Patient-specific kerma-area product as an exposure estimator in computed tomography: the concept and typical values.

Transcription

1 Patient-specific kerma-area product as an exposure estimator in computed tomography: the concept and typical values. Alexandr Malusek, Ebba Helmrot, Gudrun Alm Carlsson Department of Medical and Health Sciences Division of Radiological Sciences / Radiation Physics Faculty of Health Sciences Linköping University A. Malusek, E. Helmrot, G. Alm Carlsson (LiU) Patient-specific kerma-area product / 13

2 Introduction The concept of the patient-specific kerma-area product (PSKAP) as an exposure estimator in computed tomography (CT) was suggested by Huda 1, but no detailed information on how to evaluate this quantity in practice was given. Our aim: 1 Provide a rigorous definition of PSKAP. 2 Suggest a method for its evaluation by a CT scanner 3 Obtain typical values. 1 W. Huda, (28) Time for unification of CT dosimetry with radiography and fluoroscopy, Radiation Protection Dosimetry, Vol.128, No.2, A. Malusek, E. Helmrot, G. Alm Carlsson (LiU) Patient-specific kerma-area product / 13

3 Theory: Kerma-area product as a risk estimator reference plane x ray tube Kerma-area product, P KA : P KA = K air da beam axis A i,j K air,i,j A, K air,i,j where K air is air kerma A x ray tube P KA does not depend on the position of the reference plane if photons in the beam are neither scattered nor absorbed. K air 1/r 2 A r 2, where r is the distance from the focus A. Malusek, E. Helmrot, G. Alm Carlsson (LiU) Patient-specific kerma-area product / 13

4 Theory: Integration region is patient specific beam axis reference plane iso center position 1 patient x ray tube trajectory position 2 Only the part of the beam incident on the patient contributes to P KA. Reference plane: is perpendicular to the beam axis crosses iso-center Integration region: depends on the viewing angle may contain disconnected parts A. Malusek, E. Helmrot, G. Alm Carlsson (LiU) Patient-specific kerma-area product / 13

5 Theory: Patient-specific kerma-area product, P PSKA PKA (mgycm 2 ) projection index The patient-specific kerma-area product is a sum of projection-specific kerma-area products P PSKA = i P KA,i A. Malusek, E. Helmrot, G. Alm Carlsson (LiU) Patient-specific kerma-area product / 13

6 Theory: Implementation via a KAP meter front view x ray tube bowtie filter KAP meter side view couch detector array A KAP meter is a plane-parallel ionization chamber calibrated to measure P KA for the transmitted radiation. The KAP meter measures all radiation passing downstream the collimator rotates with the x-ray tube The integration region can be determined from the reconstructed image. A. Malusek, E. Helmrot, G. Alm Carlsson (LiU) Patient-specific kerma-area product / 13

7 Theory: Projection-specific kerma-area product x a x reference plane P KA,i = t i PKL x b t i is the acquisition time for projection i x A K air da xb = t i Ṗ KL (x) dx x a = t i Ṗ KA (I ) P KL() P KA xb x a P KL(x) P KL () dx Ṗ KA (I ) is the kerma-area product rate measured by the KAP meter as a function of tube current I P KL (x) and P KL () are the kerma-length products taken at positions x and, respectively. P KL () and P KL(x) are determined during a calibration of the CT scanner. The P KA P KL () integral is evaluated separately for each projection. A. Malusek, E. Helmrot, G. Alm Carlsson (LiU) Patient-specific kerma-area product / 13

8 Method standard CT dosimetry head phantom (R = 8 cm, H = 1 cm) Siemens SOMATOM Open CT scanner simplified formula P KA,i = t i Ṗ KL (I, x = ) 2 xc P KL (x) P KL () dx where Ṗ KA (I, x = ) is the kerma-length product rate measured at the iso-center. iso center bowtie filter x 15 mm A. Malusek, E. Helmrot, G. Alm Carlsson (LiU) Patient-specific kerma-area product / 13

9 Method: Calibration coefficient of a WD CT 1 chamber 57 mm side view top view carbonated epoxy resin air metallic electrode Measurement at SSL for beam quality Q : N KL,Q = 24.2 mgy cm nc 1 Correction for a different beam quality Q was determined from calculated N KL N KL P KL Q = wk air ɛ/(w e /e) 9 mm w 1 mm P KL calculated analytically Q calculated via the PENELOPE Monte Carlo code from energy imparted to the air cavity, ɛ x-ray spectra were analytically filtered by an aluminum filter with a thickness from to 45 mm. A. Malusek, E. Helmrot, G. Alm Carlsson (LiU) Patient-specific kerma-area product / 13

10 Results: Calibration coefficient of a WD CT 1 chamber count NKL / (mgy cm / nc) Energy spectra, 12 kv mm Al 1 mm Al 2 mm Al 3 mm Al E / kev thickness of Al / mm Energy spectra of the CT scanner filtered with, 1, 2 and 3 mm of Al for the tube voltage of 12 kv. 8 kv 1 kv 12 kv 14 kv Calibration coefficient as a function of filter thickness. Linear regression was used. Fitted lines were used to calculate correction factors for measured P KL A. Malusek, E. Helmrot, G. Alm Carlsson (LiU) Patient-specific kerma-area product / 13

11 Results: P KL measurement iso center bowtie filter x 15 mm PKL(x) PKL() kv 1 kv 12 kv 14 kv x / mm Figure: The corrected and normalized kerma-length product as a function of distance from the iso-center. x 1 c xc xc 8 kv 1 kv 12 kv 14 kv P KL(x) P KL() dx 63.6 mm 65.6 mm 67.8 mm 68.1 mm P KL(x) P KL() dx A. Malusek, E. Helmrot, G. Alm Carlsson (LiU) Patient-specific kerma-area product / 13

12 Results: the patient-specific kerma-area product, P PSKA 1 1 rotation: P PSKA =.25 Gy cm 2 P PSKA = 2P KL xc beam width at the iso-center 1.2 cm x-ray tube voltage kv tube current ma scanning time s projected range x c cm P KL mgy cm 2 8 rotations: P PSKA = 2. Gy cm 2 P KL (x) dx = 18.8 mgy cm cm =.25 Gy cm2 P KL () pitch beam width at the iso-center cm = 9.6 cm scanning time s = 8 s P PSKA = 8.25 Gy cm 2 = 2. Gy cm 2 A. Malusek, E. Helmrot, G. Alm Carlsson (LiU) Patient-specific kerma-area product / 13

13 Discussion To implement the method: the CT scanner must be equipped with a KAP meter the calculation procedure must be implemented in the scanner s software a calibration to determine the ratio P KL ()/P KA and the function P KL (x)/p KL () must be performed. A. Malusek, E. Helmrot, G. Alm Carlsson (LiU) Patient-specific kerma-area product / 13