Computed Tomography Dose Index, Includes various CTDI parameters and the way of calculating effective dose from various Computed Tomography procedures along with their conversion factor.
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Computed Tomography Dose Index
1. Dose represents the amount of energy deposited
in tissue from radiation per mass of tissue, and is
measured in J/kg = Gray (Gy).
Unfortunately, this energy damages the tissues it hits.
Consequence:
Deterministic
effect:Erythema,
Ulceration, necrosis
Stochastic Effects:
cancer
2. ā¢ Deterministic effects relate directly to the amount of radiation a
single cell receives; these require a large dose to become apparent.
ā¢ Stochastic effects can occur (randomly) with very small doses, and
even one cell can turn cancerous.
Generally speaking, in CT we are mostly
interested in the stochastic effects.
4. How is radiation dose measured in CT?
ā¢ CT dose is not measured directly on patient.
ā¢ CT dose is measured using standard phantoms.
ā¢ Measurements are then used to estimate patient dose.
6. How is Radiation dose measured?
CTDI 100 CTDI W CTDI VOL DLP Effective
dose
Dose
Distribution
Pitch
Scan
Length
Computed Tomography Dose Index (CTDI)
Measurement done at the centre of phantom of a single slice
7. CTDI
ā¢ Obtained by making
measurement in acryllic
cylinder phantom.
ā¢ Holes in centre and periphery
ā¢ Placement of pensil shaped
ionization chamber.
8.
9.
10.
11.
12. CTDI100 ā Dose Distribution
In CT: the highest dose delivered is at the
periphery
Dose is uniform on the surface but
decreases towards centre.
13.
14.
15.
16. ā¢ CTDI VOL :15mGy
ā¢ Independent of scan
length
17. Computed Tomography Dose Index (CTDI):
Measurement done at the centre of phantom
of a single slice:
cannot accommodate the anatomy covered
because we do just confine to single slice.
18.
19.
20. ā¢Volume CTDI in mGy (CTDI VOL):
ā¢X ray tube voltage (KV)
ā¢X ray tube current (mA)
ā¢X ray tube rotation time (s)
ā¢CT Pitch (P)
ā¢Phantom size (S or L)
21. ā¢ CTDI is the rate at which
you the energy is put on
the patient.
ā¢ DLP is total amount of the
energy that you are
putting the patient.
CTDI = Radiation Intensity
DLP = Total radiation used to perform a CT Scan.
22.
23.
24. CTDI Phantoms
ā¢ 16cm (S) (Head Phantom)
ā¢ 32cm (L) (Body Phantom)
Large CTDI is =
2 X Small CTDI
29. CTDI VOL āUniversal Parameterā
ā¢ CTDI VOL for Head = 60mGy on Small Phantom
ā¢ KV
ā¢ X ray filter: Head, Body or pediatric
ā¢ Model: Type of CT Scanner
ā¢ Vendor: Philips/ Toshiba/ GE
ā¢ CTDI vol >>> mAs !
31. ā¢ CTDI Vol is the radiation incident on patient, not the patient dose.
ā¢ CTDI VOL = 20mGy:
CTDI Vol is not the
indicator of any kind of
patient dose.
33. ā¢ Organ dose can be calculated. It can help to predict the
corresponding organ dose.
ā¢ Monte Carlo Simulation: The most complete computational method
for estimating organ and tissue doses is based on Monte Carlo
simulations. The simulations account for many scanner and technique
specifics,including scanner geometry, bow-tie filtration, beam
collimation, tube potential, and current as well as the CT dose index
(CTDI).
34. ā¢ Jones and Shrimpton used a simulated hermaphroditic patient (MIRD-
5 phantom) having mathematically modeled organs and tissues .
ā¢ The mathematic phantom was divided from head to mid thigh into
208 axial slabs of 5 mm thickness. Then, accounting for tube voltage
and using CT scannerāspecific data for geometry and beam shaping,
they simulated a
ā¢ CT scan and calculated absorbed doses to all organs of the body for
the irradiation of each axial slab. Summing contributions from all
slabs exposed during a particular CT examination yielded the total
organ doses.
35. Effective Dose
ā¢ Effective dose is a parameter meant to reflect the relative risk from
exposure to ionizing radiation.
ā¢ The effective dose (E) is a measure of the risk of cancer induction in
the patient from the effects of the radiation.
ā¢ It takes into account the total amount of absorbed dose received and
averages it to give a whole body effective dose.
36. Method 1: Using Organ dose estimates and
ICRP 26, 60, 103
ā¢ Tissue-weighting factors are meant to represent the relative radiation
sensitivity of each type of body tissue as determined from population
averages over age and sex and are derived primarily from the atomic
bomb survivors cohort.
ā¢ For partial-body irradiation, effective dose is the weighted summation
of the absorbed dose to each specified organ and tissue multiplied by
the ICRP-defined tissue-weighting factor for that same organ or
tissue.
37. Revisions were intended to reflect
advances in knowledge about the
radiation sensitivity of various
organs and tissues.
Tissue-weighting factors are meant
to represent the relative radiation
sensitivity of each type of body
tissue as determined from
population averages over age and
sex and are derived primarily from
the atomic bomb survivors cohort
38. ā¢ E = Effective Dose
ā¢ T = all ICRP specified tissue and organ
ā¢ W t = ICRP specified tissue weighting factor
ā¢ H t = Dose to particular organ or tissue
ā¢ E T = overall tisue
ā¢ Ez = Overall irradiated Slabs
39. Using DLP and K Coefficients from the
European Guideline
ā¢ E = k Ć DLP,
ā¢ where k
coefficient
is specific
only to the
anatomic
region
scanned.
40. Measuring Effective Dose
Effective Dose in NCCT Head: (1100X 0.0021 ) + 4.4 X 0.0021 = 2.31
msv
Effective Dose in CT IVU: ((771.7 X 3) + 4.7 ) X 0.015 = 34.79 msv
47. CT Dosimetry
CTDI 100 Effective dose
DLP
CTDIVOL
CTDIW
Measurement done on a
standard phantom using 100 cm
chamber.
Taking into account the distribution
variation based on large and small
patient.
When the scan involves pitch.
Taking account of the whole length
exposure
Risk estimation to the body
48. Automatic Tube Current Modulation
ā¢ mA is varied: Around the patient, Along the patient(scan length), &
between patient
49. Quality Reference mAs
ā¢ Quality Reference mAs = Effective mAs
ā¢ Effective mAs = mAs/pitch
ā¢ Once set in protocol and is different for different body region.
ā¢ Can convert Effective mAs in CTDI Vol i.e the Universal parameter.
ā¢ Modulation: Five Strength
52. Refrences
ā¢ Estimating Effective Dose forCT Using DoseāLength Product Compared With Using Organ Doses:
Consequences of Adopting International Commission on Radiological Protection Publication 103
or Dual-Energy Scanning, Jodi A Christner, Mayo Clinic, Available from www.ajronline.org,
DOI:10.2214/AJR.09.3462
ā¢ Videos from Walter Huda, available on Youtube.
Editor's Notes
In Radiography and Fluroscopy the entrance skin dose is higher and the radiation dose decreases as it pass through the patient. In CT the surface dose is maximum compared to centre.
More like a bell curved, peak dose is delivered only to certain central slice. There is more the tail portion
Larger the anatomy exposed , greater the biological risk.
For the same amount of incident raiation, the newborn infant and children dose will be quite large, whereas for an adult due to size of attenuation due to large mass, their dose will be much lower.