Computerized Tomography Process

Question 1

Phases of CT imaging

Answer 1

1. Scanning the patient
2. Data Acquisition
   i. Tube or tube and detector move
   ii. Multiple attenuation measurements are taken around the object
3. Image reconstruction
4. Image Display
5. Image archival (recording)

Question 2

stopping of x-rays with transfer of energy

Answer 2

Absorption

Question 3

deflection of x-rays

Answer 3

Scatter

Question 4

No. of x-ray photons falling on an object

Answer 4

Incident Intensity

Question 5

No. of photons passing through

Answer 5

Transmitted Intensity

Question 6

obtained by measuring and comparing the incident and transmitted intensities

Answer 6

degree of attenuation

Question 7

cross sectional portion of the body which is scanned for the production of CT image

Answer 7

slice

Question 8

The slice has width and therefore _____

Answer 8

volume

Question 9

determined by the width of the x-ray beam

Answer 9

width

Question 10

The image is represented as a _____ of numbers.

Answer 10

MATRIX

Question 11

A two dimensional array of numbers arranged in rows and columns.

Answer 11

Matrix

Question 12

Each individual element or number in the image matrix represents a three dimensional volume element in the object,

Answer 12

VOXEL

Question 13

The VOXEL is represented in the image as a two-dimensional element called

Answer 13

PIXEL - (picture element

Question 14

The numbers in the image matrix

Answer 14

CT numbers

Question 15

Each ____ has a number which represents the x-ray attenuation in the corresponding voxel of the object

Answer 15

pixel

Question 16

To obtain a visual image, the CT numbers are assigned

Answer 16

different shades of gray on a gray scale.

Question 17

Basic components (Data Acquisition)

Answer 17

X-ray tube
Collimators
Detector/s

Question 18

____ traverses the object and enters the detecto

Answer 18

Collimated x-ray beam

Question 19

determined from the difference between incident intensity and transmitted intensity

Answer 19

attenuation

Question 20

In the basic CT system the x-ray tube and detector are translated ____ so that the beam scans the object

Answer 20

linearly

Question 21

Imaginary line between Tube & Detector

Answer 21

Ray

Question 22

Attenuation along a Ray

Answer 22

Ray Sum

Question 23

The set of ray sums in one direction

Answer 23

View

Question 24

The attenuation for each ray sum when plotted as function of its position is called

Answer 24

attenuation profile

Question 25

Attenuation of objects with different densities will change the attenuation profile

Answer 25

Object with low attenuation | Object with high attenuation

Question 26

after a view is recorded, the tube and detector rotate a small angle and the entire process is repeated until many views have been recorded for the same slice

Answer 26

translate –rotate CT

Question 27

The image is created by reflecting the attenuation profiles back in the same direction they were obtained

Answer 27

BACK PROJECTION

Question 28

The resultant image closely resembles the original object but it shows star shaped patterns around objects and streaks

Answer 28

‘Star’ and ‘streak’ artifacts

Question 29

applied to each point along the attenuation profile to eliminate these artifacts

Answer 29

filter function

Question 30

used to create sharper (higher resolution) or smoother (lower noise) images

Answer 30

Different filter functions

Question 31

process of applying the filter function to the attenuation profile

Answer 31

CONVOLUTION

Question 32

(Super imposition of the filter function and the attenuation profile)

Answer 32

CONVOLUTION

Question 33

minimizes the artifacts by changing the back projected information

Answer 33

convolution

Question 34

stored as numerical values as a function of position in the profile. This data is used in the image reconstruction process

Answer 34

difference profile

Question 35

set of difference profiles for a complete scan

Answer 35

SCAN DATA FILE or raw data

Question 36

The actual size of a pixel is equal to the size of the area scanned

Answer 36

Field of View

Question 37

How many levels of gray can the human eye distinguish

Answer 37

20 levels of gray

Question 38

How is the low distinguishing power of the human eye solved for viewing of CT images

Answer 38

Window width and Window level

Question 39

Range of CT numbers that is displayed using the useful level of gray scale

Answer 39

Window width

Question 40

separates one CT number from another

Answer 40

Small window

Question 41

condenses more than one CT number in each of gray level

Answer 41

Large window

Question 42

Center of the range of CT numbers displayed by the window

Answer 42

Window level

Question 43

The level control moves the visible gray scale up and down the CT number scale

Answer 43

Window level

Question 44

As an honour to Hounsfield who was the pioneer of invention of CT the CT numbers are called

Answer 44

Hounsfield units

Question 45

range of Hounsfield units Is from

Answer 45

-1000 to +3000

Question 46

Standard reference points are -1000 for

Answer 46

Air

Question 47

Standard reference points are 0 for

Answer 47

Water