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1. 108 nen 1 16 T Piranha J CB2 06070130 mbes U CBCT Sahlgrenska 20090311 matningar GE CTDAGE 64 slice_one Body 40 mmml 10 48 2009 05 27 Figure 1 The dose profile from a scan in the central position in a body phantom in a 64 Slice CT 3 Measurements on wide beam CT systems with no table movement Revision C April 2009 Whether or not the parameter CTDI can be used as a predictor of the clinical patient dose has been under debate for some time To measure with the standard pencil chamber may be complicated and full of pitfalls that the standard user may not be aware of especially when measuring on CT systems that were not available when the chamber nor CTDI was invented Problem 1 The beam width For example in the Elekta Synergy the smallest possible beam width is 10 cm When a standard CTDI measurement is per formed with a pencil ion chamber the width of the beam is the same as the active length of the chamber The tails of the scattered radiation introduced in a phantom or a body at this beam width is much wider and the ion chamber will under estimate the patient dosel Problem 2 Phantom length In order to use the parameter CTDI in the calculation for the total dose given to the patient all the dose during one axial rota tion should all be contained within the length of the chamber Our measurements show that at least two phantoms were need ed to cover the whole width of the dose contribution in the head ph2. 10 and 11 since the level of the output is very varying compared to the measurement in the cen tral position We managed to capture the highest part of the pro file Here we have a peak dose rate of 22 mGy s and the average dose rate is still 0 38 mGy s The peak dose rate is almost 60 times higher than the average dose rate in a peripheral phantom position 9 Measurements on wide beam CT systems with no table movement Revision C April 2009 Figure 9 The pulsed output can be seen if the data is collected dur ing one second Figure 10 The dose distribution during one axial scan in a periph eral hole The output from this CT is pulsed as can be seen in the dose profile t Figure 11 The dose distribution during one axial scan in a periph eral hole The output during one second 4 Conclusions There is no easy solution to measuring the patient dose on wide beam CT systems yet The most convenient way is to measure the point doses at different positions and thereby create a mean value of the dose given to the patient Remember to use enough phantom length It is easy to be tricked by the manufacturers CTDI data When doing a standard CTDI measurement with a pen cil chamber the result may be very similar to the result given by the manufacturer Remember that they also might have been doing the same type of measurement so what you are comparing is not the total dose given to the patient The mean value of the
3. CT Dose Profile Analyzer software Here the profile is represented by 640 sam ples The collimation was in this case six mil limeters Figure 3 The profile measured with the CT Dose Profile Analyzer software Here the profile is rep resented with 640 data points Read more about the new feature Geometric efficiency in the manual v4 0B 50 100 50 100 Z Axis Position mm Z Axis Position mm Figure 4 Radiation profiles measured with the OSL dosimeter and the CT SD16 The probes placed in air at the isocenter of the CT scanner a The probes are placed at the center of a CTDI head phantom b Figure 5 The CT probe in a 30 cm long head phantom which was placed outside the table A helical spiral scan with the same parameters was per formed with each detector The dose peak values were normalized to 1 for an easy comparison of the profiles see figure 6 The area under the dose profile acquired with the CT probe was 1 smaller than the one measured with the LIC and the dose profiles coincide very well 3 Comparison of CTDI When it comes to comparing the CTDI or CTDI given by the CT modality the consol to the corresponding value given by the CT probe or an ion chamber there is one thing to bear in mind It is expected that these values do not coincide completely In the IEC standard 60601 2 44 2002 it is written that The displayed CTDI given by the manufacturer represents a model
4. Detector Measurement Verification Revision D May 2009 This report gives an overview of the documentation available concerning the correc tness of the data measured with the Solid State CT Detector from RTI Electronics Tanto 011 9 ae eee nee E en ne ne E ee ee ee 2 Comparison of dose profiles aE nee on nce en ee rrene rent ALATE CEP 6 25 TE 22 ne e SOD y Oale a N N E N ca pnet set ceceattocase De Dy The T BOGUE TTI Or CTD e E E CTE ee eae Ten 3 1 TheCT probe vs CT lon chamber sesiis roisia naaa aaa aA aie 3 2 Measurement COMPALISON 00 0 cccccscsscsssssesessssssseesessssscscssssvcssssssssssssssussvesesussucssassassscsusaucssesssussseseaussesssansacesceueaeessessaeeseesen 4 Comparison of axial and spiral helical scans 4 1 Theoretical derivation ardita sect atao ces a tee iene ere 42 Compare Measurements enropar acne anne aoa eels pater aa ASA Eia eei 5 Energy Dependenre abast 11 tkk k attnr tkt asat inet EA NA AP EENAA SE AE EANES AE EAEAN A Sr rereana rere rea 6 Rotational symmetry e sb stat titt ktk tb a strt 1 kere ANANASSA A A EEEE EAAAE SS S AE EEEE rnrn rnt FE T HIE Td Depende CE nanenane O EAO Meo e l c AE E EE AEE E E EE AEE A A E E E A AEE AEE SREE E E E E E E OA E A EA E EEA N EEN A 1 Solid State CT Detector Measurement Verification Revision D May 2009 1 Introduction Since the technique of using a solid state detector for measurements of the dose from CT scanners is brand new verificatio
