Comments on ICRP ref 4831-9482-1403
Radiological Protection in cone beam computed tomography (CBCT)
General:
This work gathers a large body of published work and as such is a valuable resource.
However some comments can be mentioned regarding the internal cohesion between and also with sections. Much of this variety of style and content might be traced back to the definition of terms and the interpretation of the key terms.
One principle example of this is the conflict between CBCT and MDCT.
Definition of CBCT. ‘A form of x-ray computed tomography (CT) in which the x-rays, in the form of a divergent cone or pyramid, illuminate a two-dimensional (2D) detector array for image capture.’
One might expect that the key element here is the divergent cone or pyramid involved in the creation of a reconstructed image, however the view through the document, while not constant, tends to be that the use of a flat plate technology (FPT) is the key defining element.
This raises confusion and leads to the obvious question to the reader: what modalities are in and what are out?
Chapter 2 supports the very general definition well by giving a good walk through many of the existing CBCT equipment modes – what is not included is digital breast tomosynthesis (DBT). While many would not think of BDT as a CBCT modality, it is mentioned in chapter 7 (p76) along with bCT. The justification for including bCT is that some units use FPT – however note that under the given definition for CBCT the 2D detector does not have to be flat.
This leads to the dilemma – should CBCT be defined in terms of the detector (to separate it from MDCT), or by ‘cone angle or ‘depth of reconstruction in the z axis’. The name ‘cone beam’ suggests the later (it is not ‘flat plate’ CT).
There is a case to define CBCT in terms of the z axis reconstruction length at the iso-centre. A length like 4 cm might be useful here – this would allow ‘volume imaging’ from wide angle CT scanners to be classified as CBCT – as they already are effectively. In fact some thoughts about this are expressed, somewhat curiously, under ‘current standards in radiological protection of CBCT’. It is right to discuss the criteria for CBCT in chapter 1, but it deserves its own heading.
Definition of patient dose: The term ‘patient dose’ is used 70 times in the document. However it does not feature in the definitions – surely this is an important oversight that needs to be rectified. We are told many things are not ‘patient dose’ but no definition or reference as to what it actually is.
1 Introduction:
544 (1) this attempt to distinguish CBCT and MDCT is not convincing. Is the difference really only the detector?
564: again – do the detectors have to be flat?
569: does the FPD design really lead to faster acquisition times that say an Aquilion One MCDT operating with full 16 cm detector range in the z axis at the isocentre?
583: what is a “standard” MDCT – perhaps it is better to more carefully specify the definitions of CBCT and MDCT
585: under the definition in the document both CBCT and wide angle MDCT have the same dose formalism constraints –
588: surely this is a non-substantiated generalisation. Many counter examples could be successful mentioned in this regard. The statement needs to be qualified if retained.
602 -604: I suspect some CT manufacturers would challenge this statement with scanners capable of changing exposure (including null exposure) during rotation.
669 – 684: interestingly now the scope of CBCT is being played with – Actually the z-axis length is probably not so ‘empiric’ as 40 mm has been nominated as the maximum range that CTDI can be evaluated without a correction factor being applied. While this due to the definition of the CTDI100 formalism – it may well be a good place to start if such a definition were to be used. Almost all routine CT scanning is, in my experience, with a beam width of 40 mm or less – with only volume scanning done using longer detectors in the z axis (wider angles). Such volume images are surely cone beam images. However I recommend that the definition of CBCT be determined and applied uniformly, to give the document cohesion. My personal opinion is that an MDCT scanner is most usually a CT scanner – but can be used in CBCT mode – just as an angio unit is predominantly an angio unit – but can be used in CBCT mode if needed.
2. CBCT technology
As mentioned above – if mammographic equipment is classified as possibly a member of the CBCT family– it should be covered in more detail in this chapter. In Chapter 2 there is a mention of Breast – Horizontal gantry- in table 2.1, but not of BDT.
6. Optimization
1815: Does the reference McCollough 2005 – refer to CBCT or only CT? Does the reference He et al 2010 refer to AEC? Please check – what is the evidence for AEC in CBCT? Are there any relevant references?
1937: Bismuth shielding –on line 1941; 1945 the idea of AEC for CBCT is raised – is this in fact a reality outside of MDCT units?
1965: Here we are told that most CBCT machines adjust for exposure using AEC – which appears to be in conflict with 1815 where no evidence is given of any CBCT unit working with AEC. Is this section (111) really talking about MDCT? It would help if papers that describe AEC for CBCT could be referenced so the reader can assess if this is a real and common phenomenon.
1990-2010: AEC with paediatrics – again no concrete references - AAPM 2011b is a great document – but does not talk about AEC for CBCT?
2250: Now AEC is defined (again?) after being discussed numerously before hand – at last some hint that some manufactures may be experimentally using correction graphs – no reference and a very vague idea of what may be happening. Clearly CT and fluoro have functioning AEC capability – but what about CBCT?
2264 (141) now we are talking about difficulties of dosimetry with CT AEC – but we have learnt nothing about CBCT AEC.
2868: We learn that CBCT is largely absent in dental CBCT (which is most likely the most ubiquitous form of CBCT).
2910The term bCT is introduced here with no explanation as to the abbreviation or to what it is.
3344: Table 9.1 is not referenced in the text – further – table title is not clear – is it ‘proposed QA tests for generic CBCT units’
3348: What is reference IAC 2012 – from the reference list is this – IEC 2012? – however this is a CT document leading again to unresolved confusion between what is CBCT and what is MDCT. Or is IAC another document?
Thank you for the opportunity to give comments. It is not an easy task to reign in such a diffuse topic that is so essential to our understanding but still emerging as a discipline and hence under continuous change. Currently CBCT is also a potential threat to good radiation management as many products are on the market that are high dose and low image quality as well as possibly the converse.