RADIATION DOSE ISSUES

Exposure The term exposure describes the ability of x rays to ionize air. It is measured in roentgens (R); this unit is defined as the quantity of x rays that produces 2.580 × 10−4 C of charge collected per unit mass (kilograms) of air at standard temperature and pressure (STP): 1 R = 0.000258 C/kg air. It describes how much ionization is present in the volume, but it does not tell how much energy is absorbed by the tissues being irradiated. November 2002 RadioGraphics, 22, 1541-1553

Absorbed Radiation Dose describes the amount of energy absorbed per unit mass at a specific point. It is measured in grays (1 Gy = 1 J/kg) or rads (1 rad = 100 erg/g). 100 rad = 1 Gy. Absorbed dose essentially describes how much energy from ionizing radiation has been absorbed in a small volume centered at a point it does not describe where that radiation dose is absorbed or reflect the relative radiosensitivity or risk of detriment to those tissues being irradiated.

Effective Dose takes into account where the radiation dose is being absorbed (eg, which tissue has absorbed that radiation dose) It is a weighted average of organ doses, as described in Publication 60 of the International Commission on Radiological Protection (ICRP) Unit is the Sievert (Sv) or rem. 100 rem = 1 Sv. E = effective dose, T-tissue, wT- tissue weighting factor, wR - radiation weighting factor, DT,R – average absorbed dose

CT dose Due to CT geometry dose estimation needs specialized methods In CT the dose is continuous and around the patient, not projectional There are multiple repeated exposures of an area during the scan There are multiple scans

Dose differences within a section In projectional radiography there is entry and exit side of the patient. The dose is higher to the skin at the entry and less at the exit. In CT the tube rotates 360° around the patient and dose is symmetrically distributed from the periphery to the centre There is a dose gradient from the periphery to the centre

The slope of the gradient depends on The size of the object The radiation spectrum The degree of attenuation by tissues

Large part of anatomy 32-cm phantom one detector row 2:1 radiation dose difference 120 kVp, 280 mA, 1-sec rotation time (280 mAs), 10-mm slice thickness

Small part of anatomy 16-cm phantom one detector row No dose difference 120 kVp, 300 mA, 1-sec rotation time 5mm slice thickness

Z-axis Apart from dose variations within a section there are differences along the Z- axis They describe the dose outside a section due to single tube rotation Radiation affects tissues at the section neibourghood because There is no ideal beam collimation and There is scatter generated within the patient A series of sections add up to make the total dose

Dose histogram from a single tube rotation at the scanner isocenter Z-axis

Isocenter, where the central lines of the beam dissect In CT the isocenter is in the center of the gantry opening

The stray dose depends on: Therefore The dose in a section is the sum of the dose that produces the section image and the dose the section absorbs when the surrounding sections are taken The stray dose depends on: Distance between sections Section thickness Shape of the beam

Ion chamber Measures radiation >10 keV It contains air Closed cylinder with a positive (anode) an a negative (cathode) electrode. Potential difference applied in between them The radiation causes ionization and the amount measured by the instrument

Dose measurements CT specific Multiple Scan Average Dose (MSAD) The average dose from a series of scans over an interval in length Computed Tomography Dose Index CTDI100- dose over 100mm length of ion chamber CTDIw – weighted average between peripheral and central distribution CTDIvol – takes into account protocol parameters like pitch for spiral scans or acquisition spacing for axial scans.

CTDIvol One final CTDI descriptor takes into account the parameters that are related to a specific imaging protocol, the helical pitch or axial scan spacing where N and T are as defined earlier and represent the total collimated width of the x- ray beam and I is the table travel per rotation for a helical scan or the spacing between acquisitions for axial scans.

