“Dosimetry and optimisation of Cone Beam CT for dentomaxillofacial applications”

Safety and Efficacy of a New and Emerging Dental X-ray Modality

Cone Beam CT

Ruben PauwelsOral Imaging CenterFaculty of Medicine

• Introduction CBCT• SEDENTEXCT project• CBCT dosimetry

Effective dose (phantom)Standardised dose index

• CBCT OptimisationQC phantom development & analysis

• Work in progress

• Introduction CBCT

Cone Beam CT / Digital Volume Tomography

• Dentomaxillofacial • Radiotherapy• Cardiac imaging

Cone Beam CT / Digital Volume Tomography

• Dentomaxillofacial

Cone Beam CT / Digital Volume Tomography

• Dentomaxillofacial

Cone Beam CT / Digital Volume Tomography

Advantages• 3D (isotropic voxels)• Detail (voxel size 0.08-0.4mm)• Cost, size, ease of use, … • Applications• ‘Low dose’ (generally DE=20-100 µSv)• Viewing possibilities

Cone Beam CT / Digital Volume Tomography

• Dentomaxillofacial

Can CBCT replace traditional radiographic methods?

Can CBCT replace traditional radiographic methods?

• Excessive use: collective dose ↑• Used in primary care (by dentists)• Optimisation? ALARA?

Justification Optimisation (dose & image quality) Referral criteria Training

• Introduction CBCT• SEDENTEXCT project

WP 0 Financial and administrative management

WP 1 Justification and Guideline development

WP 2 Dosimetry

WP 3 Optimisation

WP 4 Diagnostic accuracy

WP 5 Cost effectiveness

WP 6 Training and Valorisation

• Introduction CBCT• SEDENTEXCT project• CBCT dosimetry

WP 2: Dosimetry

Patient dosimetry

Protection of personnel

Standardised dose index

Phantom effective dose

In vivo dose

Mathematical models

• Introduction CBCT• SEDENTEXCT project• CBCT dosimetry

Effective dose (phantom)

Which organs are the major contributors to the effective dose for dental CBCT imaging?

Which acquisition parameters influence the effective dose, and in what way?

kV, mAs, filtration, …FOV, ROI

How can we use this info to reduce patient dose?

WP 2: Dosimetry

WP 2: Dosimetry• Thermoluminescent dosimeters (LiF:Mg,Ti)• Harshaw Reader 6600

WP 2: Dosimetry• Calibration: C → gU → Gy

WP 2: Dosimetry• Alderson Radiation Therapy phantom• 147 organ TLDs, 10 background TLDs, 20 calibration TLDs • ICRP 103 tissue weighting factors

WP 2: DosimetryOrgan ICRP 60 ICRP 103

Gonads 0,2 0,08

Red bone marrow 0,12 0,12

Colon 0,12 0,12

Lung 0,12 0,12

Stomach 0,12 0,12

Bladder 0,05 0,04

Breast 0,05 0,12

Liver 0,05 0,04

Oesophagus 0,05 0,04

Thyroid 0,05 0,04

Skin 0,01 0,01

Bone surface 0,01 0,01

Brain 0 0,01

Salivary glands 0 0,01

Remainder 0,05 0,12

WP 2: Dosimetry

Scanora 3D

Galileos

Picasso Trio

ProMax 3D

Kodak 9000 3D

WP 2: DosimetryScanora 3D Upper jaw 100x75mm

Upper and lower jawLower jaw

Galileos Upper and lower jaw 150x150mm

Kodak 9000 3D Lower jaw (part) 50x37mm

Picasso Trio Lower jaw 120x70mm

ProMax 3D Lower jaw 80x80mm

WP 2: Dosimetry

WP 2: Dosimetry

WP 2: Dosimetry

WP 2: Dosimetry

WP 2: Dosimetry

WP 2: Dosimetry

WP 2: Dosimetry

WP 2: Dosimetry

OrganContribution (%)to effective dose

Red bone marrow 15

Thyroid 44

Skin 1,4

Bone surface 1,5

Brain 1,1

Salivary glands 36,9

Large dose range Thyroid & salivary glands are crucial FOV reduction should always be considered mAs should be selected in function of diagnostic

needs Thyroid shielding & avoidance

Ideal CBCT practice?→ FOV selection (single tooth, …, full head)→ Resolution (=mAs) selection→ Training!!!

WP 2: Dosimetry

• Introduction CBCT• SEDENTEXCT project• CBCT dosimetry

Effective dose (phantom)Standardised dose index

Routine measurements → predict effective dose Conversion factors

(input: dose index + positioning) Existing methods (CTDI) Also include:

- patient (skin) entrance dose- simulations

WP 2: Dosimetry

WP 2: Dosimetry

WP 2: Dosimetry

Typically available dosimeters

WP 2: Dosimetry

Central measurement?

Ionization chamber

WP 2: Dosimetry

Accuracy?