5. ESI ONS stt ktkt b s111 tek b tta 11k bt etn casteontmerenmuenneastestsoteerdenmtnastrapeaeeremce 10 PREEN SS aA E E E EEEE A AE AEA EA AOA 10 In this paper we will refer to the first version of our CT Slice Detector as CT SD16 and second version of the detector as CT Dose Profiler released in 2009 1 Measurements on wide beam CT systems with no table movement Revision C April 2009 1 Introduction CT systems with very wide beams larger than 10 cm and no table movement have been developed from two different direc tions From one direction we have the very fast 320 Multi Slice CT from Toshiba which has a tube rotation time of 3 turns per second When the slice width is more than 64 slices the table does not move anymore From the other direction we have the medical linear accelerator therapy units which are combined with a Cone beam CT unit In these systems imaging data is typically acquired in a 200 or 360 degree rotation and a full rotation takes up to two min utes The beam widths are about 15 cm wide and the table position is fixed Unlike conventional CT scanners the X ray beam is pulsed How is the CTDI supposed to be measured on these types of CT units where the minimum collimation is as wide as the length of the CT ion chamber The CT Dose Profiler and the CT SD16 are options but how can they be used when there is no table movement And what are the values given by the manufacturer that we compare our results to And the most im
6. University Hos pital Also the CTDI was measured at a GE Lightspeed Ultra using the CT Dose Profiler and a pencil ion chamber These values were compared to the value given by the CT consol The measurements were performed in a head phantom using the protocol head routine The set kV was 120 kVp the mAs was 200 and the tube rotation time was 0 5 seconds FOV head GE Lightspeed Ultra Table 2 A comparison of ee CTDI mGy measured 20 mm 21 43 19 14 with the CT Dose Profiler in a head phantom The 1O mm 27 40 25 01 measurements were per 125mm 5 9 29 02 27 07 versity Hospital The same comparison was done using a body phantom using the protocol abdomen standard The set kV was 120 kVp the mAs was 200 and the tube rotation time was 0 5 seconds FOV body GE Lightspeed Ultra Table 3 A comparison of EE T TL mGy measured 50min 10 75 with the CT Dose Profiler and a pencil ion chamber sem ke ina body phantom The 10 mm 14 10 12 72 measurements were per aaa aa foamed ala versity Hospital 1 25 mm 14 53 3 9 Solid State CT Detector Measurement Verification 4 Revision D May 2009 4 Comparison of axial and spiral helical scans Traditionally the CTDI sible to also measure the CTDI during a spiral helical scan as is required when using the CT probe ioo HAS been measured during an axial scan with a 100 mm pencil ion chamber It is however also pos 4 1 Theoretical derivation Based on the as
7. central phantom position body during one axial on a GE Lightspeed 64 Slice CT The table attenuation is the explanation of the dip The integrated dose is 2 76 mGy and the maximum dose rate is 2 97 mGy s In the central phantom position the peak dose rate is approximately the same as the integrated dose This is because the amount of filtration is similar from all directions dur ing the rotation Figure 4 The dose distribution during one axial scan in the central hole Measurements on wide beam CT systems with no table movement 6 Revision C April 2009 ca geen 2 05 533 LI C2 notused oo lila 9751 drl s 19 Nee Swe Wd TS Figure 5 The dose distribution during one axial scan in a peripheral hole The same exposure but in a peripheral phantom position shows us a very different result see figure 5 Here the aver age dose is 9 75 mGy and the peak dose rate is 53 3 mGy s that is 18 times higher than in the central hole During the rotation when the tube is in the position closest to the detector the beam is hardly filtrated at all giving this high peak dose rate By measuring the point doses at different positions along the z axis keeping the x and y positions the same you can get an average dose in this plane However since an ion chamber integrates the total dose it detects the sum will look the same in a peripheral hole as in the central hole since it gives the mean value of the whole rotation The data from the exp
