Computed Tomography

Technique Parameters versus Radiation Dose and Image Qualitiy

Sofia Skyttner, MSc
Creative Commons-licens

Radiation Dose in CT

CTDIvol x L = DLP

  • CTDIvol, Volume Computed Tomography Dose Index
  • L = Total Scan Length [cm]
  • DLP = Dose Length Product [mGy cm]

CTDIvol:

Mean absorbed dose indicator for cylindrical PMMA phantoms of either 16 cm or 32 cm in diameter

Image Quality in CT

You want Parameter change Negative consequence
Better contrast ? ?
Better resolution ? ?
Less noise ? ?
Faster scan ? ?
No artifacts ? ?

From tube to image

CT-factors
Factors affecting radiation dose & image quality CT-factors-text

X-ray

tube voltage, tube current, scan time, helical pitch
kv-vs-noise
kV versus noise

X-ray

tube voltage, tube current, scan time, helical pitch
kv-vs-dose
kV versus dose

X-ray

tube voltage, tube current, scan time, helical pitch
kv-vs-contrast
kV versus contrast

X-ray

tube voltage, tube current, scan time, helical pitch
mAs-vs-dose
mAs versus dose

X-ray

tube voltage, tube current, scan time, helical pitch
mAs
mAs = mA * s

X-ray

tube voltage, tube current, scan time, helical pitch
mA-vs-dose
mAs = mA * s

X-ray

tube voltage, tube current, scan time, helical pitch
time-vs-dose
mAs = mA * s

X-ray

tube voltage, tube current, scan time, helical pitch
mAs-vs-noise
mAs versus noise

X-ray

tube voltage, tube current, scan time, helical pitch
axial-versus-spiral
Pitch = lenght scanned per rotation / total collimation
Z-interpolation

X-ray

tube voltage, tube current, scan time, helical pitch
ct-overscanning
Overscanning

From tube to image

CT-factors
Factors affecting radiation dose & image quality CT-factors-text

Filtration

bowtie, beam quality
bowtie_scan_field_of_view Left: SFOV 16 cm, Right: SFOV 32 cm

Filtration

bowtie, beam quality

Go to:

http://rpubs.com/sofiaskyttner/ctartifacts

and previous presentations…

From tube to image

CT-factors
Factors affecting radiation dose & image quality CT-factors-text

Patient

size, shape, centering, anatomy
ct-dose-distribution Left: CT image (32 cm), right: dose distribution (32 cm)

Patient

size, shape, centering, anatomy
different-ctdi-same-parameters
Read more: Estimated pediatric radiation dose during CT, Keith et al, Ped Rad 41(Suppl 2):S472–S482 (2011)

Patient

size, shape, centering, anatomy
diameter-vs-dose
Mean absorbed dose vs body diameter

Patient

size, shape, centering, anatomy
ssde-phantom-sizes
Future: Size-Specific Dose Estimate = SSDE = CTDIvol x factor

Patient

size, shape, centering, anatomy
ctdi-vol-AEC
CT Thorax Abdomen

Patient

size, shape, centering, anatomy
CTDI-vs-mean-body-thickness
CT Thorax Abdomen

Patient

size, shape, centering, anatomy

Go to:

http://rpubs.com/sofiaskyttner/ctartifacts

From tube to image

CT-factors
Factors affecting radiation dose & image quality CT-factors-text

Detection

slice width, total collimation
CT-overbeaming
Overbeaming

Detection

slice width, total collimation
slice-vs-dose
Slice width versus dose, keeping noise constant

Detection

slice width, total collimation
slice-vs-resolution
Slice width versus resolution

Detection

slice width, total collimation
slice-vs-noise
Slice width versus noise, keeping dose constant

From tube to image

CT-factors
Factors affecting radiation dose & image quality CT-factors-text

Recon

algorithm, helical slice width, helical weighting, special filters
recon-kernel
Reconstruction algorithm (kernel) vs noise

Recon

algorithm, helical slice width, helical weighting, special filters Spiral CT

Go to:

http://rpubs.com/sofiaskyttner/ctartifacts

From tube to image

CT-factors
Factors affecting radiation dose & image quality CT-factors-text

Post processing

DFOV, image space filters, 3D, MPR, MIPs, …

Your choice of DFOV will affect your pixel size!

Example

  • Matrix: 512 x 512 pixels
  • Wanted resolution: 0.7 mm

What DFOV should YOU choose…?

Post processing

DFOV, image space filters, 3D, MPR, MIPs, …

Optimal pixel size = half of wanted resolution

E.g.

  • Standard resolution, 0.7 mm…? Pixel size: 0.35 mm
  • High resolution, 0.5 mm…? Pixel size, 0.25 mm

But, not less…
…or risk of increased noise & image reconstruction artifacts!

Post processing

DFOV, image space filters, 3D, MPR, MIPs, …

Matrix: 512 x 512 pixels
Resolution: 0.7 mm

Optimal DFOV = optimal pixel size x matrix =

= 0.35 x 512 =
= 179 mm =
= 18 cm (rounded off for convenience)

Post processing

DFOV, image space filters, 3D, MPR, MIPs, …

Another time…

Summation

Improving one parameter…
…will worsening another…!

CT-factors

Summation

Improving one parameter…
…will worsening another…!

Example:

You want Parameter change Negative consequence
Better contrast Lower kV More noise
Better resolution Thinner slice thickness More noise
Less noise Higher mA More dose
Faster scan Lower rot.time max mAs affected
Faster scan Higher p Helical artifacts

Bonus material

The DLP formula

DLP-formula

nCTDIw spec for: kV, phantom, rotation time, per mAs

The DLP formula in detail

DLP-härledning