Phantoms only partly reflect reality They produce estimations and not exact measurements They don’t take into account specific tissue sensitivity

Dose-length product (DLP)

Factors that affect CT dose Direct effect kilovolt peak Milliamperes Rotation time Section thickness Subject thickness Pitch Dose modulation techniques Tube – isocenter distance Indirect effect Affect the image quality and indirectly the dose, i.e. reconstruction algorithms

kVp Radiographer's choice Affected by tube filtration 120 to 140kVp ≈ 35% increase in dose

mAs mA rotation time Linear dose increase mAs=effective mAs = mA Χ time/pitch When using effmAs changing one of the parameters automatically affects the others to keep effmAs constant.

Pitch Table movement in 360° / section thickness Increasing the pitch reduces dose CTDIvol the only marker taking into account pitch

Section Thickness – Single detector scanner Section thickness affects dose For thicker section more photons will pass through the patient because of wider collimation all other parameters kept stable. BUT ….. Exposure and adsorbed dose depend on the irradiated mass and in thicker sections more mass is irradiated so doses may be almost equivalent There is however more dose overlap for thin sections. Dose in single section scanner with thickness change only

Dose Efficiency vs Slice Thickness What fraction of the x-ray beam is “lost” by discarding the penumbra region depends both on colllimator design and total beam width. The size of the penumbra region depends on collimation and focal spot size, and is approximately constant for all usefull beam widths. Thus the penumbra will represent a larger percent loss (perhaps ~30%) for a 4x1.25 mm scan (5 mm total width) than for a 4x2.5 mm acquisition (10 mm total beam width). For a 4x5 mm scan (total width of 20 mm) the loss is even smaller (<10%). As more simultaneous slices are scanned and total beam widths enlarge with future cone beam scanners, the lost penumbra will be of minimal concern.

Section thickness – Multislice CT Dose depends on beam thickness and not on the thickness of the reconstructed image. All other factors kept stable this may mean an increase up to 50% for smaller beam width. As seen on table below a 5mm section can be reconstructed from 4 × 1.25 mm 4 × 2.5 mm 4 × 5 mm but CTDI is stable

Patient size All parameters stable the smaller object absorbs more radiation. This is extremely important for the paediatric patient It is due to the fact that the body is exposed to entry and exit dose as the tube is rotating and the entrance dose is not much attenuated so the smaller obbject is homogenously and highly irradiated all over Radiation dose to children is therefore a big issue

Options for reducing Dose Adjust mA to patient size and absorption Usually a maximum mA value is selected which is reduced whenever absorption is lower in a certain area or subject A 0° and a 90° scanogram is required. On these the mA value is calculated along the scanning length and the value of mA is changing during acquisition It is even possible to adjust mA in real time mode

Indirect effects on dose Slice thickness Thin sections reduce SNR. This is seen as wide standard deviation of HU numbers Noise = 1/√T, T = slice thickness Noise at 10-mm = noise at 1-mm / 3.2 This implies higher dose for satisfactory thin section SNR with increased mA or kV. Reconstruction filters High resolution filters iincrease noise and reduce SNR This may mean increased dose for better SNR when such algorithms are used.

Ways of dose reduction

Reduce Milliamper-Seconds Lowering mA by 50% reduces dose by 50% but increases noise by 41% (1/√ 2 = 1.41). Depending on the clinical needs the above changes may be acceptable or not i.e. In the lung there is high inherent contrast between lung parenchyma and solid lesions and high noise is easily acceptable In the liver there is low inherent contrast between parenchyma and lesions therefore a higher dose is necessary

Increase Pitch Dose reduction when all other parameters are kept stable BUT ………. it increases the effective section thickness which increase partial volume averaging and therefore reduces contrast between lesion and surrounding tissue.