8. 00 millimeter are integrated to give CTDI However when then green c1 and blue c2 cursor are placed at the positions where the profile is reduced to zero we see that the value between these cursors CTDI c2 c1 is 14 8 mGy compared to the value of CTDI which is 11 1 mGy The ratio between these CTDI values is given as the parameter Scatter ratio c1 c2 100 In this measurement with 40 millimeter wide beam the scat ter tails introduce 33 percent more dose than a pencil chamber can measure Further description of the software and how to interpret the dose profile is found in chapter 3 M CT Dose Profile Analyzer File Misc Measure Analyze Report Help System info Instructions Institution BIDM Phantom 1 Body 15 cm Scan length mm 300 Ready for exposure Department Pitch U 0 584 Scan time 5 8 8 40 CT Unit GE LightSpeed VCT 64 Tube rotation time s Tube current mA 200 Note Helical Scan Bowtie L i a Scan speed mm s 39 36 CT Dose Profiler DCI 08010056 l harmanda CTENAR OTANI Iv CRIN eee Measurement type Central point Method k 17 Body H Max Rec Time s 10 24 CTOI measurement Time s Settings 1 a ioo 125 150 175 20 25 250 25 300 3235 350 375 403 OO Zeatts Gon nes Dose mGy 15 01 CTD ma P iss Length mm T eN c2 C1 i Poi RE DLP mGycm 580 CTDivol eo 19 3 RT CTDl100 mGy CTDIw mGy 19 0 Scatter index CT Dose Profile Analyzer program FWHM mm
9. C eb e k h eh ON FOO O N BO O 7 We Ra SIT2H0T T SD 16 pee P i om Se er es re re ee ee cee ee Central Point Method k 1 02 Time s 0 1 2 5 8 9 10 d 0 20 40 60 80 100 120 140 160 180 198 5 Z axis mm Dose mGy 42 88 C2 C1 DLP mGycm CTDlvoi mGy CTDho mGy BEZA CTD mGy FWHM mm Scatter index B 130 100 k CTDI CTDI 100 c C1 Dare puojmer Common User Question Revision D May 2009 A summary of the most frequently asked questions about our CT Probes Solid State CT Detector Measurement Verification Revision D May 2009 This report gives an overview of the documentation available concerning the correctness of the data meas ured with the Solid State CT Detector from RTI Electronics Measurements on wide beam CT systems with no table movement Revision C April 2009 This report is a case study of measurements on different types of CT systems with wide beams and no table movement It explains the difficulties that the users are up against when using traditional measuring devices and procedures and what is required to do a correct measurement of the patient dose The Energy Dependence of the CT Dose Profiler Revision C April 2009 This report contains information of the energy dependence of the CT Dose Profiler from RTI Electronics 1 Would you please compare CT Dose Profiler and DCT10 by feature and benefit The largest differ
10. QR 10 150 6 63 E 120 8 4 Table 1 The kV and HVL of the beam qualities investigated in this paper RQR is the family of beam qualities that is used in conventional radiography without any added filtration RQT is a quality that simulates the filtration added when a meas urement is done inside a CT phantom It gives a corresponding HVL value We chose the test the energy dependence only at RQT 9 The energy dependence is displayed in terms of energy correction factors that is if this the factor is multiplied with the measured dose if the detector the detector will have no energy dependence 1 The Energy Dependence of the CT Dose Profiler Revision C April 2009 1 2 The energy correction of the CT Dose Profiler The CT Dose Profiler was compared to a reference ion chamber at PTB for the determi nation of its energy correction The result can be seen in figure 1 and table 2 The energy dependence is very small especially compared to other uncertainties involved in CT measurements The energy correction factors have been added to the software and they will be automatically applied to the measured result Figure 1 The energy correction of the CT Dose Profiler compared to the ref erence chamber at PTB Energy Correction of the CT Dose Profiler 1 100 1 050 RQR E ROT 1 000 0 950 0 900 3 4 5 6 T 8 9 10 11 RQR RQT Co Contac f RTI Electronics AB Phone 46 31 746 36 00 R
11. TI Electronics Inc Fl jelbergsgatan 8 C Fax 46 31 27 05 73 1275 Bloomfield Avenue S l Building 5 Unit 29A SE 431 37 M lndal E mail sales rti se Fairfield NJ 07004 SWEDEN www rti se USA CT Dose Beam Quality Profiler RQR 4 1 031 RQR 6 0 966 RQR 8 0 965 RQR 9 1 0 RQR 10 1 082 ROT 9 0 998 Table 2 The energy correction of the CT Dose Profiler compared to the reference chamber at PTB N 0 op R a XA u vat YY RTI Phone 800 222 7537 Phone 1 973 439 0242 Fax 1 973 439 0248 E mail sales rtielectronics com www rtielectronics com Scandinavian Quality RTI Electronics was founded in 1981 when several curious and enterprising stu dents met at Chalmers University of Technology in Gothenburg Sweden They saw their vision grow into the beginning of RTI products today world leading in X ray QA and Service instrumentation There are many reasons why RTI Electronics has become a market leader Besides fulfilling the highest user demands products from RTI Electronics are known for cutting edge innovation Other reasons include our engagement our expertise accumulated over more than a quarter of a century and our commit ment to doing it right We are convinced that You will be satisfied with Your choice of product and we would like to continue to grow together with You Copyright 2009 RTI Electronics AB CT Dose Profiler SUMMARY 200905 World Headquarters US Offic
