CT Quality Assurance and Dose Reduction Lecture PDF

CT: QUALITY ASSURANCE AND DOSE REDUCTION Dr. Anastasia Hadjiconstanti Acknowledgements: Dr. Constantinos Zervides LECTURE LOB’S 40. OUTLINE THE IMPORTANCE OF QUALITY ASSURANCE AND DOSE REDUCTION FOR CT. DOSE AND QUALITY ASSURANCE I Determination of doses is of central importance in CT for two reason...

CT: QUALITY ASSURANCE AND DOSE REDUCTION Dr. Anastasia Hadjiconstanti Acknowledgements: Dr. Constantinos Zervides LECTURE LOB’S 40. OUTLINE THE IMPORTANCE OF QUALITY ASSURANCE AND DOSE REDUCTION FOR CT. DOSE AND QUALITY ASSURANCE I Determination of doses is of central importance in CT for two reasons: 1. CT is the largest medical contributor of collective dose to the patient population. 2. Irradiation of the image receptor must be sufficient to obtain a usable image. It is difficult to achieve optimal balance in this tradeoff or know how to do it. This is especially true with CT. Assessing and reporting doses is a complex matter and doses tend to be high. CT invariably results in very non-uniform irradiation of a variety of organs with different radio sensitivities. CT contribution: - 24% of total radiation exposure - 50% of medical radiation exposure DOSE AND QUALITY ASSURANCE II So, what, exactly, does “overall” dose mean for CT? The closest thing we have to an answer at present is the effective dose (ED). The overall risk from a CT exposure may then be estimated from: Risk = 0.05 (Sv−1) × ED (Sv). Some CT machines display a crude estimate of effective dose. This is found using a three-part computation. First, they calculate the CT dose index (CTDI) for the given technique factors chosen for the current patient. DOSE AND QUALITY ASSURANCE III CTDI is a patient independent measure of machine output for the given set of technique factors. They then multiply this number by the length of the region irradiated and obtain a dose-length product (DLP). Finally, another conversion factor, obtained from Monte Carlo (numerical, statistical) calculations, transforms DLP into effective dose. CTDI I CTDI is an index of the X-ray output of a CT machine. CTDI was invented to give an average dose for a single axial slice. The starting point is the dose profile from a single narrow axial study of a CTDI phantom. This phantom is an acrylic cylinder 15 cm long. CTDI II The head and body phantoms are 16 cm and 32 cm in diameter, respectively and both contain 5 longitudinal holes. The holes are used to insert a calibrated 100-mm-long pencil ion chamber to find the dose profile. CTDI III The distribution of dose being deposited in the phantom at any instant depends on the beam angle. However, the dose profile describes the dose averaged along the central axis from a full rotation of the gantry. CTDI IV You probably expected the CT profile to be rectangular. This is not the case because two factors combine to reduce it to a bell-shape: 1. The finite size of the focal spot causes a small amount of shadow blurring at the edges of the primary beam. 2. Compton scatter within the phantom or patient. When exposed to a single, narrow-beam 360o scan of width W mm, the dose (D) along the ion chamber varies as D(z). CTDI100 was defined as the integral dose recorded by the chamber over its entire 100 mm length. CTDI V CTDI100 is measured with the ion chamber in the central hole and in one or more of the peripheral ones. A weighted average of these readings defines CTDIw. 𝟏 𝟐 𝑪𝑻𝑫𝑰𝒘 = 𝑪𝑻𝑫𝑰𝟏𝟎𝟎−𝒄𝒆𝒏𝒕𝒓𝒆 + 𝑪𝑻𝑫𝑰𝟏𝟎𝟎−𝒑𝒆𝒓𝒊𝒑𝒉𝒆𝒓𝒚 𝟑 𝟑 Another modification accounts for pitch in a real clinical helical scan. In the spiral CT technique, the term pitch is known, which is the distance (speed) of the table for one xray rotation in gantry (l) compared to the number and width of the detector (nT). CTDI VI This leads to the final CTDI result, CTDIvol, 𝑪𝑻𝑫𝑰𝒗𝒐𝒍 𝑪𝑻𝑫𝑰𝒘 = 𝒑𝒊𝒕𝒄𝒉 The CTDIvol does not reflect the total ionizing energy deposited into the scan volume. Its value remains unchanged whether 1 cm or 100 cm of patient anatomy is exposed. To better represent the overall risk for a real clinical study, the dose can be integrated along the scan length. This allows the computation of the dose-length product (DLP). DLP is approximated as the product: DLP = CTDIvol × length CTDI VIII CTDI IX Based on Monte Carlo statistical calculations, a procedure was found to transform DLP into effective dose. This involves multiplying the DPL by one of two constants. 0.002 for the head and 0.015 for the abdomen. The method of conversion from DLP to ED, is clearly adopted for convenience and extreme simplicity, not for accuracy. CTDI X Typical values for the calculated effective doses of many CT protocols are: 1–2 mSv for a head examination, 5–7 mSv for the chest, and 8–11 mSv for the abdomen or pelvis. CTDI XI The CTDIvol for a scan is a poor measure of patient dose. It offers a reproducible measure of device output. This is useful in comparison and improvement of CT parameter-setting protocols. For accurate, patient specific measurements of dose you need to employ something like the Patient Air Kerma in Tomography (PAKT) methodology. DOSE REDUCTION I Reduction of CT doses has been of extreme importance in imaging for several years. Clinically adequate images can be obtained with lower exposure levels. This is driving the development of improved technologies and the re-examination of longaccepted protocols. It is even helping to modify physician and technologist behavior. All of these are happening with little increase in cost or time. Much of the improvement is occurring because of a growing awareness of the need for it. Programs like “Image Gently”, “ESR EuroSafe Imaging” and international recommendations are helping. DOSE REDUCTION II They make physicians aware of reports that as many as 30% of CT examinations of children are: medically not necessary, or could be carried out with other safer modalities (US, MRI). Many CT protocols for specific types of studies are site- and even physician-specific. Clearly not all of them can be optimal, especially for imaging neonates and children. An effective way for a facility to reduce dose is to undertake