Milliamper-Seconds and patient size Image quality increases with higher dose and the is no image deterioration by overexposure Doses to patients are high Several methods are in use which attempt to adjust dose to patient size and weight Exposure tables for CT are available as in plain radiography

kVp reduction Dose reduction and noise increase Also changes in contrast CTDI100 (and not CTDIw) shows that the dose gradient from the periphery to the center of the object is grater in lower kV The dose to the skin is also higher at low kVp

Effective dose calculation effective dose = dose that takes into account the differences in sensitivity to radiation of the several body tissues It is calculated with several ways in phantoms

Noise lines They are oriented along the maximum absorption of radiation because only a small number of photons reach the detector in this direction They are lines either in the laterolateral or the anteroposterior direction (shoulders, pelvis) When a stable mA is used patient taking into account the lateral absorption i.e. in the shoulders the chest will be irradiated more than necessary in the AP direction The same holds true for changes in the Ζ-axis

Noise lines in the shoulders (photon starvation artifact) European Journal of Radiology Volume 76, Issue 1, October 2010

Automatic Exposure Control - AEC AEC systems adjust mA depending on body thickness along the Z-axis and within a section in order to achieve satisfactory SNR with lower dose There are 3 ways to apply AEC Patient size rotational 3-D

Patient size - AEC In CT patient is exposed in entry and exit radiation AEC adjusts mAs taking into account the patient size and this is very efficient in children Radiation dose to children is more important The disadvantage the scan is performed with stable mAs regardless of the changes in patient size along the z-axis or within the section

Z-axis AEC Beam attenuation changes along the patient High in the shoulder area Low in the chest Hogh in the abdomen This leads to different noise levels along the images when stable mAs is used. Satisfactory noise level in the chest means higher noise in shoulders and abdomen, not acceptable. Satisfactory noise level in the shoulders means higher dose in the chest Z-axis AEC adjusts the current along the scan depending on the scanogram in order to achieve stable noise in all images. Dose reduction up to 30%

Beam attenuation in Z-axis in the 2 scoutviews

Renal images α) stable mAs b) Z-axis AEC Different examinations same patient. Satisfactory images

Rotational AEC mAs changes as the tube rotates around the patient. mAs increases automatically at the tube positions where beam attenuation is higher Adjustment is calculated by Measurements in the previous section Measurements on the 2 scout views Dose reduction 20-30%

Images of the pelvis (a) without and (b) with rotational AEC. Dose in β = 27% < α. Image satisfactory

3D-AEC Combination of Z-axis and rotational AEC for even better results. Available on all modern CT scanners.

Previous slide explanation 3D-AEC in chest and (a) coronal reconstruction from CT performed with stable mAs (green line) and resultant noise (red line) (b) coronal reconstruction from CT with 3D-AEC The thick green line sows the average mAs value in the Z-axis and the thin green line changes in mAs due to patient thickness in the axial plane Image noise is relatively stable in all images along the Z-axis. (c) radiation dose distribution shows overexposure of the lungs with stable mAs when compared with (d) 3D-AEC

Dose reduction in cardiac CT Is used in patients with average risk for coronary disease and in patients with acute onset chest pain. Dose up to 20 mSv when no reduction techniques are utilized (CT abdomen 14 mSv, Lung CTA 15 mSv) It requires thin sections 0.5–0.625 mm to show the thin coronary branches and low pitch Fast tube rotation to achieve a “stationary heart” (0.3–0.4s) Data are acquired during the whole cardiac cycle only data from the diastole are used for Ct coronary angiography. the complete set of data offer functional information ejection fraction movement of the heart wall.

Different ways of ECG gating Retrospective gating Continuous data collection Reconstruction window is selected retrospectively at the middle of the diastolic phase of the ECG Prospective gating with current modulation (PECMTC). The mA is adjusted based on the ECG with a high value at the middle of the diastole and only at 30% during the remainder of the cardiac cycle. Reconstruction window in the middle of diastole Prospective gating (PEGAS). Current is applied only during the middle of the diastole on ECG. Depending on the heart rate the table moves either before the next R-wave on the ECG or with the next cardiac pulsation “step and shoot”

Prospective electrocardiogram correlated modulation of tube current (PECMTC) At the end of diastole the vessels are less affected by cardiac motion By reducing current at the rest of the cardiac cycle the dose to the patient is reduced. Current at 100% in middle diastole and at 30% in between The data at reduced current are not sufficient for vessel study but they allow study of cardiac function. More recently mA is reduced to 4% with a dose reduction by 25%

