Why is CT Dose of Interest? CT usage has increased rapidly in the past decade Compared to other medical imaging CT produces a larger radiation dose. There is direct epidemiological evidence for a an increase in cancer risk at these doses.
UNSCEAR 2000
Volume of CT exams is growing 2006* 2003 U.S. Germany % of Exams 12% 6 % % Total Dose 49% 47% * NCRP Report No. 160, 2007
Radiation Doses By Modality, 2006 CT 11.8 49 Nuclear Medicine 3.3 23 Radiography 55 19 Interventional 1.2 9 Mammography 6.8 <1 Dental 22.8 <1
CT Susceptible to Undetected Radiation Over Exposures FDA Recommendations Concerning Excess Radiation Exposure during CT Perfusion Imaging: The FDA issued an initial safety notification in October 2009 after learning of 206 patients who had been exposed to excess radiation at Cedars-Sinai Medical Center in Los Angeles over an 18-month period. The FDA has also received reports of excess radiation in >50 additional patients from other states [who were] exposed to radiation of up to eight times the expected level during their CT perfusion scans. These cases involve more than one CT scanner manufacturer.
CT Susceptible to Undetected Radiation Over Exposures
Reporting CT Doses California Senate Bill 1237 CTDI vol & DLP printed on patient s medical record CT services to be accredited to qualify for reimbursement Effective Jan, 2012 Votes: 6/23/10 Senate 19-0 8/9/10 Assembly 17-0
FDA Working on Mandate
What Are We Reporting? 1. CTDI vol CT Dose Index (volume) 2. DLP Dose-Length Product
1. Radiation absorbed dose 2. Absorbed dose in CT 3. Integral dose 4. Risk Comparison
Current Design of MSCT 3 rd generation geometry
Anatomy of a CT Scanner
Path of X-Ray Beam
1. Radiation absorbed dose 2. Absorbed dose in CT 3. Integral dose 4. Risk Comparison
Daily rad, rem Publications Gray, Sievert
1 Gy = 100 rad 1 mgy = 100 mrad 100 mrem = 1 msv 37 MBq = 1 mci
Radiation absorbed dose CT Dose Index, CTDI Integral dose, DLP Effective Dose
CT Dose Measurement Dose profile spreads beyond imaging section
Multiple Scan Average Dose Dose contribution from adjacent slices
CT Dose Measurement Single slice dose profile MSAD SINGLE SLICE DOSE MSAD can be measured from a single dose profile MSAD of central slice = Dose integrated over a single slice profile
CT Dose Index, CTDI CTDI is a MSAD to 2 FDA phantoms CTDI of head MSAD to a 16-cm plastic cylinder CTDI of body MSAD to a 32-cm plastic cylinder
CT Dose Index, CTDI X-ray output expressed as dose to a phantom FDA head & body phantoms
ion chamber (10 cm length) Area under rectangle = Area under dose profile = Dose reading of ion chamber Width of rectangle = slice thickness Height of rectangle = Dose to central slice = CTDI
CT Dose Index CTDI = Multiple scan average dose to central slice Includes dose contributions from adjacent slices Dose to a phantom, not actual dose to patient
CTDI Measurement
Helical Scanning Combine x-ray rotation with table motion X-ray beam wraps around patient in a helical path
Multi-Slice CT
CT Image of Helical Scans Interpolate data from adjacent projection planes Resolution is lower than axial scans
4-Slice CT
Multi-Slice Helical Reconstruction Interpolate multiple projection data for a given axial position.
Definition of a Screw Pitch Distance advanced in one rotation of screw
Definition of CT Pitch
Pitch
Pitch Axial Scan Pitch = 0 Pitch > 1 Pitch = 1 Pitch < 1
Pitch and Scan Overlaps
Effects of Varying the Pitch
For Spiral Scans MSAD Changes with Pitch
CTDI volume CTDIvol = MSAD for helical scans Accounts for non-contiguous exposure along patient
Interpretation of CTDI vol Not actual dose to patient An index of x-ray output expressed in terms of dose to a phantom Dose to a standard plastic phantom Useful for comparing doses from different CT scan techniques
Radiation absorbed dose CT Dose Index, CTDI Integral dose, DLP Effective Dose
Risk of Radiation Depends on total radiation energy absorbed Dose is energy concentration Dose is energy absorbed per gram tissue 1 gray = 1 Joule/Kg tissue CTDI vol does not change with scan length Radiation risk increases with scan length
Risk of Radiation Depends on total radiation energy absorbed Energy absorbed = Dose x Scan volume Scan length ~ Tissue volume irradiated
Dose Length Product, DLP DLP = CTDI vol * scan length Scan length is proportional to volume of tissue irradiated DLP and CTDI vol are shown on CT consoles
Dose Length Product, DLP DLP = CTDI vol * scan length CTDI does not change with scan length DLP changes with scan length Proportional to total radiation energy absorbed Radiation risk increases with DLP Radiation energy absorbed increases with DLP
