2CTPatients are exposed to higher radiation levels from the use of computed tomography compared to most imaging techniques
3Radiation Dose in CT How much dose is scanner delivering? Inter-scanner comparison of doseEstimate patient potential riskWeigh risk against benefitPatient exposure tables/ a requirement by regulatory agencies (JCAHO)
4CT Beam Geometry Most modern CT Emit fan shaped beam Narrow cross sectionThin beam slice across patient
6How do we measure dose? Ionization chamber Quantifies radiation exposureAir filled containerAmount of collected charge is proportional to amount of radiation passing through itCharge is removed and measured with electrometerTotal Q measured in coulombs1 C = 1.6 x 10^9 e
7Multiple Scan Average Dose Ave dose delivered to patient when a series of scans are performedGraphical
8Multiple Scan Average Dose MSAD is dose from “many” slicesSince the dose profile’s tail can extent quite far (perhaps up to 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.
9BED Index distance bed is moved between adjacent scans (mm) each scans the patient is moved a bed index distance
10Computed Tomography Dose Index (CTDI) special quantity expresses radiation dose in CTMeasured with a dosimeter inserted into a phantom that represents a patientDose is measured by scanning one slicefactors are applied to convert the measured phantom CTDI to an actual patient scana reasonable estimation of the actual dose to the patient
11CTDI and MSAD Slice dose versus procedure dose CTDI: ionization chamber used for measurementArea of dose cure for single slice dived by slice widthMSAD: use CTDI to calculate an ave dose in middle of series of scansRatio of slice width to slice spacing multiplied by CTDI
12CTDI and MSAD Affect To increase CTDI area under curve intensity ( raises height of curve) ORWidening the curve ( open collimator)CTDI Patient DoseCTDI is the MSAD at canter of a series of 14 contiguous slices
13MSAD 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 outLimit to BI increase
14BRH RecommendationA measurement based on 2 concepts CTDI and MSAD (using pencil ion chamber)Dose from CT is commonly specified in terms of CTDICTDI is derived from measurement dose from single slice
15…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 mm. We usually do not worry about the difference.
16CT 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
17CTDI/MSAD method a pencil ionization chamber 2 sized phantoms (16cm and 32cm) made of acrylic to standardize CT dose measurementsBoth phantoms have holes drilled at specific locations to accommodate the ionization chamber during dose measurementThe chamber is positioned in 1 hole at a time while the other holes are filled with acrylic plugsAn 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 phantomIt is critical to note at this point that the ionization chamber is measuring the exposure and not dosea factor is used to convert exposure to dose
18Reduction of Dose You ,the operator, must know dose You, the operator, how to keep it at minimumWhat can be done to reduce MSADHow does this affect image
20How does KV affects Dose Reducing x-ray tube voltage (kV) while keepingmAs constant, the patient dose is decreasedDropping KV from 120 to 80 kV at aconstant current (mAs) typically reducesthe 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 photonsat 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 noiseIf 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.
21Pitch and Dose Pitch = The distance the patient couch travels during one 360 degree turnAs pitch increases, the time spent in any one point inspace is decreasedPitch <1 = Higher Radiation Dose
23Shield thyroid, breast, eye lens, gonads to reduce organ dose by 30—60%
24Patient AlignmentPatient Alignment Correctly centering patient on CT gantry canreduce 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
25PerspectiveWhen a CT scan is justified by medical need, the associated risk is small relative to the diagnostic information obtainedCT scans save thousands of lives dailyCT scans greatly reduce exploratory surgeries
26Justification : 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 examinationEquipment maintenance: Having to repeat a slice or an entire examination due to equipment failure increases dose with no benefit to the patient. QA programLimit 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 effectiveDecrease 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)
27Customize 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 20mAsEducation 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 ProgramsDetector 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