Prospective electrocardiogram gated axial scanning (PEGAS) Scanners with very short rotation time for patients with heart rates >60 per minute A very small pitch is necessary which results in radiation dose increase PEGAS Prospective ECG gating with axial sections (pitch=0), no dose from overlapping tube rotations Prospective ECG gating with current applied in middle diastole only PECMTC dose reduction by 25%, PEGAS dose reduction by 68–78% PEGAS requires stable heart rate Only diastole information – no functional information

Dual source CT (DSCT) A way to reduce scan time using 2 sets of tube-detectors at 90 rotating simultaneously Scan time reduced by 50% SOMATOM Definition, Siemens Medical Solutions ? Radiation Dose reduction Dual energy scanning possible at one acquisition

256- & 320- rows of detectors This large series of detectors can cover a long Z distance and area of the subject eliminating the need for table translation when applied to cardiac CT. 64-slice (4cm coverage) with 3 rotation per cardiac cycle may produce reconstruction steps 256 (Brilliance iCT, Philips Healthcare) και 320 (AquilionOne Dynamic Volume CT, Toshiba Medical Systems) approximatelly 16cm coverage per rotation = no need for table movement Dose reduction by 50–65% compared with traditional Coronary CTA Need for further studies

Use of low kVp There are 2 advantages: Lower dose Increased contrast But Increased noise When IV contrast is administered the noise increase is balanced by the increase of anatomy contrast 100kVp instead of 120kVp good vessel resolution with reduced patient dose by 30-50% The same holds true for children and small size adults

Dose reduction with noise reduction algorithms adaptive statistical iterative reconstruction (ASIR) model based iterative reconstruction (MBIR iterative reconstruction in image space (IRIS) adaptive iterative dose reduction (IARD) The image is reconstructed with different methods than the traditional backprojection Dose reduction by 30-60%

Comparison of images (a) backprojection and (b) ASIR in different CT scanners dose length product (DLP) in ASIR study (b) 35% < (α) filtered back projection FBP ASIR image less noisy

Overscanning - Overbeaming

Overranging/overscanning Data from a further ½ rotation are needed in order to reconstruct the first and the last image of a plan.

Overbeaming Z-Axis Dose Profile for 10 mm slice The beam width must be wider than the detector coverage in order to achieve homogenous beam density over all rows of detectors. This is due to the diverging rays of the beam A second factor may be understood by referring back to the dose profile for the single slice scanner, for which the slice thickness was taken as the profile’s full width at half maximum (FWHM) size. If we were to divide this 10 mm slice into 4 slices scanned by a 4-slice MSCT with 4 x 2.5 mm detector configuration, it is clear the the “outer” two slices would receive considerably less radiation and would thus be considerably noisier. The only way to avoid this is to widen the x-ray beam so that all four slices are uniformly irradiated. Now, however, we waste the entire penumbra (i.e., edge region) of the dose profile, which can no longer contribute to image formation. Depending on what fraction of the total x-ray beam this represents, this represents another 10-30%

AAPM/RSNA Physics Tutorial for Residents: Topics in CT Radiation Dose in CT1. Michael F. McNitt-Gray, PhD. November 2002 RadioGraphics, 22, 1541-1553 Impact of new technologies on dose reduction in CT. Ting-Yim Leea, , and Rethy K. Chhem. European Journal of Radiology Volume 76, Issue 1, October 2010, 28-35 Radiation Dose Associated With Common Computed Tomography Examinations and the Associated Lifetime Attributable Risk of Cancer. Rebecca Smith-Bindman, MD; Jafi Lipson, MD; Ralph Marcus, BA; Kwang-Pyo Kim, PhD; Mahadevappa Mahesh, MS, PhD; Robert Gould, ScD; Amy Berrington de Gonza´ lez, DPhil; Diana L. Miglioretti, PhD. ARCH INTERN MED/VOL 169 (NO. 22), DEC 14/28, 2009