TLD

WP 2: Dosimetry

Sensitivity?Calibration?

Film

WP 2: Dosimetry

FOV size and shapeUniform Distribution?(Practical) application for CBCT?

DAP

WP 2: Dosimetry→ Combine measurements→ Assesment of distribution (anthropomorph / homogeneous)

• Introduction CBCT• SEDENTEXCT project• CBCT dosimetry

Effective dose (phantom)Standardised dose index

• CBCT Optimisation

Technical image quality assessment Image quality vs. Radiation dose! Diagnostical performance < technical parameters Optimising beyond available options Optimising on others levels: also in progress

Monte Carlo simulations (G.Zhang, Leuven)Reconstruction (e.g. artefact reduction)

WP 3: Optimisation

What are the most relevant image quality parameters in dentomaxillofacial imaging, and how are they best measured using a test object?

How do CBCT devices relate to each other for these parameters?How do CBCT pixel values correlate to actual densities and CT

numbers (Hounsfield Units)? Are pixel values uniform throughout the reconstructed volume? Which are the most relevant acquisition parameters affecting the stability and linearity of pixel values?

Are CBCT datasets truly isotropic? How can metal artefacts be evaluated and which parameters

influence their appearance?

WP 3: Optimisation

WP 3: Optimisation

Development of QA tools

PMMAcylinder

Inserts

WP 3: Optimisation• Spatial resolution (XY and Z)• Point spread function• Line spread function• Contrast resolution • CT number• Artefact analysis• Geometric accuracy (XY and Z)• Homogeneity • Uniformity• Dose?

Scanora 3D

Galileos

Picasso TrioProMax 3D

Kodak 9000 3D

WP 3: Optimisation

3D Accuitomo

i-CAT SkyView

WP 3: Optimisation• Spatial resolution (XY and Z)• Point spread function• Line spread function• Contrast resolution • CT number• Artefact analysis• Geometric accuracy (XY and Z)• Homogeneity • Uniformity• Dose?

WP 3: Optimisation

WP 3: Optimisation

WP 3: Optimisation

XY plane

Resolution < 3 lp/mm Artefacts Saturation XY plane

→ line pair pattern on PMMA background

WP 3: Optimisation

WP 3: Optimisation• Spatial resolution (XY and Z)• Point spread function• Line spread function• Contrast resolution • CT number• Artefact analysis• Geometric accuracy (XY and Z)• Homogeneity • Uniformity• Dose?

WP 3: Optimisation

0.1 mm 0.25 mm

Steel wire in air

WP 3: Optimisation

WP 3: Optimisation

i-CAT HR

mm5,24,84,443,63,22,82,421,61,20,80,40

pixe

l val

ue

0,24

0,22

0,2

0,18

0,16

0,14

0,12

0,1

0,08

0,06

0,04

0,02

0

Cauchy distribution

WP 3: Optimisation3D Accuitomo

mm4,64,44,243,83,63,43,232,82,62,42,22

pixe

l val

ue

0,13

0,12

0,11

0,1

0,09

0,08

0,07

0,06

0,05

0,04

0,03

0,02

0,01

0

Galileos

mm5,65,24,84,443,63,22,82,421,61,20,80,40

pixe

l val

ue

0,4

0,36

0,32

0,28

0,24

0,2

0,16

0,12

0,08

0,04

0

i-CAT LR

mm6,45,64,843,22,41,60,80

pixe

l val

ue

0,26

0,24

0,22

0,2

0,18

0,16

0,14

0,12

0,1

0,08

0,06

0,04

0,02

0

WP 3: OptimisationKodak 9000 3D

mm65,65,24,84,443,63,22,82,421,61,20,80,40

pixe

l val

ue

0,064

0,056

0,048

0,04

0,032

0,024

0,016

0,008

0

Scanora 3D

mm65,65,24,84,443,63,22,82,421,61,20,80,40

pixe

l val

ue

0,13

0,12

0,11

0,1

0,09

0,08

0,07

0,06

0,05

0,04

0,03

0,02

0,01

0

SkyView LR

mm5,24,84,443,63,22,82,421,61,20,80,40

pixe

l val

ue

0,18

0,16

0,14

0,12

0,1

0,08

0,06

0,04

0,02

0

WP 3: Optimisation

Picasso Trio

mm65,65,24,84,443,63,22,82,421,61,20,80,40

pixe

l val

ue

0,3

0,28

0,26

0,24

0,22

0,2

0,18

0,16

0,14

0,12

0,1

0,08

0,06

0,04

0,02

0

SkyView HR

Histogram Cauchy

mm5,65,24,84,443,63,22,82,421,61,20,80,40

pixe

l val

ue

0,16

0,14

0,12

0,1

0,08

0,06

0,04

0,02

0

i-CAT HR

mm5,24,84,443,63,22,82,421,61,20,80,40

pixe

l val

ue

0,24

0,22

0,2

0,18

0,16

0,14

0,12

0,1

0,08

0,06

0,04

0,02

0

WP 3: Optimisation

WP 3: Optimisation• Spatial resolution (XY and Z)• Point spread function• Line spread function• Contrast resolution • CT number• Artefact analysis• Geometric accuracy (XY and Z)• Homogeneity • Uniformity• Dose?

