IAEA RER/9/135 COURSE ON OPTIMIZATION IN COMPUTED TOMOGRAPHY Sofia, Bulgaria, Tube current modulation and dose reduction : How TCM works

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1 IAEA RER/9/135 COURSE ON OPTIMIZATION IN COMPUTED TOMOGRAPHY Sofia, Bulgaria, 2017 Tube current modulation and dose reduction : How TCM works Dean Pekarovič UMC Ljubljana, Institute of Radiology Quality and Safety office Plain Radiography one kv value one mas value What should be on Image? Pulmo, Fat, Air, Bones Do we have same attenuation in all parts of the Image? 1

2 kv increase will Decrease the mas required to achieve a constant detector exposure (AEC on). Decrease ESD and effective dose. Decrease image contrast. Increase scattered radiation. 79 Kv 0,71 mas DAP 0,128 dgy cm 2 EI: 250,9 DI: -0,45 24,98 µgy 67 Kv 0,71mAs DAP 0,085 dgy cm 2 EI 191,5 DI -0,71 34,57 µgy Radiologist to decide contrast ribs/pulmo Ref: ICRP 121, Radiological protection in paediatric diagnostic and interventional radiology mas increase will Increase detector exposure (without AEC) Increase radiation dose to the patient. Improve image quality increased CNR and SNR?. noise / higher mas 75 Kv 1,2 mas DAP 0,224 dgy cm 2 DI 3,23 NO AEC 75 Kv 0,63 mas DAP 0,084 dgy cm 2 DI -0,9 NO AEC 2

3 Lookin for proper ImQ DX 65 kv DAP: 0,021 dgycm 2 0,71 mas DI : 0,35 63 kv DAP: 0,032 dgycm 2 0,9 mas DI : 1,15 58 kv DAP: 0,053 dgycm 2 1 mas DI : 2,26 newborn 3 girl weeks, girl, 2760 g 7 days 2750 g 62 kv DAP: 0,054 dgycm 2 1,21 mas DI : 3,04, + Cu Filter 70 kv DAP: 0,021 dgycm 2 0,56 mas DI : - 0,42, + Cu Filter What we need is how to control the ImQ and Dose kv mas EI DI DAP µgy mgy/s ,53-0,32 0, ,1 256,4 0,12 0,958 71,9 16,6 90 1,2 284,8 0,57 1, ,5 0,32 1, ,8 1246,5-0,03 1, , ,6 217,9-0,6 1, ,6 238,5-0,2 2, ,6 kv mas EI DI DAP ,28 0, ,26 0, ,7-0,07 0,883 How to improve DI 1mAs cca 0,4 DI 1 Kv cca 0,3 DI DI % CsJ Nastavljeni protokoli mas focal Size range kv spot target EI OTROK SMALL 0, ,22 PC 1 KG 68 MEDIUM 70 0, ,22 LARGE 70 0, ,22 range mas spot target EI kv focal SMALL 0, ,22 OTROK PC 2 KG 70 MEDIUM 72 0, ,22 LARGE 72 0, ,22 range mas spot target EI kv focal SMALL 1 226,22 OTROK PC 3 KG 71 0,5 MEDIUM 73 0, ,22 LARGE 73 0, ,22 range mas spot target EI kv focal SMALL 0, ,22 OTROK PC 4 KG 72 MEDIUM 74 0, ,22 LARGE 74 0, ,22 range mas spot target EI kv focal SMALL 0, ,22 OTROK PC 5-10 KG 75 MEDIUM 75 0, ,22 LARGE 75 0, ,22 range mas spot target EI kv focal SMALL 1 226,22 OTROK PC KG 75 0,8 MEDIUM 77 0, ,22 LARGE 77 0, ,22 range spot target EI kv mas focal SMALL 0, ,22 OTROK PC KG 80 MEDIUM 80 0, ,22 LARGE 80 0, ,22 range mas spot target EI kv focal SMALL 0, ,22 OTROK PC KG 80 MEDIUM 85 0, ,22 LARGE 85 0, ,22 range mas spot target EI kv focal SMALL 1 226,22 OTROK PC KG 85 0,9 MEDIUM 90 0, ,22 LARGE 95 1, ,22 3

