slide1 n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Introduction to Computerized Tomography Dr / Ibrahim Alshikh PowerPoint Presentation
Download Presentation
Introduction to Computerized Tomography Dr / Ibrahim Alshikh

Loading in 2 Seconds...

play fullscreen
1 / 30

Introduction to Computerized Tomography Dr / Ibrahim Alshikh - PowerPoint PPT Presentation


  • 64 Views
  • Uploaded on

Introduction to Computerized Tomography Dr / Ibrahim Alshikh. References for the present lecture. Historical perspective. CT scanner components. Generations of CT machine. CT number. Window level and width. Use of contrast medium. References for the present lecture.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Introduction to Computerized Tomography Dr / Ibrahim Alshikh' - lemuel


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
slide2

References for the present lecture

Historical perspective

CT scanner components

Generations of CT machine

CT number

Window level and width

Use of contrast medium

slide4

Historical perspective

Godfrey Hounsfield:

built the first CT scannerin 1972

Alan Cormack:

devised the mathematicalcomputations (1964)

by which an image could be reconstructed

from the data generated from the CT scanner

They were awarded the 1979 Nobel Prize for Medicine

slide5

Gantry :

(can tilt up to 30o)

Detector array

Patient support couch

X-ray source

Computer

Data acquisition time

Reconstruction time

Operating console

Gantry

Aperture

Table feed per rotation (not per sec)

Slice thickness

Pitch =

Patient support couch

Computed tomographic

scanner components

(Table)

Crystal scintillation detector (CsI, CdWO4)

-100%; can’t pack together

Gas-filled detector (Xe or Xe/Krypton)

-50% efficiency; pack together

Refs. 2, 3

slide6

Only accommodate a human head

Scan time for a single slice: 6 min

(4.5 min for image acquisition,

1.5 min for image reconstruction)

First – generation machine

Translate-rotate scanner

The first scanner:

A 13 mm slice with 3 line pairs/cm

spatial resolution

and used an 80x80 image matrix

slide7

The second scanner: drawback

Linear arrangement of detectors:

Detectors in the middle of array

were a different distance from

the radiation source than those

at the ends Increase the

scatter radiation and degrade

the image quality

Can accommodate the whole body

Scan time for a single slice: 20 second

Second– generation machine

Translate-rotate scanner

Detectors

Gantry

Gantry

Aperture

X ray tube

slide8

X-ray source

Fan beam

(Curved)

Detector array

Third-generation CT-scanner

slide9

Can accommodate the whole body

Scan time for a single slice:  one second

Curved detectors solve the differential

magnification problem of linear detectors

Ring artifact- if a single detector in the array was defective

X-ray tube

Third – generation machine

Rotate-rotate scanner

The third scanner:

Contain  30 detectors and

cover between 30o~60o with

a single projection

Detectors

Gantry

Gantry

Aperture

X ray tube

slide10

Can accommodate the whole body

Scan time for a single slice: < one second

Fourth – generation machine

Rotate-only scanner

The fourth scanner:

Detector array consisted of

several thousand elements &

provided 360o of coverage –

avoid the ring artifact

Detectors

Patient ‘s radiation dose is increased

Gantry

Gantry

Aperture

X ray

tube

slide11

Angulation

Gantry

X-ray tube

90o

270o

Table

movement

Detector

180o

Aperture

Conventional CT machine-

4th generation

slide12

X-ray tube

Gantry

Rotation

Step-wise

table

movement

3rd scan level

2nd scan level

1st scan level

Slices for conventional CT

In conventional CT, a series of equally spaced is required sequentially through a specific region, e.g. the head.

There is a short pause after each section in order to advance the patient table to the next preset position.

The section thickness & overlap/ intersection gap are selected at the outset.

The raw data for each image level is stored separately.

The short pause between sections allows the conscious patient to breathe without causing major respiratory artifacts

Ref. 3

slide13

Principal difference: patient couch moves

continuously during image taking

This movement produces image data for

a portion of a spiral

Scan time is further decreased by

increasing the pitch – affect image quality

Development of slip ring allows for continuous

movement of X-ray source – scan time is further

decreased because X-ray source can rotate faster

without the heavy cables

Fourth – generation machine

modification

Helical or Spiral CT

slide14

Slices for spiral CT

In spiral CT, images are acquired continuously while the patient table is advanced through the gantry.

