CT Dose Estimation. John M. Boone, Ph.D., FAAPM, FSBI, FACR Professor and Vice Chair of Radiology. University of California Davis Medical Center

Transcription

1 CT Dose Estimation John M. Boone, Ph.D., FAAPM, FSBI, FACR Professor and Vice Chair of Radiology 1 University of California Davis Medical Center

2 CT Dose Estimation Introduction The CTDI Family of Metrics Dose Length Product Effective Dose Dose Reporting Summary 2

3 Introduction 3

4 Introduction 4

5 The radiation from CT has caught the eye of the public 5

6 California SB-1237, SB-58, AB-510 Mad River Hospital Alex Padilla Cedars-Sinai Dose reporting required as of July 1,

7 CT Dose Estimation Introduction The CTDI Family of Metrics Dose Length Product Effective Dose Dose Reporting Summary 7

8 CTDI: Computed Tomography Dose Index Tom Shope where it all began. 8

9 CTDI L = 1 nt +L/2 -L/2 D(z) dz 100 mm CTDI 100 = +50mm 1 D(z) dz nt -50mm 9

10 CTDI: Measurement tools 100 mm long ion chamber Phantoms 10

11 100 mm pencil chamber PMMA plug 32 cm body PMMA phantom peripheral hole center hole 16 cm head PMMA phantom also, pediatric body phantom 11

12 Measuring CTDI 100 in the real world 12

13 32.1 mgy 13

14 100 mm 100 mm pencil chamber and thin CT x-ray beam 100 mm CT x-ray beam and small radiation meter 14

15 center CTDI 100 periphery CTDI 100 Weighted CTDI: center periphery CTDI w = ⅓ CTDI ⅔ CTDI

16 helical CT acquisition 16

17 Dose 1 pitch low pitch high pitch

18 Weighted CTDI: center periphery CTDI w = ⅓ CTDI ⅔ CTDI 100 Volume CTDI: CTDI vol = CTDI w pitch 18

19 General Electric Siemens CTDI vol is a useful parameter because it is required to be displayable on all modern CT scanners due to IEC regulations. It is also stored in the header of the CT images. 19

20 CTDI is a good measure of CT x-ray tube radiation output, but is not a stand-alone metric for patient dose etcetera. 20

21 CTDI body phantom CTDI is a good measure of dose to a 32 cm diameter, 1.19 g/cm 3 piece of plastic. Most patients will be smaller, and have higher doses. Larger patients will have lower doses (at the same techniques) 32 cm cm r = 1.19 waistline typical patient 28 cm cm waistline

22 AAPM Report 204

23 circle of equal area (weighted) 5.40 mgy = CTDIvol (32 cm phantom) water equivalent diameter

24 Dose patient CTDI vol f Size-specific dose estimate (SSDE) SSDE To perform patient size-specific dose estimation, the conversion factor above is multiplied by the value of CTDI vol. 24

25 5.40 mgy = CTDI vol (32 cm phantom) SSDE = CTDI vol f 13.0 mgy =

26 CT Dose Estimation Introduction The CTDI Family of Metrics Dose Length Product Effective Dose Dose Reporting Summary 26

27 Which scan has more dose? L D = De Dm to first order, the dose is the same L 27

28 Dose Length Product (DLP): L DLP = CTDI vol L (mgy cm) DLP is related to the total energy deposited in the patient 28

29 c a x-ray beam profile along z

30 b L f(z) -L/2 D L (z) +L/2

31 -L/2 D L (z) +L/2

32 D L (z) Dose profiles as a function of Scan Length scan length

33 D(L) Equilibrium Dose as a function of Scan Length D eq

34 TG-111 Method

35 TG-111 Method

36 TG-111 Method

37 L

38 AAP Task Group 200 & ICRU Phantom

39 ICRU Method: Real Time X-ray Meter 39

40 beam profile [ RECT ] 40

41 CT Dose Estimation Introduction The CTDI Family of Metrics Dose Length Product Effective Dose Dose Reporting Summary 41

