Advances in Computed Tomography - PowerPoint Presentation

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Advances in Computed Tomography

Dr. Hussein Ahmed Hassan

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Contents

CT Hardware: An Update

,

Dual Energy CT

CT Perfusion Imaging

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Dual Energy CT

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INTRODUCTION

Conventional CT scanners, operating at a single energy, provide morphologic imaging only, with little material-specific information in body imaging.

Dual energy CT utilizes the principle that different materials show different attenuation at varying energy levels, and this difference in attenuation can be used for tissue characterization.

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Dual energy CT

scans

are

a

relatively new form

of

CT

scanning

that use

separate

X-ray

energies to make images. Images can be generated:

by the

simultaneous

use

of

two

X-ray

tubes (‘dual

source

’

)

;

by using an

X-ray

detector with

separate layers

to detect two different

energy ranges

(‘dual layer’);

or

by using

a single scanner

to

scan

twice using

two different

energy levels (electronic

kVp

switching

)

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A to C Dual energy scanners

the first generation DS-DECT

scanners (A), the second

generation DS-DECT scanner

(B) and SS-DECT scanner (C).

In a first generation DS-DECT

scanner, two tubes are placed

in the gantry at an angle of 90°

to each other. Tube A has

a higher

kVp

(140) and had a larger detector array of FOV 50 cm. The smaller detector array (26 cm FOV)

is paired against the lower energy tube (80 KV). In a second generation DS-DECT scanner, the smaller detector (FOV 33 cm) is paired with a higher energy tube with

a selective photon shield. The SS-DECT scanner has a single tube and a detector array with 50 cm FOV. There is rapid switching of kVp in the tube

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Advantages

CT

angiography.

Dual energy

scans

can amplify the iodine

signal of

contrast agents, improving the delineation

of

arteries.

They

can also

better

distinguish iodine from

calcium, therefore allowing better

bone subtraction around vessels; for example, at the skull

base.

CT of the kidney, ureter and bladder (CT KUB). Dual energy CT KUB scans

can reliably distinguish urate from non-urate

calculi.CT imaging around metal implants. Dual energy CT can significantly reduce the streak artefact normally associated with metal implants and allow

better visualisation; for example, around spinal rods

or hip replacements.

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With

fast

kV-switching, Dual Energy data can be acquired by rapidly

switching

the

tube

voltage between CT projections.

Disadvantages

lower

number

of

projections are available to create each image; reduced image

quality

In addition,

only

the kVp can be modulated

between

individual projections.

Resulting

over-

exposure in the highkV projections

or

under-exposure in the low-kV

projections

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Idealized dual

layer

detector technology:

In reality a

certain

amount of

high and low-energy photons are

registered

in both

layers

which

significantly

reduces spectral separation

DisadvantagesDetector

not able to distinguish between high and low energy photons. Both high and low energy photons

are absorbed in both layers

The construction of this detector requires two photodiodes, which significantly increases electronic noise. Leads to inferior image

quality for dual and single energy images.

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Slow

kV-switching: Both

kV

and

mA

are

switched

between half rotations

of

the

gantry,

either in

sequence or in spiral

modesDisadvantages

The time needed to

switch from 80 kV to 140 kV and adjust the mA is typically in the order of 100

ms.During

this time, the patient is exposed to radiation that does not provide useful

information.Thus,

this method does not follow the ALARA

(“as low as reasonably achievable”) principle.

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Dual

Energy

imaging means that the

system

uses

two

X-ray

sources simultaneously

at different energy

levels.

This

makes it possible to

differentiate

between fat, soft

tissue,

and

bone,

and

also

between

calcifications

and contrast material

(iodine) on

the basis

of

their unique

energy-dependent

attenuation

profiles.

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Image Display in DECT

Images generated in DECT can have two types of display:

Material density display

In DECT, the material density display can be iodine density display or water density display.

Monoenergetic

image display .The

monoenergetic

or

pseudomonochromatic

display are energy-specific display. Images are processed at any given

kVp

from the dual energy data sets, which resemble images physically acquired after scanning at that given kVp.

