Dual Energy CT:Approaches and Applications

Dianna Cody, Ph.D.

Professor

Department of Imaging Physics

Objectives• How does it work?

– Approach by vendor

• What is it good for?– Established applications

– Emerging applications

Dual Energy CT• Recycled idea from 1980’s

• Bone QCT Densitometry

• Acquisition of attenuation data at two distinct energy levels

• Produce two sets of images

• Data processing for basis materials

Material DecompositionInterleaved High- and

Low-kVp ProjectionsData Acquisition

Low kVp Projections High kVp Projections

80 kVp 140 kVp

split

Filtered Back Projection

40keV –140keV

....)()()()()()()()()( 1

2

1

2

1111 iPiPiPiPiPiiPiiP highlowhighlowhighlow

....)()()()()()()()()( 2

2

2

2

2222 iPiPiPiPiPiiPiiP highlowhighlowhighlow

water

iodine

FBP

Attenuation-to-material density transformation

Monochromatic Generation

Iodine Density Water Density

70 keV

Iodine Projections Water Projections

Blended Images

How to acquire data at two energies? Varies by vendor

• GE – FAST kVp switching

• Philips – Two layer detector

• Siemens – Two x-ray tubes

• Toshiba – Two sequential rotations

Important Issues

• Temporal match of data acquired at separate kVs

• Adjust mA for kV? (Increase for low kVp, decrease for high kVp)

• Field of View limitations?

• Dose impact?

• Post-processing options

GE Approach

• Newest scanner model only (HD750)

• Fast kVp switching: 140 kV & 80 kV

• Full field of view

• 1/3 projections obtained at 140 kV

• 2/3 projections obtained at 80 kV

• Excellent temporal matching

• “Tissue Signature” function

GE Approach - Dose• Measured on HD750 June 2009

• 120kVp, 300mAs, 40mm beam– CTDIw 13.7 mGy

• 80 & 140 kVp, 600mAs, 40mm beam– CTDIw 31 mGy

• DE dose about 13% higher than 120kVp predicted by mAs change alone

Dose Penalty?• YES – difficult to obtain dual energy CT

images at same dose as single average 120kVp scan

• BUT – if standard practice is C-/C+ scans, a single pass dual energy CT scan may provide virtual non-contrast images

• Small dose increase may justify the additional information obtained

Philips Approach• Two-layer detector

• No extra x-ray tube technology

• Excellent temporal matching

• Full field of view

• Dose?

Siemens Approach

• Available on Definition

• Field of view limited (depends on model)

• Two x-ray tubes operated simultaneously at different kVps

• Adjust mA value independently

• Temporal matching somewhat limited (90° out of sync)

• Cross-scatter

• Dose?

Toshiba Approach• Dual rotations

• One at low kVp, next at high kVp

• Then move forward

• Limited to axial mode?

• Adjust mA for kVp?

• Temporal matching worst-case

• Full Field of view

• Dose?

Established Applications

Kidney Stone Characterization

Automated Bone Removal (Angiography)

Potential Applications

Liver & Kidney Lesion Characterization

75 keV - With Contrast

Images Courtesy of Dr. Amy Hara, Mayo Clinic, Scottsdale, AZ

Water Density(virtual non-contrast)

Iodine Density

Liver Fat Quantification

Virtual Non-Contrast Images

Calcified Plaque Processing

Pulmonary Embolism Detection

Metal Artifact & Beam Hardening Reduction

80 kVp 80 keV

Metal Artifact Reduction

140 kVp 70 keV w/MAR

80kVp

140kVp

80keV

More?• Improve detection of small pancreatic

lesions

• Discriminate between mucinous and nonmucinous pancreatic cystic lesions

• Characterize solid lesions (tumor vsfibrosis)

• Proton dose estimation in spine (fatty replacement of bone marrow)

Tissue Signature

70 keV (mono)

m1 (water) m2 (iodine)

Tissue Signature

• Optimize imaging parameters for new targeted tracers?

• Useful for discriminating solid lesions?

• Other applications yet to emerge?

Summary

• Vendors have diverged in approach to dual energy CT

• Advantages & Disadvatages

• Established applications

• MANY emerging applications…!

END• Questions?

• Dianna Cody, Ph.D.

• dcody@mdanderson.org