42 Dose Length Product (DLP): L DLP = CTDI vol L (mgy cm) DLP is related to the total energy deposited in the patient 42

43 Most modern dosimetry is Monte Carlo based = person mathematical model of person 43

44 CT geometry is modeled and organ doses are estimated 44

45 tissue weighting factors from ICRP 103 effective dose organ doses computed absorbed Organ dose w t E Gonads Bone marrow Colon Lung Stomach Bladder Breast Liver Esophagus Thyroid Skin Bone surface Brain Salivary Glands remainder = Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y YY msv 45

46 other minor details kerma absorbed dose equivalent dose effective dose K D D w r = H H w t = E mgy mgy msv msv 46

47 other minor details quantities kerma absorbed dose equivalent dose effective dose K D D w r = H H w t = E mgy mgy msv msv units 47

48 other minor details CTDI vol is the air kerma measured inside the phantom kerma absorbed dose equivalent dose effective dose K D D w r = H H w t = E mgy mgy msv msv 48

49 other minor details Incident kerma leads to absorbed dose estimate through the Monte Carlo process kerma absorbed dose equivalent dose effective dose K D D w r = H H w t = E mgy mgy msv msv 49

50 other minor details correction for different types of radiation such as a, b, n: w r for x-rays is 1.0 kerma absorbed dose equivalent dose effective dose K D D w r = H H w t = E mgy mgy msv msv 50

51 other minor details corrects for organ radiosensitivity using tissue weighting factors, w t. kerma absorbed dose equivalent dose effective dose K D D w r = H H w t = E mgy mgy msv msv 51

52 The short-cut calculation of E (abdomen) E = DLP abdomen Effective dose (msv) and dose length product (mgy.cm) for abdomen CT and the clinical indication: rule out abscess. The conversion factor is (1/59.1 =) msv / (mgy.cm) Included, data from 46 scanners, 21 single slice, 25 multislice Effective dose calculated with the ImPACT CT Dosimetry calculator

53 Effective Dose per DLP (AAPM TG-96) DLP k E Region of Body Head and neck Head Neck Chest Abdomen / pelvis trunk k (msv/[mgy-cm])

54 CT Dose Estimation Introduction The CTDI Family of Metrics Dose Length Product Effective Dose Dose Reporting Summary 54

55 WORKFLOW in RADIOLOGY All images are digital (on PACS) Radiologists look at images on computers As they look at the images, they dictate their findings using voice recognition software In California, CTDIvol & DLP need to be included into the Radiologist s Report 55

56 The Structured Dose Report with Radimetrics Feed: Report: Gaga, Lady MRN: EXAM DATE [5/24/ :35 pm] INDICATION: [] DOSE: [Dose information for this CT examination: Series 1: (no contrast) CTDIvol = 12.4 mgy DLP = 496 mgy-cm Diameter = 28.2 cm SSDE = 16.1 mgy Series 2: (with contrast) CTDIvol = 13.2 mgy DLP = 577 mgy-cm Diameter = 28.2 cm SSDE = 17.2 UC Davis Health System CT scanners..low dose CT practice.] text and format setup by institution FINDINGS: [] IMPRESSION: []

57 UC DOSE: Dose Monioring Software Institutional CT dose assessment 57

58 CT Dose Estimation Introduction The CTDI Family of Metrics Dose Length Product Effective Dose Dose Reporting Summary 58

59 Summary CTDI vol : (mgy) Is a measurement of dose to a large plastic phantom, and for normal size patients CTDI vol underestimates their dose. Should not be considered as the patient dose Uses two reference phantoms, 16 cm and 32 cm, depending on scan protocol. The 16 cm reference phantom is used for all heads and for some pediatric bodies (depends on scanner) DLP: (mgy cm) Includes CTDI vol and scan length Scales linearly with effective dose

60 Summary Size Specific Dose Estimate (SSDE) Is an automatic way in which to correct for patient body size, to produce a more accurate dose estimate. Effective Dose (E) Is not a physical dose metric, but rather embodies organ radio-sensitivities Is controversial to use for specific patients 60

61 CT Dose Estimation Introduction The CTDI Family of Metrics Dose Length Product Effective Dose Dose Reporting Summary 61