WP 3: OptimisationPTFE cube

PMMA surrounding

ESF↓

LSF↓

MTF

Gumbel distribution

WP 3: Optimisation

i-CAT HR

mm1211109876543210

pixe

l val

ue

0,03

0,028

0,026

0,024

0,022

0,02

0,018

0,016

0,014

0,012

0,01

0,008

0,006

0,004

0,002

0

Gumbel distribution

WP 3: OptimisationGalileos

mm1211109876543210

pixe

l val

ue

0,072

0,064

0,056

0,048

0,04

0,032

0,024

0,016

0,008

0

i-CAT HR

mm1211109876543210

pixe

l val

ue

0,03

0,028

0,026

0,024

0,022

0,02

0,018

0,016

0,014

0,012

0,01

0,008

0,006

0,004

0,002

0

i-CAT LR

mm1211109876543210

pixe

l val

ue

0,06

0,056

0,052

0,048

0,044

0,04

0,036

0,032

0,028

0,024

0,02

0,016

0,012

0,008

0,004

0

Kodak 9000 3D

mm131211109876543210

pixe

l val

ue

0,028

0,026

0,024

0,022

0,02

0,018

0,016

0,014

0,012

0,01

0,008

0,006

0,004

0,002

0

WP 3: OptimisationScanora 3D

mm1211109876543210

pixe

l val

ue

0,026

0,024

0,022

0,02

0,018

0,016

0,014

0,012

0,01

0,008

0,006

0,004

0,002

0

SkyView HR

mm1211109876543210

pixe

l val

ue

0,1

0,09

0,08

0,07

0,06

0,05

0,04

0,03

0,02

0,01

0

SkyView LR

mm1211109876543210

pixe

l val

ue

0,09

0,08

0,07

0,06

0,05

0,04

0,03

0,02

0,01

0

Picasso Trio

mm1211109876543210

pixe

l val

ue

0,032

0,028

0,024

0,02

0,016

0,012

0,008

0,004

0

WP 3: Optimisation

WP 3: Optimisation• Spatial resolution (XY and Z)• Point spread function• Line spread function• Contrast resolution • CT number• Artefact analysis• Geometric accuracy (XY and Z)• Homogeneity • Uniformity• Dose?

WP 3: Optimisation• Air• Aluminium• Hydroxyapatite (50, 100, 200)

WP 3: Optimisation• Spatial resolution (XY and Z)• Point spread function• Line spread function• Contrast resolution • CT number• Artefact analysis• Geometric accuracy (XY and Z)• Homogeneity • Uniformity• Dose?

WP 3: OptimisationCNR = MPV1-MPV2 (SD1+SD2)/2

WP 3: Optimisation

WP 3: Optimisation

WP 3: Optimisation

• Pixel value: relation HA 50,100,200?• Work in progress

WP 3: Optimisation

WP 3: Optimisation

WP 3: Optimisation• Spatial resolution (XY and Z)• Point spread function• Line spread function• Contrast resolution • CT number• Artefact analysis• Geometric accuracy (XY and Z)• Homogeneity • Uniformity• Dose?

WP 3: OptimisationTitanium rods

WP 3: Optimisation

WP 3: Optimisation

WP 3: OptimisationSmoothing + subtracting background

2 ROIs, ignore black streaks

WP 3: OptimisationMeasure along intervals, keep extreme values

Distribute (e.g. Gumbel) and evaluate parameters

ProMax 3D

84 kV 9 mA

ProMax 3D

84 kV 14 mA

WP 3: Optimisation→ Inter-device variability→ Intra-device variability?

WP 3: Optimisation• Spatial resolution (XY and Z)• Point spread function• Line spread function• Contrast resolution • CT number• Artefact analysis• Geometric accuracy (XY and Z)• Homogeneity • Uniformity• Dose?

WP 3: Optimisation

Cfr. spatial resolution

WP 3: Optimisation• Spatial resolution (XY and Z)• Point spread function• Line spread function• Contrast resolution • CT number• Artefact analysis• Geometric accuracy (XY and Z)• Homogeneity • Uniformity• Dose?

WP 3: Optimisation

WP 3: Optimisation

Small FOV (truncation, …) Non-homogeneous slices

• Introduction CBCT• SEDENTEXCT project• CBCT dosimetry

Effective dose (phantom)Standardised dose index

• CBCT OptimisationQC phantom development & analysis

• Work in progress

Dose index Effective dose / organ dose analysis (+paediatric) QC phantom: second prototype Image analysis: artefacts, LSF, MTF Pixel homogeneity / uniformity Compare devices & evaluate differences