4 From plain to CT CT -problem 4

5 Tube Current Modulation Purpose is to ensure the same picture quality regardless of the characteristics of the patient. = same CNR in all images Tube Current Determines the number of electrons accelerated across the x-ray tube per unit time Units: milliamperes (ma) CTDI vol is directly proportional to Tube Current CTDI vol Tube Current REF : AAPM Computed Tomography Radiation Dose Education Slides 5

6 TCM along z axis mas changes automatically 140mAs 55mAs 110mAs 130mAs Tube Current Modulation (TCM) / Automatic Exposure Control (AEC) Automatically adapts the Tube Current or Tube Potential according to patient attenuation to achieve a specified image quality Automatic adjustment of Tube Current may not occur when Tube Potential is changed Centering the patient in the gantry is VITAL for most AEC systems AEC aims to deliver a specified image quality across a range of patient sizes. It tends to increase CTDI vol for large patients and decrease it for small patients relative to a reference patient size. The use of Automatic Exposure Control may decrease or increase CTDI vol depending on the patient size and body area imaged and image quality requested REF : AAPM Computed Tomography Radiation Dose Education Slides 6

7 TCM - Principe Reference data is collected from attenuation of anatomical structures during scanned projection radiograph -SPR (one or two) Data collected from previous rotation. TCM -principle TCM could be defined as a CT technique that performs automatic modulation of tube current in the x, y plane (angular modulation), or along the scanning direction, z- axis, (longitudinal modulation), or both (combined modulation). Is really total Automatic? The modification is done according to each patient s size, shape and attenuation of body parts being scanned. The operator must select a required image quality level and then the system can adjust the tube current to obtain the predetermined image quality with improved radiation efficiency. Ref :Kalra MK et all, Computed tomography radiation dose optimization: scanning protocols and clinical applications of automatic exposure control.curr Probl Diagn Radiol 2005; 34:

8 Start info.. TCM needs input WHAT is needed Image quality = radiologist input, but team should be involved; + radiographer + medical physicist Different vendor solutions : Noise = SD CT-numbers on the Image (GE, Toshiba) Reference ma or mas for a standard patient for specified quality (Philips, Siemens) TCM idea AP diameter : 36 cm LAT diameter : 28 cm 8

9 Scan where Tube Current Modulation was used Blue Curve Represents actual instantaneous ma Red Curve Represents avg ma for each image Yellow Curve Represents avg ma over entire scan Overall avg is used for CTDIvol reported in Dose Report Dose Modulation -AEC Many CT scanners automatically adjust the technique parameters (and as a result the CTDI vol ) to achieve a desired level of image quality and/or to reduce dose Dose Modulation and Reduction techniques vary by scanner manufacturer, model and software version. Tip : read the manual and then test the AEC. 9

10 Variations in ma modulation Spatial ma modulation Longitudinal -Z axis modulation Angular / x-y axis / transverse modulation Combined / x-y-z modulation Temporal ma modulation ma adapted at different time points using ECG gating (Cardiac CTA);not included in this PPT TCM Modulations Longitudinal (Z direction ) Is an TCM feature that adjusts the Tube Current as patient size and attenuation changes of the anatomic region in the longitudinal direction. The CT Localizer Radiograph is used to estimate patient attenuation. REF : AAPM Computed Tomography Radiation Dose Education SlideS 10

11 TCM Modulations Angular (X,Y direction) Is an TCM feature that adjusts the Tube Current as the x-ray tube rotates around the patient according to the size, shape, and attenuation of body region being scanned to compensate for attenuation changes with view angle. Angular Tube Current Modulation is used to adjust the Tube Current to attempt to deliver similar dose to the detector at all view angles. REF : AAPM Computed Tomography Radiation Dose Education SlideS 180 -mirror Ref: M.K.Karla, Automatic Exposure Control in Multidetector-row CT TCM Modulations Angular + Longitudinal (x, y, z direction) Is an AEC feature that incorporates the properties of both Angular and Longitudinal Tube Current Modulation to Adjust the Tube Current based on the patient s overall attenuation. Modulate the Tube Current in the angular (X-Y) and longitudinal (Z) dimensions to adapt to the patient s shape. Ref: M.K.Karla, Automatic Exposure Control in Multidetector-row CT REF : AAPM Computed Tomography Radiation Dose Education SlideS 11

12 All in one Ref : Marcus Söderberg; Automatic exposure control in CT Principles are similar, but..? How it works on my CT modality 12