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A and B Material density display. Iodine overlay map (A), created by overlapping the iodine density images over

monoenergetic

image, highlight the organs containing iodine (colored red). The gradient of color varies according to the degree of enhancement of the organ. The water density display (B) is equivalent to virtual unenhanced image. Note that both calcium and iodine appear dense on a routine single energy CT, but they can be differentiated on iodine map images as calcium will not be highlighted on a iodine map image (for example, the calculus in this image)

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A and B Simple hepatic cysts, imaging performed on a second generation DS-DECT scanner. Simulated

monoenergetic

image display at 70 kV generated from a dual energy dataset (A) shows

hypodense

focal lesions in segment III and V. Iodine overlay maps (B) show them to be dark (not taking up iodine)

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Virtual

monoenergetic

images generated from a DS-DECT scanner. It is evident from the images that the attenuation of iodine and the image contrast are high at lower energy images, although the images appear more noisy. As the

kVp

increases, the contrast difference between various tissues becomes negligible and the 160 kV image becomes identical to a

noncontrast

image

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A and B Renal calculi characterization by DECT. Images acquired at 100 and 140 kV in a DS-DECT scanner. A reference dual energy ratio of 1.13 is taken to differentiate uric acid from

nonuric

acid calculi. The uric acid calculus (A) shows a dual energy ratio less than 1.13 and are colored red (arrow). The

nonuric

acid calculi show a dual energy ratio above the reference line and are colored blue (B)

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A to E Focal hepatic

steatosis

. The area of focal fat deposition is seen as a

hypodensity

in segment 4b adjacent to the

falciform

ligament (arrow in A).

Monoenergetic

display (B) derived from a DS-DECT scanner (

Somatom

Definition Flash, Siemens) with tin filtration shows the increase in attenuation in higher kV images ( attenuation of 23.8 at 70 kV, shown with a yellow line and 34.2 at 140 kV, shown with a red line) (C), which is suggestive of fatty change. In comparison, normal enhancing liver parenchyma shows a decrease in attenuation (D, E) in the higher energy images (attenuation of 82.1 at 70 kV and 64.1 at 140 kV images)

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CT Perfusion Imaging

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Perfusion computed tomography

Perfusion computed tomography (CT) allows functional evaluation of tissue

vascularity

.

It measures the temporal changes in tissue density after intravenous injection of a contrast medium (CM) bolus using a series of dynamically acquired CT images

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Technique of CT Perfusion

Step I involves acquisition of unenhanced CT images to cover the entire region of interest.

Step II involves selection of the slice for dynamic imaging. The selected slices should be chosen to cover the maximum tumor area. The total tumor coverage area is 2 cm for 16MDCT and 4 cm for 64MDCT and up to 9 cm for 128 MDCT scanner.

Step III involves contrast enhanced dynamic image acquisition.

Step IV involves post-processing of CT data to generate colored perfusion maps of blood flow (BF), blood volume (BV), mean transit time (MTT) and permeability surface area product. Time attenuation curves showing the enhancement characteristics of the artery and tumor during the first pass and delayed phase of perfusion CT acquisition can be obtained.

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A to D Normal cerebral perfusion CT maps representing CBF (A), CBV (B), MTT (C), and TTP (D) respectively

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A and B Perfusion CT maps representing CBV (A) and MTT (B) in a patient showing a central infarct core (CBV = 1.2

mL

/100

mL

) in the region of the left basal ganglia with a large surrounding ischemic penumbra (

rMTT

= 210%) revealing CBVMTT mismatch (relatively preserved CBV with increase in MTT values)—case of acute ischemic stroke

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A and B

Noncontrast

axial CT images reveal large areas of subtle loss of

graywhite

matter differentiation in the left

frontoparietal

region—case of large hemispheric infarct

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Perfusion maps (same case as in

Noncontrast

axial CT images ) show extensive area of perfusion defects involving the left cerebral hemisphere with a central infarct core revealing severely reduced CBV and CBF values and surrounding areas of ischemic penumbra demonstrating relatively preserved CBV, moderate reduction in CBF and prolongation of MTT, TTP values

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CONCLUSION

Computed tomography hardware thus is a rapidly evolving field, constantly introducing newer technologies for the user as the vendors continue to innovate.

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Thank you