1. An exercise in how to assess the CTDIw , DLP and effective dose
for single- and multi-slice computed tomography systems
Hilde M. Olerud 1) and Wendy Garborg 2) Departments of Medical Physics and Technique 1 and Radiology 2 at the Norwegian Radium Hospital, Montebello, N-0310 Oslo, Norway
Hiii, all that really matters is the collimation and the total current time product....
Pst, Urvin, what about dose calculations in helical CT ?
The CT dose index - CTDI
The CT dose index 1 measured with an ionisation chamber with 10 cm active length is defined according to formula (1) , where T’ is the total nominal slice thickness, corresponding to the total active detector length for multi-slice scanners and the nominal slice thickness for single-slice scanners. CTDI10cm measured free in air along the axis of rotation normalised to the current time product (mGy/mAs), depends principally on the radiation quality (tube voltage and filtration), distance from focus to the rotation axis, and the collimation of the beam.
The weighted CT dose index -CTDIw
The weighted CT dose-index, CTDIw , is defined according to formula (5), where CTDI 10cm,p is measured 1 cm beyond the surface and CTDI 10cm,c is measured in the centre, respectively, of a phantom with diameter (a) 16 cm (head) and (b) 32 cm (body) (Figure 1). These values are provided in the scanner specifications (product data), usually related to the current time product recommended for normal use in the clinic. If normalised to the current time product (100 mAs) the notation nCTDIw is used. CTDIw represents an estimate of the mean absorbed dose in the CT slice. The formalism includes two ways of defining it further: (a) CTDIw as measured for one axial scan related to the total nominal slice thickness, or (b) CTDIw related to the energy deposit per tube rotation, i.e. the CTDIw is corrected for the pitch value (or axial increment). The latter is the CTDIw value provided on the CT monitor when ordering a new examination. Then ”large FOV” corresponds to the 32cm phantom, and ”small FOV” to the 16cm. phantom.
The definition of pitch
The pitch may be defined in two ways (a) related to total active detector width, T’, or (b) related to the active detector width per data channel. GE use the latter definition of pitch, and we may use the notation pitchz for this 2. We prefer the first definition, and the two definitions of the CTDIw is then related by formula 6 and 7.
The multi-slice detector
The LightSpeed QX/i detector consists of 16 detector elements, each of 1.25 mm z-width. There are always four output signals from the detector, i.e. four data channels. One channel is based on the added signal from one, two, three or four detector elements, i.e. the active detector width is 1.25, 2.5, 3.75 or 5 mm. Once the signals are added into one data channel, they cannot be separated later. There are only two possible pitch values for the LightSpeed QX/i scanner, the high quality (HQ) mode correspond to pitch=0.75 (pitchz =3) (over-lapping slices) while the high speed (HS) mode corresponds to pitch=1.5 (pitchz =6) (inter-leaved). The explanation of how this take place is shown in Figure 2. For example will ordered 5 mm slice thickness with (a) table speed 15 mm/rotation, HQ mode, and (b) table speed 7.5 mm/rotation, HQ mode, in both cases result in pitch=0.75, because the detector configuration automatically changes from (a) 4x5 mm, to (b) 4x2.5 mm. That means the selection of table speed and scan mode determines the detector configuration and therefore also the collimation of the beam. Faster table speeds means shorter scan times, and larger volume covering during “one breath hold”, but the expense is less resolution along the patients length axis.
The dose length product - DLP
The Dose-length product in the units of mGycm1 is a measure for the total energy deposit in the patient, and may then be calculated via one of the expressions in formula 8, where T’ is the total nominal slice thickness, i.e the slice thickness (single-slice) or total active detector width during rotation (multi-slice), A is the tube current (mA) and t is total exposure time (s) for the scan serie i, that is used in the calculations when the CTDI is normalised to the current time product (mAs), otherwise one simply multiplies with #rotations=total time/rotation time=total length/tablespeed.
Figure 1: Equipment for the measurement of CTDI
Figure 3: The NRPB phantom
Learning objectives and summary
Our hospital installed one single slice CT (GE HiSpeed CT/i) and one multi-slice CT (GE LightSpeed QX/i) in the summer of Both systems have software that provides the CTDIw and DLP automatically when ordering an examination. This exercise will guide you, step by step, on how to calculate these dose figures manually, and also what is important for the assessment of the effective dose to patients. It is also essential to establish the basic understanding of the multi-slice CT technology, the relationships between table speed, detector configuration and pitch. To illustrate these objectives, we focused on one single-slice- and two multi-slice protocols, that were designed to give rouphly the same patient dose. The results are summarised in Table1. The necessary formulas for the calculations are framed below.
ACKNOWLEDGEMENTS TO
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.... not exactly true but....
Steps towards an understanding
The “High speed” (HS) scan mode for the multi-slice system (Pitchz=3), corresponds to pitch=1.5 on the single-slice system (inter-leaved), while “High quality” (HQ) (Pitchz=6), corresponds to pitch 0.75 (over-lapped).
The distance from tube focus to isocenter is 630mm for the single-slice- and 541mm for the multi-slice system. That means the normalised dose free in air at the axis of the rotation, CTDIair (mGy/mAs), is higher for the multi-slice system, and the current time product (mAs) must be decreased accordingly to get the same dose in the CT slice.
