4 30 cm (FUJIFILM Medical Systems USA, Inc., Stamford CT) Elimination of OD between film calibration and optical scanner calibration fitted equations B. Optical scanner calibration Optical scanner type: Microtek ScanMaker 9600XL (Science-based Industry Park, Hsinchu, Taiwan) Dose = f (grey levels) Pixel value profiles obtained from digitized films can be converted to dose profiles

6 , Materials and methods C. Determination of dose profiles using a solid state detector CT-SD16 slice detector coupled with the Barracuda x-ray multimeter (RTI Electronics, AB, Mölndal, Sweden) Instant acquisition of dose profiles via specialized software Profiles of beam collimations available only in sequential mode could not be obtained D. Calculation of free-in-air geometric efficiency GE free in air + a a a = mm D(z) dz D(z) dz + a D(z) dz 140mm D(z) dz Where: D(z): Dose value along z-axis a: Nominal beam-width along z-axis at the isocenter + a a + D(z) dz D(z) dz : Dose integral corresponding to the nominal detector width mm 140 mm D(z)dz : Total dose integral.

7 Results Dose profiles obtained with film and the CT- SD16 solid state probe for the 4 and 1. mm collimations. Measurements were performed at 80 kvp, body mode and the small focal spot in use Geometric efficiency for 80 kvp, body mode measured by a) film, b) CT-SD16. Values for small and large focal spot are illustrated. Insets exemplify the percent difference in GE caused by focal spot swap.

8 Results GE for all available collimations and beam qualities as measured with film and the solid state detector. Measurements were performed with the small focal spot in use The differences of the values of GE determined with film from the corresponding values determined with the CT-SD16 detector. The dashed lines mark the 5% difference level B: Body mode, H: Head mode

9 Conclusions Radiographic film Can produce detailed dose profiles Tedious method. Calibrations and post processing are needed to obtain dose profiles Can measure GE of beam collimations available only in sequential mode or static beams CT-SD16 Easy to use. Immediate dose profile acquisition Can produce detailed dose profiles Can measure GE of scanning beams only GE Depends on the geometrical characteristics of the beam Wide beam collimations have higher GE values than the thin ones Increasing focal spot size is correlated with GE deterioration which is more pronounced for thin collimations GE is not affected by changes in beam quality

10 References 1. M.A. Lewis, Multislice CT: opportunities and challenges, Br. J. Radiol. 74, (001). M. K. Kalra, M. M. Maher, T. L. Toth, L. M. Hamberg, M. A. Blake, Jo-Anne Shepard and S. Saini, Strategies for CT Radiation Dose Optimization, Radiology 30, (004) 3. K. Perisinakis, A. E. Papadakis and J. Damilakis, The effect of x-ray beam quality and geometry on radiation utilization efficiency in multidetector CT imaging, Med. Phys. 36, (009) 4. International Electrotechnical Commission. Amendment 1 to IEC , edition : medical electrical equipment, part 44 particular requirements for the safety of x-ray equipment for computed tomography. International Standard. Geneva, Switzerland: International Electrotechnical Commission, 00; 1 7