12. XQ over the distance 100mm is multiplied with the calibration factor c and the pitch p Measurements in the central hole in a phantom fulfils the assumption that the dose rate is independent of the pitch 5 Solid State CT Detector Measurement Verification Revision D May 2009 4 2 Comparing measurements 25 Dose profiles from helical scans and axial sequential scans measured with the CT probe were compared see figure 7 The area under the dose profile differed with 1 02 between the two methods Helical scan ee omen Figure 7 Dose profiles acquired with the CT probe during a helical scan and during axial sequential scans Signal Haa a kad aa TO d 5 Energy Dependence The energy dependence of the CT Dose Profiler is corrected for automatically in the software To read more see further in this documentation x LU 300 200 100 100 200 300 z axis mm 6 Rotational symmetry The detector chip inside the CT probe is rotational symmetric This is very important since we want to exclude that variations in the reading of the beam intensity during the rotation is related to the probe itself Figure 8 shows the result of a typical test of a CT Dose Profiler made at RTI Electronics The deviation between min and max dose rate is in this case 2 1 per cent However since we have a falling load during the expo sure the actual deviation dur ing the rotation is even smaller Figure 8 Th
13. and not the value measured on a particular CT scanner 3 1 The CT probe vs CT ion chamber There are two things to be aware of when the CTDI measured with the CT probe is compared to the value measured with a CT ion chamber For those who are using our first version of the probe the CT SD16 the correction of the angular dependence must be activated in the software Otherwise the measured dose profile will be too low and as a consequence also the given CTDI value The second thing is that some CT ion chambers have an energy dependence which is not flat over the kV range when measur ing in a CTDI phantom The energy dependence of the CT probe is automatically corrected for in the software 3 2 Measurement comparison A couple of examples of real life measurements can be seen in table 1 2 and 3 below This data was collected by the physicists at Link ping University Hospital The CTDI was measured with the CT SD16 and the CT Dose Profiler and compared to the value given by the CT consol The CT scanner used was a Siemens Somatom Sensation Open The measurements were performed in a body phantom using the protocol abdomen routine The set kV was 120 kVp the mAs was 200 and the tube rotation time was 1 second FOV body Table 1 A comparison of Siemens Somatom Sensation Open CTDI _ mGy measured vol C ee with the CT SD16 the CT 18 22 Dose Profiler and the CT scanner The measure a aa E 28 8 mm 6 5 17 94 Link ping
14. antom For the body phantom at least three were required According to Dixon the required lengths are 180 and 300 mm respectively The same measurement as in figure 1 was performed with three body phantoms in a row The dose contribution from the scat ter tails have increased to 71 compared to CTD R CT Dose Profile Analyzer ay C L File Misc Measure Analyze Report Help System info Instructions Institution BIDMC Phantom 3_Body 45 cm Scan length mm 600 Ready for exposure cid AE E woe Scantime s J 14 5 Room CT room Collimation NT mm 40 Tube voltage kVp 12 CT Unit GE LightSpeed VCT 64 Tube rotation time 9 L Tube current mA 200 Note 7 Helical 3 phantom Bowtie L 2 Scan speed mm s 39 36 CT Dose Profiler JOCI 08010056 151 1 R Tester Measurement type ke 17 Body 71H Max Rec Time s 20 48 CTDI measurement Time s 6 Settings 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 8061 Z axis mm mR 9 el CTDI 2 TUIS 20 80 CIDE ma zos Length mm T 7 Piti T DLP mGycm 1230 RTI CTDho mGy 118 CTDiw mGy 201 CT a PICS AA E o FWHM mm 93 3 Seder pes 117 Piranha J ems Lies 5 U CBCT Sahlgrenska 20090311 matningar GE CTDIGE 64 slice_thre Body 40 mm xml 10 47 2009 05 27 Figure 2 The dose profile from a scan in the central position in a composition of three body phantoms in a 64 Slice CT Measurements on wide beam CT systems with no
15. chamber Figure 11 and 12 show the angular de pendence of the CT Dose profiler and a CT ion chamber from Victoreen Since no software correction needs to be applied when using the CT Dose Profiler it makes it more suitable for measuring point doses Normalized dose lenaht Figure 12 The angular dependence of a standard 100 mm pencilion 30 20 10 0 10 20 0 chamber from Victoreen 7 Solid State CT Detector Measurement Verification Revision D May 2009 Myf RTI 8 Conclusions e The conclusion from this report is that the solid state CT probe from RTI Electronics can be used as a substi tute for the standard 100 mm pencil ion chamber e The dose profile measured by the CT probe is very similar to the profile measured with TLDs OSL and a liq uid ion chamber Since all data given by the software is derived from the dose profile we can thereby conclude that they are derived from a correct measurement e The CTDI measure with the CT probe is comparable with CTDI measured with a pencil ion chamber e The procedure to measure in a helical scan instead of an axial scan gives the same CTDI value e The CT probe is rotational symmetric and independent of the beam energy and angulation since this is taken care of by the software Read more about the corrections applied to the measurements in chapter 12 in the CT Dose Profile Analyzer Manual 9 References 1 K Perisinakis The effect of x ray beam quality and geomet
16. e RTI Electronics AB Phone 46 31 746 36 00 RTI Electronics Inc Phone 800 222 7537 n 1275 Bloomfield Avenue Phone 1 973 439 0242 Fl jelb tan 8 C Fax 46 31 27 05 73 R cies al Building 5 Unit 29A Fax 1 973 439 0248 SE 431 37 M lndal E mail sales rti se Fairfield NJ 07004 E mail sales rtielectronics com SWEDEN www rti se USA www rtielectronics com
