ImPACT were invited to deliver the following presentation at the International Conference on Quality Assurance and New Techniques in Radiation Medicine (QANTRM) meeting organised by the IAEA in Vienna, 13 - 15 November 2006.
Since its introduction into clinical practice in the early 1970’s, CT has proved to be a valuable diagnostic tool. Advances in CT technology, particularly the introduction of helical scanning in the early 1990s, and subsequently multi-slice scanners in the late 1990s, have lead to increased applications and greater use of CT. The ability to acquire currently up to 64 data slices simultaneously, as well as the increase in tube technology and data handling, has lead to a large degree of flexibility in scanning parameters that can be used.
One of the costs of the high level of diagnostic information that can be obtained is radiation dose, and recent estimates have shown that the radiation dose from CT scanning can contribute almost 70% to the population dose from medical exposures. Dose reduction in CT is therefore an important issue, however this must not be achieved at a detriment to the diagnostic quality which is the primary aim of the CT scan. The quality assurance of the whole CT scanning procedure encompasses technical quality, as assessed by quality control procedures, as well as skilled and appropriate use of the scanner. Quality control procedures are important for ensuring the basic physical quality of the CT scanner, however in order to obtain optimum diagnosic quality images at a dose that is as low as is reasonably achievable (ALARA), a good understanding of the scanner and the effect of scan protocols is essential.
This first part of this presentation will outline some of the quality control procedures, for example noise, dose image width, spatial resolution, that are essential for ensuring that the scanner is operating to its required standard. An understanding of the influence of scanner parameters will be given. There are a number of resources that can be accessed to establish a quality control system on a CT scanner. These cover issues such as ‘what to do and when’, ‘how to do it and why’, and ‘practical tips and pitfalls’. It is important to understand the difference between acceptance testing, commissioning and quality control. Many tests may be similar, however practical contstraints, such as time available on the scanner, mean that some tests have to be adapted for the quality control procedures. For example a sophisticated spatial resolution test, for which many images from many scan protocols are undertaken, then analysed by in-house software, with results taking some time to analyse, may be totally inappropriate for a routine check on deterioration. Many of the tests that would be undertaken on a single slice scanner are similar to those carried out on multi-slice scanner. However there are five aspects to be considered when testing a multi-slice scanner. Firstly, is the phantom or test object long enough to ensure that the wider beam and all the simultaneously aqcuired slices are tested? Secondly, do you need to measure all of the extnesive number of slices or configurations of slices that are available? Thirdly, all the multi-slice scanners have narrower sub-millimetre slices. Can your test object measure down to that thickness ? fourthly, can you deal with the amount of data that you will be generating by imaging many slices of data simultaneously. Fifthly, at what stage, with scanners operating at larger numbers of simultaneously acquired data slices, do you abandon axial scanning and just test in helical mode? These aspect all have to be considered when establishing a quality control testing procedure.
The frequency of the tests has to be considered, as well as which personnel should be assigned the tasks. The UK Institute of Physics and Engineering in Medicine has produced a report Report, ‘Recommended standards for the routine performance testing of diagnostic x-ray imaging systems’  which gives guidance on this. It also provides guidance for ‘supsension’ and ‘remedial’ levels of parameters. Phantoms that are to be used consist of commercially available phantoms, such as the CATPHAN or RMI phantoms, those supplied by the scanner manufacturer, and those made in-house. The scanner manufacturer’s phantoms are often easiest to use and may have quality control software available on the scanner which automatically scans and detects aspects of the scanner that are out of specification. However where a quality control system is to be applied across a number of different scanners external phantoms are preferable.
The second part will focus on the quality assurnace of the whole CT scanning process, by looking at the more complex issues of optimization of CT scanning protocols. One important aspect is to define the diagnostic image quality for the particular clinical examination. To do this, the scan length, contrast requirements, patient motion constraints, size of structures imaged and contrast of these structures need to be considered.
It is well accepted that it is important to keep dose as low as reasonably acceptable or achievable, the ALARA principle. However what is relatively unknown, and poorly addressed until recently is the level of acceptable noise for specific clinical examinations. The ability of the human eye and brain to interpret diagnostic information is very important in establishing an appropriate image quality. A number of studues have been established which are investigating this. For example a study adding simulated noise to brain scans established that certain diagnoses could be still obtained at half the recommended mAs settings . For high contrast imaging, such as lung studies, this could be a low as a tenth of the original dose. Some of the manufacturers have initiated work on this subject by making simulated noise programs available to certain test sites for full clinical studies.
Scan parameters such as tube voltage, tube current, scan time, x-ray beam collimation, pitch, imaged slice width and reconstruction algorithm will affect the image quality and dose. The setting of each of these parameters needs to be carefully considered to achieve the defined image quality whilst minimizing the radiation dose. For example low contrast resolution increases with narrower slices when the object is smaller than the slice thickness, however to keep image noise constant the dose has to be increased with narrower slices.
Finally, an understanding of tube current modulation software, how it works and its limitations, is essential for obtaining optimum image quality and appropriate dose. These packages generally allow the initial tube current to be adapted to the size of the patient prior to scanning. They also ensure that during the scan the tube current is modified so that areas of high attenuation, such as the shoulders, have adequate high tube current, whilst for areas of low attenuation, for example the lungs, it is reduced. Appropriate quality control testing, and a good understanding of the ability of the new technology in multi-slice scanners, enables good quality assurance systems to be established for the whole scanning process.
-  Institute of Physical Scientists in Medicine (UK), IPEM report 91 (2005), Recommended standards for the routine performance testing of diagnostic x-ray imaging systems
-  IPEM report 32 part iii (2003) Measurement of the performance characteristics of diagnostic imaging systems in medicine. Part iii computed tomography
-  American College of Radiology CT Accreditation program (www.acr.org) Med. Phys, 31 (9) September 2004
-  A J BRITTEN, M CROTTY, H KIREMIDJIAN, A GRUNDY, AND E J ADAM, The addition of computer simulated noise to investigate radiation dose and image quality in images with spatial correlation of statistical noise: an example application to X-ray CT of the brain Br. J. Radiol. 2004 77: 323-328.