13 Siemens : Care Dose 4D AEC System Operator-Controlled Z-Axis Angular X,Y Vendor Name Parameter Parameter Explanation Principles Technique Modulation Modulation Angular modulation of tube current in the x, Image quality y, and z axes on the Siemens CAREDose4D reference mas mas that would be used for basis of patient size x-y-z/combined Yes Yes an average-sized patient relative to the mas specified by the user for a standard-sized reference patient Implies need for image quality equal to that obtained with the use of specified reference mas in a standard adult (70 kg -80 kg) or child (20 kg). Head Protocols only x,y modulation Quality Reference mas is NOT the max or min. Siemens : Care Dose 4D The degree to which the tube current is adjusted for patient size can be selected, using very weak to very strong in 5 steps (high degree of ma adjustment), compensation settings. Ref : Siemens and Sodberg, Automatic exposure control in CT 13

14 Philips : Dose Right AEC System Operator-Controlled *miss new technique with x, y and z axis modulation Z-Axis Angular X,Y Vendor Name Parameter Parameter Explanation Principles Technique Modulation Modulation Modulation of tube current on the basis of Z-DOM D-DOM Reference image Image quality expressed in patient size to achieve with ACS Phillips Dose Right Ref mas/slice terms of noise level of an the same image noise Yes Yes ACS existing optimal clinical level as in a previously image defined reference image Baseline mas is used as a reference to obtain constant image noise along the z axis. DoseRight ACS (automatic current selector) provides patients based AEC(object size), by use of a reference image DoseRight DOM (Dose Modulation) Z-Dom: Z-axis AEC D-Dom: Tube current is set so that 90% of images will have equal or lower noise than the reference image, with remaining 10% of images in a series having equal or higher noise than the reference image. No ACS for Head CT protocols. Philips : Dose Right* Additional Parameter: Reference Noise Index DoseRight 3D-DOM (three dimensional dose modulation) combines angular and longitudinal patient information to modulate dose in three dimensions (x-y-z-axis). It incorporates modulation of tube current time product (mas) according to changes in individual patient s size and shape in the transverse (x-y-axis; angular) direction during helical scans, in addition to changes in the craniocaudal or caudocranial (z-axis; longitudinal) direction, as the tube rotates. 14

15 GE : AutomA 3D AEC System Operator-Controlled Z-Axis Angular X,Y Vendor Name Parameter Parameter Explanation Principles Technique Modulation Modulation Measure of image quality/ Modulation of tube noise level defined current only in the Auto ma Noise index relative to uniform water longitudinal direction z axis/longitudinal Yes No phantom to maintain a constant NI Measure of image quality/ Modulation of tube GE noise level defined current in the x, y, and Smart ma Noise index relative to uniform water z axes to maintain a x-y axis/angular Yes Yes phantom constant noise index Auto ma 3D* Noise index x-y-z/combined Yes Yes GE : defined with NI (Noise Index) NI : Std.Dev. of HU number in water phantom with standard algorithm used NI CTDI vol Auto ma use last scout if more then one were performed. Smart ma MUST works with Auto ma GE : AutomA 3D Noise Index is Image Quality Parameter which sets the image noise in the image. Scout is used to determine patient attenuation characteristics and size and along with Noise Index the ma per rotation for the acquisition is determined. Noise Index will vary based on the slice thickness selected due to the difference in image noise relative to slice thickness. The same NI should never be used across all slice thicknesses. Minimum and maximum ma - Range of allowed ma to achieve desired noise index 15

16 Toshiba Sure Exposure 3D AEC System Operator-Controlled Z-Axis Angular X,Y Vendor Name Parameter Parameter Explanation Principles Technique Modulation Modulation User prespecifies image quality on the basis of Standard deviation of pixel a patient-equivalent Sure Exposure Target Image quality values in an image water phantom, and x-y-z/combined Yes Yes 3D level Higher SD= higher noise mas is modulated on Always ON the basis of patient Toshiba size to maintain image quality Sure Exposure z axis/longitudinal Sure Exposure 3D is Longitudinal Modulation XY Modulation is Angular Modulation 2 possibilities for radiographer : Standard Deviation SD Image Quality level High, Standard.. SD of pixel values measured in a patientequivalent water phantom. Toshiba Sure Exposure 3D LL (50cm), L (40cm), M (32cm), S (24cm), SS (18cm) AEC setup allows tube current to be limited by max. and min. values. Be careful with reconstructed slice thickness and reconstruction kernel. 16