a comprehensive comparison of its own protocols. DOSE REDUCTION III Also, they need to consider those employed at other facilities for the same machine type. Dose reductions of up to 50%, still with adequate image quality, have been reported. This can be achieved from a simple change of kVp. With conscious support from radiographers, we can try to go beyond that using trial and error. Reducing mA-s/rotation for a study by 10% is a way. If everyone is satisfied with the images, stay with the change. DOSE REDUCTION IV Thus, to create a dose reduction program you need the following: Select a different diagnostic modality that: can obtain the needed information without ionizing radiation, considers cost, considers possible examination delay. Consider the overall clinical impact of a study. If the CT procedure is not likely to change the patient treatment or follow-up significantly, do not perform it. DOSE REDUCTION V If you do image with CT, fit the scanning parameters to the patient’s body shape, size, and age. Tolerate images that may have some degree of noise as long the study can deliver. Encourage your organization to re-appraise the benefit-dose tradeoff of the current scanning protocols. Organizations around the world have begun actively collaborating in re-examining scanning parameters. Technique charts have been published listing ways to modify current adults' protocols to make them suitable for children. CT vendors incorporate various forms of automatic exposure control circuits (mA modulation). DOSE REDUCTION VI DOSE REDUCTION VII There are variations on this theme, such as: involving control of the kVp, to reduce exposure of the female breast, etc. Another possibility is to sculpt the edge of the beam at the end of the region of clinical interest. This cuts down the irradiation of tissues outside the region of interest. QUALITY ASSURANCE I Despite its obvious importance, there is not a great deal of guidance available on CT QA. Vendors provide a handbook of recommended QA activities and carry out routine preventative maintenance, if under contract. Most daily, weekly, and monthly QA activities are undertaken by CT technologists. Semi-annual and annual QA is the domain of a qualified medical physicist. The entire QA program and remediation actions are typically the responsibility of the chief physicist. Several national and international organizations offer general QA guidelines. QUALITY ASSURANCE II The American College of Radiology has set up an accreditation program for CT. This involves the evaluation of: the individual CT at a facility, all interpreting physicians, medical physicists, and technologists working with it. It is the device that is accredited, not the entire facility, and re-accreditation is required every three years. QUALITY ASSURANCE III There are three general categories of actual tests dealing with: sample clinical images, measurements on a highly specialized ACR CT phantom, and dose assessments with the CTDI phantom. The first of these three requires the submission of a set of clinical images of various types. This includes details of the protocols that are used routinely at the facility. The second category of tests involves measurements on a specially designed ACR CT phantom. QUALITY ASSURANCE IV QUALITY ASSURANCE V The phantom consists of four modules which allow for assessment of:  the average HU value for pure water for a range of machine operational settings;  linearity of CT number with the attenuation coefficients of a number of materials within;  image uniformity and Poisson noise throughout the field of view;  low-contrast resolution by way of a contrast-detail test device;  and high-contrast resolution down to 0.4 lp/mm. QUALITY ASSURANCE VI QUALITY ASSURANCE VII QUALITY ASSURANCE VIII QUALITY ASSURANCE IX QUALITY ASSURANCE X The third category of tests calls for CTDI measurements for three distinct protocols. The adult head, with the small CTDI phantom, should yield a reading below the pass/fail criterion of 80 mGy. For the adult abdomen protocol with the large phantom, CTDIvol has to be below 30 mGy. The pediatric head phantom, with the site’s standard pediatric setting, must be below 40 mGy. The pediatric abdomen, with the site’s standard pediatric setting, must be below 20 mGy. QUALITY ASSURANCE XI QUALITY ASSURANCE XII SUMMARY I Determination of doses is of central importance in CT for two reasons: 1. CT is the largest medical contributor of collective dose to the patient population. 2. Irradiation of the image receptor must be sufficient to obtain a usable image. CTDI is a patient independent measure of machine output for the given set of technique factors CTDI is an index of the X-ray output of a CT machine. CTDI was invented to give an average dose with a single axial slice. Reduction of CT doses has been of extreme importance in imaging for several years. SUMMARY II Clinically adequate images can be obtained with lower exposure levels. This is driving the development of improved technologies and the re-examination of longaccepted protocols. Despite its obvious importance, there is not a great deal of guidance available on CT QA. Several national and international organizations offer general QA guidelines. It is the ACR CT Accreditation Program requirements, however, that now serve as the gold standard. REFERENCES Authors Title Computed Tomography: Physical E. Seeram Principles, Clinical Applications, and Quality Control Medical Imaging and Radiation C. Martin, P. Dendy and Protection for Medical Students R. Corbertt and Clinical Staff Edition Publisher Year ISBN 4th Edition Saunders 2015 9780323312882 2nd Edition The British Institute of Radiology 2014 9780905749549 M.A. Haidekker Medical Imaging Technology 1st Edition Springer 2013 9781461470724 A.B. Wolbarst, P. Capasso and A.R. Wyant Medical Imaging: Essentials for Physicians 1st Edition Wiley-Blackwell 2013 9780470505700 L.E. Romans Computed Tomography for Technologists: A Comprehensive Text Wollters Kluwer Health / Lippincott 2010 Williams & Wilkins 9780781777513 st 1 Edition ACR CT Accreditation Phantom Instructions