RAD309 Patient Dose

CT Patients are exposed to higher radiation levels from the use of computed tomography compared to most imaging techniques

Radiation Dose in CT How much dose is scanner delivering? Inter-scanner comparison of dose Estimate patient potential risk Weigh risk against benefit Patient exposure tables/ a requirement by regulatory agencies (JCAHO)

CT Beam Geometry Most modern CT Emit fan shaped beam Narrow cross section Thin beam slice across patient

How do we measure dose? Ionization chamber Quantifies radiation exposure Air filled container Amount of collected charge is proportional to amount of radiation passing through it Charge is removed and measured with electrometer Total Q measured in coulombs 1 C = 1.6 x 10^9 e

Multiple Scan Average Dose Ave dose delivered to patient when a series of scans are performed Graphical

Multiple Scan Average Dose MSAD is dose from “many” slices Since the dose profile’s tail can extent quite far (perhaps up to 60-70 mm away), any particular slice of tissue may receive some additional dose from several other slices being scanned on either side. The total accumulated dose from the “exam” consisting of many contiguous slices is called a “Muliple Scan Average Dose (MSAD)”, and is typically 25-40% higher (depending on collimator design) than the dose from a single slice.

BED Index distance bed is moved between adjacent scans (mm) each scans the patient is moved a bed index distance

Computed Tomography Dose Index (CTDI) special quantity expresses radiation dose in CT Measured with a dosimeter inserted into a phantom that represents a patient Dose is measured by scanning one slice factors are applied to convert the measured phantom CTDI to an actual patient scan a reasonable estimation of the actual dose to the patient

CTDI and MSAD Slice dose versus procedure dose CTDI: ionization chamber used for measurement Area of dose cure for single slice dived by slice width MSAD: use CTDI to calculate an ave dose in middle of series of scans Ratio of slice width to slice spacing multiplied by CTDI

CTDI and MSAD Affect To increase CTDI area under curve intensity ( raises height of curve) OR Widening the curve ( open collimator) CTDI Patient Dose CTDI is the MSAD at canter of a series of 14 contiguous slices

MSAD and BI BI MSAD Large gap b/w slices (slices further apart) Radiation spread over a large area (ave. Dose) BUT relevant tissue can be missed out Limit to BI increase

BRH Recommendation A measurement based on 2 concepts CTDI and MSAD (using pencil ion chamber) Dose from CT is commonly specified in terms of CTDI CTDI is derived from measurement dose from single slice

…to use pencil chamber methods To measure the CTDI, we measure all the dose under the dose profile by using a very long (100 mm) ionization chamber, which is placed in one of the holes in the acrylic phantom. Two phantoms have been standardized by FDA for these CTDI measurements: a head phantom 16 cm in diameter and 15 cm thick; and a body phantom 32 cm in diameter and 15 cm thick. Note that the CTDI is not exactly the same as the MSAD (although they are usually very close). The reason why is that the MSAD requires that all of the significant parts of the dose profile “tails” be measured. The CT ionization chamber measures only 100 mm: this usually represents almost all of the profile, but the profile tail may still be significant up to a range of 140-150 mm. We usually do not worry about the difference.

CT DOSIMETRY dosetools Typically during an inspection of a CT scanner, the CTDI will be measured at the center and at 1 cm depths in both the head and body phantoms. dosetools

CTDI/MSAD method a pencil ionization chamber 2 sized phantoms (16cm and 32cm) made of acrylic to standardize CT dose measurements Both phantoms have holes drilled at specific locations to accommodate the ionization chamber during dose measurement The chamber is positioned in 1 hole at a time while the other holes are filled with acrylic plugs An exposure is made and recorded. This is done for all holes so that dose measurements can be obtained for a number of positions in the phantom It is critical to note at this point that the ionization chamber is measuring the exposure and not dose a factor is used to convert exposure to dose

Reduction of Dose You ,the operator, must know dose You, the operator, how to keep it at minimum What can be done to reduce MSAD How does this affect image

Factors Affecting Patient Dose KVp mAs Pitch Collimation Bed Index Beam Geometry Detector Setup Other: repeats, shielding, alignment, patient size...etc..

How does KV affects Dose Reducing x-ray tube voltage (kV) while keeping mAs constant, the patient dose is decreased Dropping KV from 120 to 80 kV at a constant current (mAs) typically reduces the patient dose by about 60% The contrast of a lesion relative to the surrounding background generally increases as kV is reduced, but the noise (mottle) level also increases since there are fewer photons at the lower kV (hence the lower patient dose). The lesion contrast-to noise ratio (CNR) generally gets worse as the kV is reduced at a constant mAs in CT—the increase in contrast is less than the corresponding increase in noise If one reduces the kV and increases the mAs, however, it may be possible to maintain image quality (CNR) while reducing patient doses. CT optimization with respect to x-ray tube voltage is being investigated by several researchers, with initial findings suggesting that the use of lower kV values is promising for CT performed using iodinated contrast material [2, 3]. To summarize, reducing the CT kV at a constant mAs will always reduce the patient dose, and would probably reduce the lesion CNR. When iodine is administered to a patient, reducing the kV is likely to reduce patient doses with no adverse effect on CNR.

Pitch and Dose Pitch = The distance the patient couch travels during one 360 degree turn As pitch increases, the time spent in any one point in space is decreased Pitch <1 = Higher Radiation Dose

Pitch is 0.75 Image shows 25% overlap

Shield thyroid, breast, eye lens, gonads to reduce organ dose by 30—60%

Patient AlignmentPatient Alignment Correctly centering patient on CT gantry can reduce radiation dose by as much as 56% When the patient is in the incorrect position the patient must be moved and the CT scan repeated

Perspective When a CT scan is justified by medical need, the associated risk is small relative to the diagnostic information obtained CT scans save thousands of lives daily CT scans greatly reduce exploratory surgeries

Justification : is the scan necessary for ongoing patient care Justification : is the scan necessary for ongoing patient care? Can the examination be replaced by a low- or no-dose examination Equipment maintenance: Having to repeat a slice or an entire examination due to equipment failure increases dose with no benefit to the patient. QA program Limit the scan boundaries to area of interest. careful positioning of the scan volume, source to object distance, limiting the scan coverage to the area of interest, or angling the gantry away from sensitive structures can be very effective Decrease the exposure: The correct balance between dose and image quality .The guidelines also give reference values for patient dose for particular examinations. Customize the exposure to patient size: adjust the exposure factors to compensate for changes in patient size (mAs for children examinations)

Customize scan parameters to examination type : High resolution (fine detail) scans such as sinuses, inner ear and skeletal structures can be performed with lower exposure factors as spatial resolution is relatively independent of dose levels. for example, report dose levels as low as 20mAs Education and research: Radiographers review CT protocols at their institution and compare image quality and dose to the ICRP guidelines. education on dose reduction techniques in departmental CT training Programs Detector sensitivity: When purchasing CT scanners ask questions about scanner performance and radiation dose. Don't assume that all manufacturers scanners will have similar detector sensitivity and performance