17. e maximum deviation from max to min signal during a rotation must be within 5 In this example it is 2 1 Solid State CT Detector Measurement Verification 6 Revision D May 2009 7 Angular Dependence The angular dependence refers to the change in signal when the probe is not aligned with the ta ble see figure 9 Figure 9 Testing the angular dependence In our first version of the CT probe the CT SD16 the software contains a correction for the angular dependence of the detector chip Without this correction the signal would change in the way as can be seen in figure 10 When using the CT SD16 for measuring CTDI this correction is automatically applied to the profile However if point doses are measured during an axial scan the correction is not applied CT SDD2 7 mm Al 5x5 cm2 field 117 kV Angular dependence Free in air c T Normalized sianal Normalized to 0 degrees Degrees Degrees Figure 10 The angular dependence of the CT SD16 with Figure 11 The angular dependence of the CT Dose Profiler out the correction applied The left part of the graph rep resents the side of the connector side of the chip and the right side represents the front part of the chip lon chamber 2 5 mm Al 20x20 cm2 field 117 kV The detector chip inside the second version of the probe the CT Dose Pro filer is more or less independent of the angle of radiation and in that sense more similar to an ion
18. electing the correct beam quality or the closest available beam quality the energy correction for the detector is ap plied For the CT Dose Profiler the value is within 5 for the range of 60 to 150 kV for the RQR listed For the CT SD16 it could be more than 10 6 What is the best way to measure when the CT scan ner isn t present in the database Select a CT scanner similar to the one that you are going to perform the measurements on and change the name 7 Why does the beam quality and k factor only change when you double click on the database settings Shouldn t those settings change when you select a different CT unit or Phantom in the options below the database Right now L can double click something in the database but if I change the phantom from body to head it won t affect the beam qual ity or k factor unless I use the database settings again That s correct The database is like a template and you can therefore use it for much more combinations that is in the database itself Once you double click on a template from the database you can change the name and other things but not the used beam quality or the k factor since they are used to calculate the correct dose value If the measurement is stored the program uses the CT database index next time you read this file and use the same correction again a even if you have selected your own name of the CT system Common User Question 1 Revision A May 2009 Solid State CT
19. ence between these two types of probes is that the CT Dose Profiler is a solid state detector whereas the DCT 10 is an standard pencil ion chamber and they have there fore totally different ways of measuring the radiation that hits them The DCT10 does not measure dose it measures dose time length during an axial scan and can therefore not be used to measure dose or dose rate to a point Normally the chamber is only partly irradiated and can only measure beams that are maximum 100 mm wide which is also the length of the chamber It is less sensitive than the solid state detector and it can be harder to get stable values especially when using a body phantom The CTDI standard has been based on the use of CT ion chambers and axial scans but it have been shown that measurements during a helical scan using a point detec tor also gives the CTDI value required The CT Dose Profiler measures dose or dose rate to a point and when you move it using the table movement it collects the dose profile Therefore it has no limitation regarding the width of the beam The software also calculates other useful data from the dose profile We believe that the CTDI approach itself will disappear since new CT scanners have such wide beams that the 100 mm CT ion chamber cannot collect the complete dose contribution 2 What are the different procedures of CT dose measu rement between CT Dose Profiler and DCT10 With the CT Dose Profiler and the CT SD16 yo
20. l beam width Dose mGy Dose mGy Solid State CT Detector Measurement Verification 2 Revision D May 2009 2 2 The CT probe vs OSL A way to acquire the dose profile within a phantom is by using an OSL dosimeter This probe consists of an OSL coated film sub strate placed within a light tight plastic cylinder This cylinder can be inserted into a CTDI phantom A single exposure is made with the patient table in stationary position After exposure the probe is returned to the manufacturer for reading with a precision laser The result is a digital radiation dose profile that can be integrated to determine CTDI 2 There is an excellent agreement between the dose profiles generated by the OSL dosimeter and the CT SD16 detector 2 3 The CT probe vs Lic As part of a master thesis the CT probe was compared to a liquid ionization chamber LIC This chamber is able to measure dose profiles and has the same detector area length 0 3 mm as the CT probe The LIC and the CT probe were placed in a specially built 30 cm long PMMA phantom with a diameter of 160 mm The phantom was positioned free in air see figure 5 Figure 6 Dose profiles from the LIC and the CT probe normalized to a peak value of 1 Peak value normalized to 1 300 200 100 0 100 200 300 z axis mm 3 Solid State CT Detector Measurement Verification Revision D May 2009 Figure 3 is a corresponding graph acquired with the
