PEPconnect

Optimizing CT Image Quality - Part 1 - CT Dose

This web-based training will introduce Radiologic Technologists to the fundamentals of CT dose and those parameters commonly used to quantify dose in CT.  The 30 minute presentation will discuss CT Dose Index (CTDI), kV, mA, rotation time, pitch and phantom size, and how these individually and collectively relate to dose.  Following the training, the user will demonstrate their acquired knowledge of CT dose and dose parameters by completing a summary test.  

Optimizing CT Image Quality CT Dose     Walter Huda, PhD Professor of Radiology Medical University of South Carolina     CT Dose Index [CTDI] CTDI & X-ray Techniques CTDI & Patient Dose Acrylic Cylindrical Phantom adsg Hole locations in center and periphery... ... for placement of pencil-shaped ionization chamber. CTDIvol DLP (Dose Length Product) CTDI volume measured in mGy (CTDIvol) Depends on: X-ray tube voltage (kV)   X-ray tube current (mA) X-ray tube rotation time (s) CT Pitch (P) Phantom Size (S or L) CTDIvol 15 mGy Independent of Scan Length DLP = CTDIvol x Scan Length (units are expressed in mGy-cm) CTDIvol - Radiation Intensity   DLP - Total Radiation Used Source? — — 400 mAs 60 mGy @ 120 kV 21 mGy @ 80 kV CTDIvol is a Universal Metric Regardless of: Voltage (kV) X-ray filter Model type Vendor Phantom Size mAs (axial) Effective mAs (helical) Voltage (kV) 16 cm (S) 32 cm (L) Measurements taken with a large phantom will be half the value of those taken with a small phantom. CTDIvol (S) 60 mGy CTDIvol (L) 11 mGy CTDIvol (L) 11 mGy   ↵ CTDIvol (S) 60 mGy CTDIvol (S) 22 mGy   ↵ CTDIvol (S) 60 mGy CTDIvol is directly proportional to: Tube current (mA) Tube rotation time (s) mAs CTDIvol is: directly proportional to tube current (mA) directly proportional to tube rotation time (s) inversely proportional to pitch directly proportional to the effective mAs           - Effective mAs = mAs / Pitch CTDIvol ∞ kV 2.6 80 kV  ► 140 kV CTDI Increases >400% CTDIvol is the radiation incident on the Patient NOT the patient dose CTDIvol = 20 mGy Organ Dose Total CTDIvol (i.e., Incident Radiation) Patient Size Factor Scan Length Factor Embryo Dose Estimates in Body CT Huda et al. American Journal Roentgenology 194 (2010) 874-880 Abdomen/Pelvic Scan (Pregnant) CTDIvol is 20 mGy Normal-sized patient Absorbed Dose to Fetus = 28 mGy + Chest Scan no significant change to fetal dose but DLP is increased Organ Doses are obtained from CTDIvol Total Dose (Effective Dose) is related to Dose Length Product (DLP) Effective Dose (mSv) North American Natural Background Radiation ~ 3 mSv/Year Cosmic - 12% Terrestial - 12% Internal - 16% Radon - 60% Effective Dose (E) = Dose Length Product (DLP)  X  k factor (k) CT effective dose per dose length product W. Huda, D. Magill Medical Physics - Volume 38, March 2011, pp 1261-1265 Adult Head CT Exam CTDIvol: 60 mGy DLP: 1000 mGy-cm x   k [0.002 mSv/mGy-cm] ____________________ 2 mSv - Effective Dose One Year-old Head CT Exam CTDIvol: 30mGy DLP: 360 mGy-cm x   k [0.005 mSv/mGy-cm] ____________________ 1.8 mSv - Effective Dose Adult Chest CTDIvol: 7.5 mGy DLP: 150 mGy-cm x   k [0.02 mSv/mGy-cm] x   k [0.02 mSv/mGy-cm] ____________________ 0.02 mSv - Effective Dose Adult Body Size Correction Fixed CTDIvol and DLP CT Dose Index (CTDI) CTDIvol is “Radiation Intensity” DLP is the total amount of radiation used to perform a CT scan (NOT patient dose). CTDI & X-ray Techniques CTDIvol directly proportional to mAs (axial), effective mAs (helical) and kV 2.6 CTDI & Patient Dose Total CTDIvol → Organ Dose Total DLP → Effective Dose

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