The x-ray tube describes an apparent helical path around the patient.

If table advance is coordinated with the time required for a 3600 rotation (pitch factor) data acquisition is

complete and uninterrupted

X-ray tube

Imaging

volume

Gantry

This technique is helpful when data are reformatted to create other 2D views: sagittal, oblique, coronal or 3D

Continous

table

movement

slide15

1st generation CT

2nd generation CT

Multiple

pencil beam

Pencil beam

Multiple

detectors

Single detector

Summaries of generation of

CT machines

Third generation CT scanner:

Both the X-ray tube & detector

array rotate around the patient

Fourth generation CT scanner:

The X-ray tube rotates within a stationary ring of the detectors

Spiral CT: The X-ray tube & detectors move in a continuous spiral motion around the patient as the patient moves continuously into the gantry in the direction of the red solid arrows

slide16

Advantage of spiral technique

Conventional CT

Liver

Spiral CT

Advantage of spiral technique

Lesions smaller than the conventional thickness of a slice can be detected

Small liver metastases (7) will be not being included in the section

The metastases would appear in reconstructions from the dataset of the helical technique

slide17

X-ray source

Fan beam

attenuation

coefficients

Grey scale

Hounsfield

Scale

(CT no.)

(Curved)

Detector array

Attenuation Coefficient

(Attenuation)

Attenuation

coefficient

slide18

Attenuation coefficient and CT no.

for biological tissues at 60 keV

Attenuation

Coefficient

 (cm-1)

Tissue

CT Number (HU)

slide21

CT No.

Front

Frontal sinusmastoid air cellsCT number -1000Hu

CT number -80Hu

soft tissue,CT number 40Hu,

CT number800-1000Hu

slide22

Window level (center) & width

Window level: CT no; Width: range between 2 CT no

Modern equipment has a capacity of 4096 gray tones, which represent different density levels in HUs. (The density of water was arbitrarily set a 0 HU and that of air at -1000 HU)

Monitor can display a maximum of 256 gray tones

Human eye is able to discriminate only ~20

Densities of human tissues extend over a fairly narrow range (a window) of the total spectrum (10-90HU), it is possible to select a window setting to represent the density of the tissue of interest

slide23

Window level and width

Density levels of different types of tissues

The mean dentistry level of the window should be set as close as possible to the density level of the tissue to be examined.

The lung, with its high air content, is best examined at a low HU window setting

Bone require an adjustment to high levels

The width of the window influences the contrast of the images: the narrower the window, the greater the contrast

Lung window

Bone window

slide24

Lung window

If lung parenchyma is to be examined, e.g. when scanning for nodules, the window center will be lower at about -200HU, & the window width (2000HU). Low density pulmonary structures can be much more clearly differentiated

slide25

Brain window

Density values of gray & white matter differ only slightly. The brain window must be very narrow (80-100HU-> high contrast) and the center must lie close to the mean density of cerebral tissue (35HU) to demonstrate these slight differences

slide26

Bone window

Brain window

Bone window

Bone window should have a much higher center, at about +300HU, and a sufficient width of ~1500HU

Metastases in the occipital bone would only be visible in the appropriate bone window

but not in the brain window

Brain is invisible in the bone window: small cerebral metastases would not be detected

slide27

Window level (center) & width

Density levels of different types of tissues

Density levels of almost all soft-tissue organs lie within

a narrow range between 10 and 90 HUS

The only exception is the lung and this requires a special

window setting (lung window)

For hemorrhage

Density level of recently coagulated blood lies about 30HU above that of fresh blood.

This density drops again in older hemorrhages or liquefied thromboses.

Ref. 3

slide28

Before

CM

After

CM

Oral administration of contrast media

Without contrast medium (CM), it is difficult to distinguish between the duodenum (130) & the head of the pancreas (131, right figure) & other parts of the intestinal tract (140) would also be very similar to neighboring structures

After an oral CM, both the duodenum & the pancreas can be well delineated (Figures below)

slide29

Summaries

Knowing basic knowledge of CT:

Historical perspective

CT scanner components

Generations of CT machine

CT number

Window level and width

Use of contrast medium