Changing from pitch 1.5 to 0.75 means you have to halve the current time product (mAs) to get the same dose.
The agreement between the sensitivity profile and the dose profile is better for the widest detector configurations, that is why the normalised CTDIair is somewhat higher for the 4x2.5 mm configuration, and the mAs must be further decreased from 90 to 80 to get the same dose in the slice.
Helical scan technique with pitch 1.5 corresponds to axial scan technique with axial increment factor 1.5. You can achieve that from various combinations of slice thickness (w) and couch increment (c), i.e. w/c=5/7.5 or w/c=10/15. In the same way pitch 0.75 corresponds to w/c=10/7.5.
For the effective dose calculations with CTDOSE it is a challenge to adapt the clinical scan volume to the mathematical phantom (Figure 3). It is also a matter of course to determine the number of slices, n, that corresponds to the equivalent axial scan technique you have chosen (the scan length L = c(n-1)+w).
Other input parameters to the CTDOSE program are the mAs value per rotation, and the CTDI value for the slice thickness or detector configuration that applies to the actual helical scanner and helical scan technique. 1 2 3 4 5 6 7
The exposure and absorbed dose
The absorbed dose, D, is defined by the quotient between the mean energy imparted by ionising radiation to the matter in a volume element, and the mass of the matter in this volume element. The absorbed dose to air may be derived from measurements with ionisation chambers via the exposure, X, and the absorbed dose in other media may be derived from the ratio between the mass-energy-absorption-coefficients (formula 2 and 3). The absorbed dose is measured in units of gray (Gy = J/kg). Wair/e is the energy required to release one unit of charge in dry air, Wair/e=33.97 J/C. The conversion factor from the Röntgen unit of exposure to SI dose units, may be derived from the above formula: 1 R=8.76 mGy.
Organ absorbed doses and the effective dose
The effective dose, E , is given by ICRP 3 , where wT are the tissue weighting factors and DT is the mean absorbed dose to organ or tissue T (formula 4). Values of tissue weighting factors have been established on the basis of all available literature on stochastic risk from radiation, and apply to a reference population of equal numbers of both sexes and a wide range of ages. In the definition of effective dose they apply to workers, to the whole population, and to either sex. The values of wT are chosen to have a sum equal to unity, so that a uniform equivalent dose over the whole body gives an effective dose numerically equal to that uniform equivalent dose. This means that the concept of effective dose can be used to compare non-uniform exposure situations to uniform exposure situations with respect to stochastic risks. The calculation of the effective dose requires assessments of organ absorbed doses for twelve particular organs or tissues. Today the predominant method for assessment of organ absorbed doses is the application of conversion coefficients established by use of Monte Carlo simulations. The Monte Carlo method, in this context, is a computational model in which physical quantities are calculated by simulating the transport of X-ray photons. In the computer program, single photons from the X-ray tube are followed through the imaging chain by allowing them to interact (be scattered or absorbed) within the patient.
The NRPB software
There is software available for the calculation of the effective dose based on absorbed dose to selected organs, and the CTDI free in air is an important input parameter for such calculations. National Radiological Protection Board (NRPB) has provided conversion coefficients from the simple measurement of the CT dose index free in air at the axis of rotation in the absence of the patient (NB given as dose to ICRU muscle) 4. A program called CTDOSE for calculations of organ absorbed doses and effective doses, based on these conversion coefficients is available (Heron Le JC, National Radiation Laboratory, Christchurch, New Zealand). The doses calculated by means of this method apply to an adult hermaphrodite phantom consisting of 208 slabs, each 5 mm thick (Figure 3). The method is primarily applicable to axial scan techniques and scanners on the marked around 1990, but may be applied for modern scanners by using scanner matching data provided by the Impact group at St.George’s hospital in London. There are other windowsbased software obtainable more recently, but CTDOSE was used to illustrate the principal points for what matters in the assessment of effective dose.
Figure 2: How the choice of table speed and scan mode determine the detector configuration and later possibilities in reconstructed slice thickness’ for ordered 5 mm slices on the multi-slice system
REFERENCES
European guidelines on quality criteria for computed tomography. Report EUR Brussels: Office for Official Publications of the European Communities (1999)
McCollough C and Zink E. Performance evaluation of a multi-slice CT system. Med Phys 26: (1999)
International Commission on Radiological Protection Radiological Protection and Safety in Medicine. ICRP Publication 73. Oxford: Pergamon Press (1996)
Jones DG and Shrimpton PC. Survey of CT practice in the UK. Part 3: Normalised organ doses calculated using Monte Carlo techniques. NRPB-R250. Chilton, Didcot: National Radiological Protection Board (London, HMSO) (1991)
Table 1: Calculation of the CTDIw, DLP and effective dose for one single-slice – and two multi-slice CT protocols, when the tube currents were set to give rouphly the same dose to the patient.All scans were taken
at 120 kV, 1 sec rotation time and large field of view.
Table 2: The scan mode and table speed determines the detector configuration for the multi-slice system
RSNA, Chicago Illinois November 26-December 1, 2000