Multi-detector computed tomography

Draft document: Multi-detector computed tomography
Submitted by Toru Fujita, JSRT - Japanese Society of Radiological Technology
Commenting on behalf of the organisation

Comments of the Japanese Society of Radiological Technology (JSRT) on the draft of Multi-detector computed tomography in Dec. 2006 We appreciate the remarkable efforts of the International Commission on Radiological Protection (ICRP) and also express special gratitude for the openness and sincerity in revising the draft in response to public comments. We highly appraise the draft and hope our comments would contribute to make the excellent publication even better. #1. Technical comments: 1-1. Sub-clause: APPENDIX A A1.CT Dose Index (CTDI) p70, 1776(Eqn.3) “CTDIW = 1/3 CTDI100,center + 2/3 CTDI 100,edge. (Eqn. 3)” Amendment: Not “edge” but “peripheral”. CTDIW?1/3CTDI100,center?2/3CTDI100,peripheral Reason: As long as there is no specific reason, a technical term needs to unify with previous publications. Not “edge” but “peripheral” is used in ICRP Publ.87 p41, IEC (60601-2-44 IEC: 2001+A1:2002(E)), pp22 This sentence is being quoted from "International Electrotechnical Commission and 2002;". Therefore the technical term should not be changed to prevent confusions. 1-2. Sub-clause: APPENDIX A, A1.CT Dose Index (CTDI) p70, 1768 “CTDI100 is acquired using a 100-mm long, 3 cm3 active volume CT “pencil” ionization chamber and” Amendment: It is desirable to mention the importance of the energy calibration of ionization chamber for adjusting effective energy as defined the transmission for aluminum. Reason: The response of ion chamber depends on photon energy distribution. Therefore, although accuracy does not need to be high, it is desirable to mention that the energy calibration is essential for radiation measurement using an ion chamber even in a hospital. 1-3. sub-clause : APPENDIX A?A1.CT Dose Index(CTDI) p70, 1781 air kerma (mGy) or exposure (R) to absorbed dose Amendment: It should be replaced to “air kerma (mGy) or exposure (R) to absorbed dose in air” Reason: Polymethylmethacrylate is used for measuring CTDI. However CTDI is defined as absorbed dose in virtual air within Polymethylmethacrylate. It might be adequate to consider the fundamental dose definition and dose standard. 1-4. Sub-clause: APPENDIX Amendment: It is desirable to add the explanation of “dose efficiency”. Reason: “Dose efficiency” is displayed at every MDCT. This term is an important index for planning radiation dose control. Therefore it is expected to explain “dose efficiency” precisely in APPENDIX. 1-5. Sub-clause: 1.3.Difference between SDCT and MDCT p 12, 335-345 helical pitch, pitch factor and so on. Amendment: These terms should be explained more precisely by using the proper fundamental technical terms. Reason: The Publication is not only for experts but also for various general practitioners. Therefore, technical terms should be explained to clarify the meaning and to understand easily. 1-6. Sub-clause: 2.6.Responsibilities for patient dose management p24, 637 Professional societies of referring physicians and of radiology should work together with medical physics experts to survey the practice, estimate the magnitude of unjustified usage and evolve strategies for avoidance of unjustified exposures. Amendment: It should be replaced to “Professional societies of referring physicians and of radiology should work together with medical physics experts and technologist (operator) to survey the practice, estimate the magnitude of unjustified usage and evolve strategies for avoidance of unjustified exposures.” Reason: It is hard to say that medical physics expert is easily available in all countries, and there are many countries where technologist is playing a role of medical physics expert. Therefore, it should write medical physics expert and technologist concurrently. 1-7. Sub-clause: 3.4. Tube potential (kVp) p45, 1213 The use of lower kVp (80-100) for dose reduction has also been recommended for chest and abdominal MDCT in newborn and infants (Siegel et al., 2004). Amendment: This sentence should be reconsidered. Reason: It is not reasonable to reduce radiation dose by decreasing tube voltage (this term should not be replaced with kVp or kV) for all cases in chest and abdominal MDCT in newborn and infants, because there are many factors regarding with radiation dose and image quality. We agree that appropriate tube voltage is key factor for adequate examination, but it seems to recommend to adjust not only tube voltage but also tube current, beam time and pitch factor for optimizing the radiation dose to patients. 