Example Neuroradiology Diagnostic Reference Levels*!"#$%&#'%(&) *+,-.)/$012),34)/$01)5$2)!"#$%&'()'*+(,-((./0-(1'2(34!5( 6-7-( 8*9'(*&+(:%&#:':( ;7( ;,-( <'2$'=2*>(?2*#@*( A-( B,-( *European Guidelines on Quality Criteria for Computed Tomography, EUR 162, May 1999. http://www.drs.dk/guidelines/ct/quality/htmlindex.htm
Why Not Report DLP Alone? Redundancy in dose reporting? DLP = CTDI vol x Scan Length No, loosely speaking DLP ~ Stochastic risk CTDI vol ~ Deterministic risk Can have small DLP, large CTDI vol If scan length is short Example: Brain Perfusion Scans
Loosely Speaking CTDI vol Estimates deterministic risk DLP Estimate stochastic risk
Types of Risk Deterministic Risk Injury occurs definitely above threshold dose 2-3 Gy cataract Stochastic Risk Probability to develop cancer Linear no threshold model
Radiation Effects: Stochastic and Deterministic Detrimental Effects Threshold Patrick Colletti, MD Radiation Dose
Linear Extrapolation Cancer risk = 0.004 % chance for 1 msv
Radiation absorbed dose CT Dose Index, CTDI Integral dose, DLP Effective Dose
Effective Dose Not a physically measurable quantity A calculated equivalent wholebody dose For partial body exposure Mathematically derived from the DLP
Effective Dose An equivalent wholebody dose to compare stochastic risk from radiation Knowledge of stochastic risk was derived from wholebody exposures Accounts for Different organ sensitivities to radiation Different types of radiation
CT Effective Dose 1. Measure CTDI 2. Calculate CTDI vol = CTDI / pitch 3. Calculate DLP = CTDI vol x scan length 4. Calculate Effective dose = DLP x CF
Some Effective Doses, msv Chest, PA 0.02 Pelvis, AP 0.70 CT, head 2 CT, chest 8 CT, abdomen 10 CT, pelvis 8 CT, Pulmon Angio 15 CPTA, stent, etc. 15 Mammogram (4-view) 0.2 FDG 10 mci 7.0 MDP 20 4.2 MIBI 20 6.7 Ga-67 5 12.0 Tl-201 3 17.0 V/Q 10/3 1.5 Renal 5 1.3 Background radiation 3.0 msv/yr Nuclear medicine technologists 2.0 Flight Crew, 700 hrs between LA-DC 3.0 New York Grand Central Station 3.0 Radiologists 0.7 Interventional Radiologists 17.0
Methods for CT Dose Reduction Reduce mas Dose increases linearly with mas Reduce KV X-ray output ~ (KVp) 2 Reduces KV 30%, reduces dose by 50%
Methods for CT Dose Reduction Reduce scan length Decreases DLP Decreases effective Dose Increase pitch Decreases CTDI Use Automatic Exposure Control (AEC) Understand how dose modulation works Caveat: Improper use with certain types of studies can drastically increase dose (i.e. CT Brain Perfusion)
CT Brain Perfusion: Automatic Exposure Control Know the ma and kvp being used! Recommend: kvp = 80 ma = 100 Make sure Automatic Exposure Control is OFF! SureExposure (Toshiba) Care Dose (Siemens) Auto ma (GE) DoseRight (Phillips) Check cine scan time: ~ 50 seconds with 25 phases/acquisitions Verify estimated CTDI and DLP on consoles prior to initiating scan
CT Stroke Protocol: Expected Radiation Dose Standard non-enhanced CT ~ 2.0 2.5 msv 4 slice CT Perfusion ~ 1.5 2.5 msv Though deposited in a smaller region of the brain CT Angiogram ~ 4.5 msv Total CT Stroke Protocol ~ 9 msv ~ 4 noncontrast CT head doses
CT Brain Perfusion: Radiation Report on Console/PACs Expected Values for CTP: CTDIvol (mgy) ~ 180-200 DLP (mgy-cm) ~ 570-650 Scanner Console displayed as CTDIvol and DLP per phase (25 phases) CTDIvol ~ 7-8 mgy DLP ~ 24-26 mgy-cm Total Exam (NeCT, CTP, CTA): DLP (mgy-cm) < 3500
What Can Technologists do to Reduce Patient Dose in CT? Limit the scan length Adjust mas according to patient size! One size does not fit all Use automatic exposure control when appropriate! Understand how dose modulation works Do not overly rely on the manufacturer! Mfg suggested techniques tend to be high
What Can Manufacturers Do? Utilization of low dose techniques Improve AEC algorithms GE Auto ma Siemens Care Dose Philips DoseRight ToshibaSureExposure Prompt user attention to: High CTDI and DLP Alarm or scanner disable feature for doses above certain threshold
What Can Physicians do? Reduce unnecessary scans Provide written CT protocols to technologists Use ALARA principles when establishing protocols Approval by Radiation Safety Committee When possible actively participate in the CT scan procedure Monitor and report dose on all CT scans during time of study interpretation to detect and correct errors
Dose Limits NCRP is working on acceptable range of CTDI vol and DLP Europeans have a reference guideline Use in-house dose references Inform radiation safety officer of unusual values