21. l 2009 2 2 To measure the dose to a point Absorbed dose is a measure of the energy deposited to a unit mass of absorbing material In this paper we will refer to the ab sorbed dose as the point dose According to the definition of CTDI it describes the summation of all those dose contributions along a line which is parallel to the axis of rotation of the scanner z axis If we go back to the basics maybe we can use the point dose instead to estimate the patient dose Problem 4 No peak dose data When doing a measurement of the dose to small volume organs with the pencil chamber you perform a single axial rotation with the chamber centred z 0 inside a phantom By moving the chamber to the different positions in the phantom you ac quire measured data from both the peripheral and central holes This leads to the incorrect conclusion that the dose from this single axial rotation is the same regardless of which hole it is measured in The peak dose rate during a rotation is very dependent on where in the phantom the detector is placed With the pencil cham ber an integral over the total rotation is measured and it cannot differentiate between a narrow profile with high peak dose or a broad profile with low peak dose if they have the same area The peak dose rate can be very important to know since it is the dose rate that may for example hit the eye of a patient during a CT scan Figure 4 shows how the dose rate is distributed in the
22. ler is placed in the centre of the phantom The dip in the right part of the profile is because of the table attenuation If the collection time is reduced to 1 s the pulses can be reso luted see figure 9 The measurement was performed when the tube was above the table The average dose rates in figure 8 and 9 this value is found in the display called Average mGy s range C1 to C2 between the two pictures coincide 0 34 and 0 36 mGy s if we set the cursors as in figure 8 and calculate the mean dose value on the same part of the rotation In figure 8 the maximum dose rate value is approximately 0 5 mGy s and that is not corresponding to the maximum value of almost 2 75 mGy s in figure 9 The explanation is that because a limited amount of samples the memory depth of the electrom eter the graph can not resolute each individual peak when the exposure time is as long as 60 s and the result is a mean value However when we have a shorter time each pulse is displayed correctly The peak dose rate in the central phantom position is found to be 2 75 mGy s more than 7 times higher than the average dose rate Figure 10 below shows the same type of measurement as in fig 7 performed in a peripheral phantom position Again the av erage dose rate is fairly constant at 0 38 mGy s To see the true peak dose rate we collect data during a short period see figure 11 It is not possible to compare the average dose rate values between figure
23. mu lated an ion chamber of appropriate length The corresponding measurement on the Varian Clinac IX Cone beam CT can be seen in figure 7 The profiles are a bit similar but for this Cone beam CT the peaks are not the turns of the CT tube they are a result of the pulsed output from this machine In fact almost the whole profile is acquired during one turn tube rotation time 60s 3 2 To measure dose to a point The point dose can very easily be measured with the CT Dose Profiler The diode is rotational symmetric in the x y plane and it has negligible angular dependence in the z direction which is important in order to measure dose to a point We recommend to use the CT Dose Profiler for this type of meas urements since the CT SD16 has an angular dependence The software will give the dose contribution on a graph on a time scale The peak value or the summarized contribution from one turn can be read out If the X ray generator has puls es they are seen and the pulse frequency and pulse length can be checked In figure 8 to the right we have the scan from one axial rota A T S ndl HI CVE T ia P O and a Nd __ 1912 me art cl pi Td yX pai ase i ay 19 48 Figure 8 The dose distribution from the Varian Clinac IX during one axial scan in the central hole Measurements on wide beam CT systems with no table movement 8 Revision C April 2009 ueg Allil tion in the Varian Clinac IX The CT Dose Profi