1-8. Sub-clause: 4.1Justification of examination p51, 1385 Introduction of informed consent for radiation risks, although challenging, may help to increase awareness about CT radiation dose and perhaps decrease some “unnecessary” CT from being performed. Amendment: As a consequence, introduction of informed consent for radiation risks, although challenging, may help to increase awareness about CT radiation dose and perhaps decrease some “unnecessary” CT from being performed. Reason: We agree the importance of the informed consent. However, it is likely to cause misunderstanding to mention about effect of the informed consent on education to the medical staff as this sentence. It should be emphasized that the effect for staff occurs of accompanying as a result the informed consent. Therefore, "As a consequence”, might be added in the beginning of the sentence. 1-9. Sub-clause: 4.2.Training issues p52, 1415 The radiologists and CT technologists must be trained to adapt CT scanning techniques based on clinical indications (standard dose CT indications such as CT for liver metastases or low dose CT indications for screening CT studies, pediatric CT, kidney stone CT) and to assess associated radiation doses with different scanning parameters. Amendment: Term unification between “operator” and “technologist” Reason: Two terms “operator” and “technologist” seem to be same meaning in this draft. Therefore it is expected to unify the terms. 1-10. Sub-clause: 4.3.1.Chest CT p53, Table 4.1A. 1474 CTDI vol 2.0 mGy 10.1 mGy about CTDIvol at chest CT Amendment: In general, it should indicate the size of phantom clearly when indicate dose with CTDI. Reason: It seems that CTDIvol in the table has been measured using a phantom of 16cm size. To understand the data correctly, it is expected to put the basic conditions of the measurements. 1-11. Sub-clause: 4.3.6.CT guided interventions p65, Table 4.5A, 1662 “CT fluoroscopy” Amendment: It should describe the exposure time. Reason: There are no data on exposure time. However for CT fluoroscopy, without exposure time it is difficult to interpret CTDIvol of the procedures. Therefore it should add the exposure time and rotation time. 1-12. Sub-clause: 4.3.8.CT of the pregnant patients p68, Table 4.6. 1712 Fetal dose at 3 months 0.07 cGy 1.5 to 1.7 cGy 0.4 to 0.72 cGy Amendment: The unit of fetal dose at 3 months should not be "cGy" but “mGy”. Reason: All other organs doses are indicated in mGy. It should unify prefix to avoid confusions. 1-13. Sub-clause: APPNDIX A A3.Organ dose and effective dose, p73, 1858 Effective Dose = k • DLP (Eqn 7) Comment It seems desirable to indicate the height and physique of this conversion to estimate effective dose from DLP. 1-14. Sub-clause: APPNDIX A A4.Dose estimation tools, p74, 1882 Modern CT systems display the CTDIvol and DLP information for every scan acquisition. From these values, an estimate of effective dose may be obtained, as discussed above. For more complete calculations of organs dose, data from Monte Carlo dose calculations must be used. These are available from different sources as: the NRPB of the United Kingdom (Hart et al., 1994; Shrimpton et al., 1991)); the GSF of Germany (Zankl et al., 1991; Zankl et al., 1993; Zankl 74 and Wittmann, 2001); CT-EXPO (Stamm and Nagel 2002). Several software programmes have been developed to integrate the dose to target organs for each slice irradiated in the CT exam (Kalender et al., 1999) and those from ImPACT ( Comment There are many computer programs to calculate the equivalent doses of X-ray examination by the Monte Carlo method. However some programs are only available for a reference man (Westerner) model. The condition of physique might bring the difference that could not be disregarded especially in x-ray CT. Therefore it should mention about the limitation of these applications. #2. Editorial comments: 2-1. Sub-clause: 1.2. Introduction to MDCT Technology p8, 236-237 It is describes that “The number of slices, or data channels, acquired per axial rotation continues to increase with 64-detector systems now common.” Amendment: The term now common” should be deleted. Reason: We expect that 64-detector systems will increase the share of the world market. However, it is not still common. 2-2. Sub-clause: 3.1.1.Genaral descriptors of image quality, p27, 715 artefacts Amendment: artifacts Reason: The technical term should be unified. 2-3. Sub-clause: 3.4. Tube potential (kVp) p45, 1204 Amendment: 3.4. Tube voltage (kV) Reason: The X-ray tube voltage is defined by the peak value, and the tube current is defined by the geometric mean value. It is general to show the unit symbols with [kV] and [mA], and these values are displayed with [kV] and [mA] on the CT console screen. Making only X-ray tube voltage a special unit symbols causes confusion. [kVp]?[kV]; The terms are necessary to be unified as same unit. This discrepancy is same in Fig.3.3.