Dosimetry in Diagnostic Medical Imaging: State of the Art and New Developments

Transcription

1 Dosimetry in Diagnostic Medical Imaging: State of the Art and New Developments Annalisa Trianni Medical Physics Department Udine University Hospital

2 Content Understand dosimetric quantities to be used in X-Ray imaging for different modalities Recognize what different parameters represent Understand the limitations Introduce new dosimetric concepts proposed

3 Measurements and Units Equipment Performance Fluence Flux Exposure Modality Specific Units EI, DI, ESAK ESAK rate, KAP CTDI, DLP AGD Measurements and Units Patient Dose Absorbed Dose Organ Dose

4 Equipment specific dose quantities

5 XA Projection Exposure Index and Deviation Index KAP air kerma in a plane, integrated over the area of interest ESAK and iak and air kerma incident on patient w and w/o BKS respectively Provided by the system Provided by the system or measured

6 Cumulative air kerma (CK) at the Interventional Reference Point (IRP) Fluoroscopy

7 Computed Tomography CTDI Measured in a acrylic cilindric phantom with a pencil ionization chamber. Represents the average dose in a phantom over the central scan of a series of scans DLP CTDI vol L Describes the cumulative dose for the entire examination

8 Equipment specific dose quantities WHAT DO THEY REPRESENT Standardized parameters to evaluate output Useful tools to compare different equipment, protocols, procedures

9 From operational quantities to patient dose

10 XA Projection Equipment specific dose quantities (iak, KAP) could be used to estimate organ and effective dose: Several programs based on MC codes (i.e. PCXMC) Anatomical data based on antropomorphic phantoms E calculated using either ICRP 60 and ICRP 103 w T E = å w T H T = w T T å T å R w R D T,R Tissues of Reference Male and Reference Female Tissue Specific Radiation Specific

11 Mammography Absorbed Dose to breast (AGD) estimated using simplified models (Dance et al.): Energy imparted to breast tissue calculated with MC code Compression Paddle CC LAT AGD = i AK g c s Breast thickness 0,5 cm skin

12 CT 1. CTDI vol and DLP as dose indicators Limitations: They represent average dose to a phantom (32 cm or 16 cm) positioned at the centre of the scan and exposed in the same conditions (e.g. average ma used throughout the scan) They do not account for patient size, shape and composition

13 CT 1. CTDI vol and DLP as dose indicators Use: Description of scanner output Comparison among different protocols, technical parameter settings, different scanners (attention to the phantome used!) and different centers Optimization Indication of how output is adjusted for patient size (e.g. peds protocols) Relative comparison (i.e. Reference Levels)

14 CT 2. DLP to E conversion k factors calculated using MC calculations and defined per anatomical region Limitations: E = DLP k Formula NO is based SCANNER on approximation SPECIFIC of a curve fit for several scanners between E (based on Monte Carlo simulations) and DLP Patient model was an hermaphrodite standard man size NO PATIENT SPECIFIC

15 CT 3. Organ dose estimation Organ dose estimated using programs based on MC codes parameterization of many scanners Several programs based on MC codes (i.e. ImPACT CT, ImpactDose,CT Expo ) Anatomical data based on antropomorphic phantoms: SCANNER SPECIFIC NO PATIENT SPECIFIC 1 st gen.: STYLIZED Figure 1: Saggital, 2coronal, nd gen.: and transverse VOXEL slice through the voxel-based phantom. 3 rd gen.: BRIP

16 Effective Dose WHAT CAN E BE USED FOR? RISK ASSESSMENT As stated in the ICRP INDIVIDUAL The Commission intended effective dose for use as a principal protection quantity for the establishment of radiological protection guidance. It should not be used to assess risks of stochastic effects in retrospective situations for exposure in identified individuals, nor should it be used in epidemiological evaluations of human expos Cancer risk does not only depend on tissue-specific dose, dose rate, Accurate and quality of radiation estimation (neutron, gamma, of x-rays, organ..) but also dose on sex, age and time since exposure

17 CT Size-Specific Dose Estimate CTDIvol does not account for patient size different values for patients of different size means scanner output is being (usually properly) adjusted for patient size AAPM Report 204 (2010) describes a method to take into account patient size Report describes coefficients based on Lateral Width and AP thickness to calculate an effective diameter to be applied to CDTI vol SSDE again is not patient dose

18 Example 120 kvp 100 mas 120 kvp at 200 mas 9 cm 27 cm 32 cm phantom CTDI vol = 10 mgy 32 cm phantom CTDI vol = 20 mgy

19 CT Size-Specific Dose Estimate CTDIvol does not account for patient size different values for patients of different size means scanner output is being (usually properly) adjusted for patient size AAPM Report 204 (2010) describes a method to take into account patient size Report describes coefficients based on Lateral Width and AP thickness to calculate an effective diameter to be applied to CDTI vol SSDE again is not patient dose

20 Example 120 kvp 100 mas 120 kvp at 200 mas 9 cm 27 cm 32 cm phantom 32 cm phantom CTDI vol = 10 mgy SSDE = 13.2 mgy CTDI vol = 20 mgy SSDE = 13.2 mgy Different CTDI vol Equivalent SSDE

21 CT Patient-specific dose estimate (PSDE) WA Kalender To account for the total scatter proposal for combining information coming from CT images where organ could segmented with the best fitting mathematical phantom Fast MC to obtain the whole body dose distribution describing also TCM and filters Contouring to obtain organ dose REALLY NEEDED FOR EVERY PATIENT?

22 Background NCRP 160 (2007) e Mettler et al. (2008)

23 Background

24 Fluroscopy Skin dose 1. Direct Measurments using large area detectors, TLDs array,

25 PSD (mgy) Fluoroscopy Skin dose Estimations based on CK using correlation coefficients previously assessed based on measurements y = 1,0503x + 179,63 R 2 = 0, CK (mgy)

26 Fluoroscopy Skin dose Assessment from CK at IRP through the equation: æ SD = CK ç è SIRPD SPD ö ø 2 t BSF f - factor To evaluate the MSD it s necessary to account for field overlapping

27 Fluoroscopy Skin dose Skin dose mapping softwares CK translated to the location of the patient s skin Computational model Antropomorphic phantom

28 Take Home Message Different equipment specific dosimetric quantities are available They are useful quantities to evaluate output and compare modalities, equipment, protocols, procedures, They are not patient dose Great effort of medical physics community in the direction of accurate organ dose estimation

29 Thank You