24. n of the method is needed This report summarizes the results from comparisons with well known techniques The Solid State CT Detector CT SD16 or CT Dose Profiler from RTI can be used as a substitute for the traditional pencil ionization chambers Fur ther on in the report it will be referred to as the CT probe 2 Comparison of dose profiles The values measured with the CT probe are all derived from the measured dose profile If we know that we get a correct dose profile we can also trust the values that are derived from it 2 1 The CT probe vs TLD K Perisinakis has compared dose profiles measured with a CT probe and a TLD array The pictures below are from his work Although the graphs in figure 1 contain very few data points the profiles seem to coincide very well Figure 1 Free in air dose profiles determined us ing TLD array solid line and the CT SD16 detector dotted line The profiles were obtained using 120 kVp small focus and a head mode of operation The twin vertical lines indicate the nominal beam width Figure 2 shows what typical dose profiles acquired free in air look like These profiles also contain too few data points to show the profiles in detail The numbers next to each profile represents the collimation number of slices x width of each slice Figure 2 Free in air dose profiles determined using TLD array at 120 kV and all available beam collima tions The twin vertical lines indicate the nomina
25. osure cannot be investigated at a certain point See further how this can be done in chapter 3 2 7 Measurements on wide beam CT systems with no table movement Revision C April 2009 3 Measurements with the CT Dose Profiler 3 1 To measure CTDI To be able to measure the CTDI with the CT Dose Profiler or CT SD16 a table movement is required During the scan through the beam the thin detector 250um collects the total dose profile and the CT Dose Profile Analyzer software gives the dose profile From this profile all parameters are calcu lated such as CTDI anywhere in the profile allowing the CTDI being calculated but it is also possible to set the cursors at any length even to include the complete scatter tails also See figure 1 for the required dose profile In our study since the X ray units don t have any table move ment we created a movement of the probe by connecting it to a motor The motor pulled the probe through the phantom with a constant speed We want to stress that this motor is not available as a product it was only used during this investiga tion Figure 6 shows this type of measurement performed on a Toshiba Aquilion 320 using a total phantom length of 820 mm and a collimation of 160 mm Each dip you see in the dose profile is when the tube goes underneath the table in other words one turn Dickson has done the same type of measurement in his study using a small ion chamber 0 6 cc Farmer and si
26. portant question how can we get a good estimation of the dose given to the patient The measurements referred to in paper have been performed on a Varian Clinac IX at the Sahlgrenska University Hospital in Goteborg Sweden on a Elekta Synergy X ray Volumetric Imager XIV Cone Beam System at Rikshospitalet in Oslo Norway and on a Toshiba Aquilion 320 at Toshiba Japan Measurements on wide beam CT systems with no table movement 2 Revision C April 2009 UL IIH 2 Measurements with a standard CT pencil ion chamber 2 1 To measure CTDI CTDI was introduced in 1995 as a practical indicator of the patient dose The wide spread use of the 100 mm pencil cham ber seems to have been an temporary solution that became the standard This chamber measures the accumulated dose in a phantom in a series of consecutive scans covering a specific scan length of L 100 mm So what is the problem with CTDI It underestimates the dose for typical clinical body scan length of 250 500 millimeter Already at a beam width of 20 millimeter when measuring in a phantom the complete scatter tails become too long to be measured by the pencil chamber The picture below shows a dose profile acquired from a 64 Slice CT with the CT Dose Profiler and the CT Dose Profile Ana lyser software The collimation the beam width is 40 millimeter and the detector is placed in a CTDI body phantom The measured dose rate values within the red dotted cursors separated by 1
27. ry on radiation utilization efficiency in multide tector CT imaging Med Phys 36 4 April 2009 2 J A Bauhs T J Vrietze A N Primak M R Brusdewitz C H McCollough CT Dosimetry Comparison of Measurement Techniques and Devices RadioGraphics 2008 28 245 253 3 B Cederquist Evaluation of two thin CT profile detectors and a new way to perform QA in a CTDI head phantom Master Thesis in Radiation Physics at Goteborg University 4 CT Dose Profile Analyzer Manual version 4 0B Contact World Headquarters US Office Hh RTI UERN emer Measurements on wide beam CT systems with no table movement Revision C April 2009 This report is a case study of measurements on different types of CT systems with wide beams and no table movement It explains the difficulties that the users are up against when using traditional measuring devices and procedures and what is required to do a correct measurement of the patient dose L Trou TO eeen A A EN E AIAN E AAA EIEE NANE ENE EEA EAEE OE ONR 2 2 Measurements with a standard CT pencil ion chamber 3 RR IR Ome WELTON E AAAA 3 2 2 To measure the dose t a pOInT asss t 1111 ktkt kb ab sts 111r rke EAEE aS rnrn kerensa r erreren 6 3 Measurements with the CT Dose Proniler ccccscessessessessessecsscssessessessessessssusssessessesstssssseesessstssesetsseeseesees 8 BM TO VS re C Deia E i A A A A Stanstoteaccet 8 3 2 To measure dose t a DOI 8 d SOIC