17 Image Quality Parameters -TCM GE NI (Noise Index) Philips mas/slice Siemens -ref. mas Ref :M.P.Supanich,Tube Current Modulation,3rd CT Dose Summit Scan and Reconstruction parameters Which parameter effect TCM? AEC system Tube Voltage Rotation time Pitch Slice Thickness Reconstruction kernel Localizer** CARE Dose 4D * Yes Yes Yes Dose Right Yes Yes Yes Yes Yes AutomA 3D Yes Yes Yes Yes Yes SureExposure 3d Yes Yes Yes Yes Yes Yes *New Versions of CARE Dose 4D, a change in the tube volatage will result in a change in tube current by the AEC ** add to original Table Ref : 17

18 Localizer -SPR Start and end of scan range. Used for appropriate tube current modulation (AEC in use). Incorrect position in iso center leads to inadequate tube current modulation (higher or lower). Incorrect position lead to incorrect geometry and size of scanned object. 6 cm from Center TH 0 kv 120 mas 190 AEC ON Localizer Both has same exposure parameters kv 120 mas 35 Length 256 mm CTDI vol (32cm) 0,13mGy DLP 3 mgycm Table Height Δ 6cm 1. Spiral mode kv 120 mas/ref mas 204/180 Length 117 mm Slice thickness 5 mm CTDI vol (32cm) 13.6 mgy DLP 145 mgycm 22 % difference in dose from 6 cm table height difference. TH +6 cm kv 120 mas 230 AEC ON Positioned above isocenter (+6 cm) 2. Spiral mode kv 120 mas/ref mas 219/180 Length 117 mm Slice thickness 5 mm CTDI vol (32cm) 16.7 mgy DLP 178 mgycm 18

19 Isocenter noise and CR Cylindrical 16 cm CTDI-head phantom scanned in center position and while lowered by 60 mm. Noise (1SD HU) increases vertically across the phantom in lower position as the beam shape is non-optimally targeted in the scan. isocenter isocenter noise increase slight contrast change Noise (1SD HU) y-coordinate (mm) centered 60 mm below Contrast (HU) y-coordinate (mm) centered 60 mm below Ref : Eurosafe, M. Kortesniemi, D. Pekarovic, D.Sheppard Spr DIRECTION 19

20 Scanning Outside The Localizer Localizer is basic source for TCM If scan region extends outside of localizer we have a problem. Four possible outcomes in scan region not covered by localizer : Tube Current goes to maximum Tube Current goes to minimum Tube Current stays what it was at edge of localizer Tube Current goes to manual setting Ref : TCM, M.Supanich,3rd CT Dose Summit One or two and direction of the localizer 20

21 TCM performance on QA phantom Ref :Iball: A QA test for CT AEC systems, Journal Of Applied Clinical Medical Physics Example how to start 120 kv 146 mas ma ma ma ma ma ma ma ma , ,5 54, ,5 112,5 54, , , , , ,5 54, ,5 58,5 101,5 54, , ,5 54, , ,5 59, ,5 59, ,5 55,5 59, , , , , ,5 83,5 53, ,5 80,5 53, , ,5 53, , , ,5 53, ,5 55,5 60, , , , , , , , ,5 62,5 134,5 54,5 61, ,5 62, ,5 61, ,5 62, , ,5 62,5 120 kv 53 mas 21

22 Example 26 Months ma kv 100 mas 30 Thorax wo CM Topogram Cr/Ca 100kV 35 mas Helical 100 Kv 30 mas AEC off g/rot : 0,5 s CTDIvol: 1.23 mgy DLP : 23 mgy cm Example kv 100 mas ma ma kv 100 mas Months Thorax +Adomen wo CM Topogram Cr/Ca 100kV 100 mas Helical 100 Kv 100 ma AEC off g/rot 0,5 s CTDIvol : 4.50 mgy DLP : 182 mgy cm (+Abd) 22