28. sumption that the dose rate in the point of measurement is independent of the position of the tube the dose from a complete rotation can be expressed as the signal Q per sample time ts multiplied by the rotation time F and a calibration factor c which converts Q z to D z the dose See equation 1 1 Do LO ey For one axial scan CTDI is defined as the integral of the dose profile along the rotational axis divided by the total nominal beam collimation3 This is expressed in equation 2 2 CTDI PO Jo ee The total nominal beam collimation is N the number of simultaneously acquired images multiplies with the nominal beam width d Combining equation 1 and 2 gives gt CRI D dz Gy a dz Gy The CT probe samples the signals on its way through the radiation field After every sampling time t and sampling length Az a signal Q is collected which represents the dose in that part of the z axis The integral can be changed to a sum of all signals per sampling time 4 CTDI c gt 2 Az Gy Az is the distance between two sample points and it depends on the table speed T and on the sampling time 5 Az T t Combining this with equation 4 gives CL 6 CTDI Sy 2 0 Gy The pitch is defined as _ TF 1 P Ng Also if n is the number of samples made during 100 mm then CTDI can be given by 100 8 CTDI pc So Gy The CTDI can be measured during a helical scan if the collected signal
29. table movement 4 Revision C April 2009 Hh RTI eg Allla In figure 3 the result of the same measurement performed in a composition of five body phantoms is shown The dose contribu tion from the scatter tails is now 76 higher compared to CTDI Since the scatter contribution differs little between three and five phantoms we conclude that three phantoms are required to include all dose contribution Figure 3 The dose profile from a scan in the central position in a composition of five body phantoms in a 64 Slice CT The five phantoms mounted together Problem 3 You cannot measure dose free in air To measure the dose free in air on a Multi detector CT with a pencil chamber correctly you need to know the projection of the collimation In other words the true width of the beam that hits the patient the beam FWHM The standard way is to calculate CTDI by dividing the measured integral with the collimation T but the factor that should be used instead is the FWHM For Multi Detector CT the FWHM is greater than T There is a penumbra beyond the active detectors to ensure that they are fully irradiated For example on a GE Lightspeed 16 Slice scanner the CTDI is 27 greater than the actual in air central ray dose for T 10mm and 56 larger for T 5 mm The true dose is essentially constant for the different collimator settings 5 Measurements on wide beam CT systems with no table movement Revision C Apri
30. total absorbed dose measured with the CT Dose Profiler was compared to the reading of TLDs and the deviation was within the uncertainty of the measurements 10 5 References 1 R L Dixon Restructuring CT Dosimetry A realistic strategy for the future Requiem for the pencil chamber Med Phys 33 10 October 2006 2 H D Nagel Radiation Exposure in Computed Tomography 3 A Palm E Nilsson Absorbed Dose and Dose Rate using Varian OBI 1 3 and 1 4 CBCT system To be pub lished V 0 L _ Contact i pat w RTI RTI Electronics AB Phone 46 31 746 36 00 RTI Electronics Inc Phone 800 222 7537 T 1275 Bloomfield Avenue Phone 1 973 439 0242 Fl jelb tan 8C Fax 46 31 27 05 73 A R Moan ax Building 5 Unit 29A Fax 1 973 439 0248 SE 431 37 M lndal E mail sales rti se Fairfield NJ 07004 E mail sales rtielectronics com SWEDEN www rti se USA www rtielectronics com The Energy Dependence of the CT Dose Profiler Revision C April 2009 This report contains information of the energy dependence of the CT Dose Profiler from RTI Electronics 1 1 Definition of Beam Qualities We made the comparison for beam quality RQR which is defined in the IEC report 61267 Medical diagnostic X ray equipment Radiation conditions for use in the determination of characteristics In addition we also used beam quality RQT Beam Quality KV anuz values RQR 4 60 S Sap 30 3 02 ROR 8 100 4 0 pe 120 5 02 R
31. u measure dur ing one scan in Helical spiral mode with the same protocol as is used for the patient The software calculates CTDI and DLP you get the true dose profile and can see the total dose even outside the 100 mm window With the chamber DCT10 you have to make up to five axial scans and then manually calculate the CTDI and DLP You will not get the dose profile and can very easily underestimate the dose to the patient The time and the beam width cannot be measured since no spatial information can be acquired from the ion chamber 3 What is the difference between measuring CTDI using the five phantom positions A B C D E and using the Central Point Method at only point A uw rom all five phantom positions and calculated a weighted mean In the standard procedure the software uses the CTDI value to get the CTDI The Central Point Method uses the constant ratio contant for each type of CT scanner and phan tom size between CTDI and CTDI The software gives you CTDI oo with the constant ratio the k factor to give you CTDI and in the central position This value is then multiplied CTDI r Read more about this in the user s manual 4 Can the customers add their own data to the data base The CT database file can be updated edited with new CT models Normally that is done at RTI when a new releases is made 5 How important is the beam quality when you make a free in air measurement By s
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