23 Example ma ma 30 % mas deviation in Z plane kv 100 mas Months Thorax wo CM Topogram Cr/Ca 100kV Helical 100 Kv 30 ref/ 27 mas AEC on g/rot 0,5 s CTDIvol 4.50 mgy DLP 182 mgy cm Example kv 100 mas Months Thorax wo CM Topogram Cr/Ca 120kV Helical 100 Kv 20 ref/ 15 mas AEC on g/rot 0,5 s CTDIvol : 0.80 mgy DLP : 14 mgy cm 23

24 When should it be avoided? A great tool, but sometimes.. Head exams? Brain Orbits Sinuses CT Perfusion Little or no table motion No chance to change cross section shape low manual technique Team Therefore, it is paramount that radiologists know how to use the AEC systems in their own scanners. AEC systems do not reduce radiation dose per se; rather, they control radiation exposure relative to the required image quality. Only Radiologist? 24

25 EMAN - WP 8 :Training & Education 1.The medical practitioners requesting a CT examination. This group requires knowledge about indications for CT, its alternatives and the associated risks and benefits. 2. The core CT team that defines and optimizes the set of standard scan protocols on a specific scanner (radiographer, medical physicist and radiologist).this team will usually start with a standard set of protocols provided by the manufacturer and adapt it to the local needs. This team requires in-depth knowledge of scan parameters and how to optimize them. 3. The professionals (radiologists, radiographers) define the CT protocols. This group has to have knowledge when not to use CT, but another image technique, according to patient clinical indication. They are ultimately responsible for the individual choice of the correct protocol associated with each of the set of available standard protocols at a specific scanner / institution. 4. The radiographers that actually perform the examination. This group requires knowledge about individual routine adaptations required for each patient, such as centring of patients, adapting scan range, adapting protocol to patient size, optimizing modality performance in order to obtain the best diagnostic image at the lowest possible dose. How to evaluate Understanding CTDIvol Dose Reference Levels ( will be disuseed..) 25

26 CT Dose Numbers CTDI vol DLP Effective Dose Why CTDI vol? CTDI vol provides information about the amount of radiation used to perform the study. CTDI vol is a useful index to track across patients and protocols for quality assurance purposes. CTDI vol can be used as a metric to compare protocols across different practices and scanners when related variables, such as resultant image quality, are also taken in account. 26

27 DLP The Dose Length Product (DLP) is also calculated by the scanner. DLP is the product of the length of the irradiated scan volume and the average CTDI vol over that distance. DLP has units of mgy*cm. CTDI Absorbed dose in the slice. Absorbed dose including scatter contributions from outside the slice (CTDI). The CTDI is calculated as the integral of the absorbed dose along the z axis, divided by the nominal slice thickness S. Ref : Easy Guide to low dose,siemens 27

28 CTDIw There are different ways to calculate the CTDI. One of them is to consider the differences between the absorbed dose in the periphery and in the centre of the patient s body by a weighted sum of the central and peripheral CTDI values. Ref : Easy Guide to low dose,siemens CTDIvol -DLP The Dose Length Product (DLP) is the product of CTDIvol and the examination range. The Dose Length Product (DLP) is also calculated by the scanner. DLP is the product of the length of the irradiated scan volume and the average CTDI vol over that distance DLP has units of mgy*cm Ref : Easy Guide to low dose,siemens 28

29 Effective dose Region of body E = E DLP DLP Effective dose E - measured in Sievert (Sv) units, includes the sensitivity to radiation of the different organs. It is the sum of the equivalent doses in all irradiated organs multiplied by the respective tissue weighting factors wi. Normalised effective dose, E DLP (msv mgy -1 cm -1 ) Head Neck Chest Abdomen Pelvis Normalised values of effective dose per dose-length product (DLP) over various body regions. ICRP 60 ICRP 103 The Recommendations of the International Commission on Radiological Protection of 2007 (ICRP 103) has different coefficients than that of 1990 (ICRP 60). In particular, gonads are less radiosensitive and the breast is more radiosensitive than previously assumed. The effective dose E is an approximate measure that was introduced to compare the stochastic risk of a nonuniform exposure of ionizing radiation with the risk caused by a uniform exposure of the whole body. 29

30 Conclusions Users must possess a good understanding of the concepts of noise index, standard deviation, reference images, and reference ma(mas) as they relate to each AEC system. Use User manual and ask vendor for additional papers. Evaluate your own AEC system, for each software and scanner type. Don`t accept one solution. Understanding will improve reports and CT Dose will become controllable. Thank you 30