Ct principles

1. BY
DR LIVINUS CHIBUZO ABONYI
BSc, PgDMed US, MSc, PhD
RDG 402: PRINCIPLES OF COMPUTED TOMOGRAPHY
1ST SEMESTER, 400L.

2. COURSE OUTLINE
• IMAGE DIGITIZATION
• COMPUTED RADIOGRAPHY
• BASIC PRINCIPLES OF COMPUTED
TOMOGRAPHY
• BASIC PRINCIPLES OF NMR
• CARE & MAINTENANCE OF RADIOGRAPHIC
EQUIPMENTS

3. STUDY OBJECTIVE
Students are expected to understand:
 the principle of Image digitization
 Principle of Computed Radiography
 Principle of Computed Tomography
 Basic Components & Functions of CT.
Scan Machine
 Principles & Components of NMR Unit
 Care & Maintenance of medical
Imaging Equipts.

4. DIGITAL IMAGING
Digital imaging shares with conventional the following:
o Same technique/procedure
o No discernable difference by the patient.
o Phosphor plates are used instead of conventional X-ray films
Images are sent to a workstation to make the details more
comprehensible.
A digital image is made up of:
 Precisely defined number of points – called Pixels.
o Each pixel has a defined location in an X & Y axis.
 A grey-value or colour level which is equally defined.

5. A defined image therefore has different number of image
points both in X & ϒ dimensions.
This is called the matrix. One image depth refers to the
number of possible colours or grey tones on the displayed
image.
A digital image can therefore be displayed at all locations
where the information about its composition is available.
This is the basis for transmitability/reproducibility of digital
images; provided a new platform is provided.

6. SPATIAL RESOLUTION
A unit of an image has a homogeneous colour or a single grey-value.
Details within an individual pixel cannot be displayed.
For a higher resolution, the size of the individual points must be small.
For a greater detail, there must be as many points are possible per cm (or a unit
distance).
Spatial resolution is a function of the number of pixels per cm.
The total number of pixels depends on the size of the image.
If the number of pixels per image is predetermined; resolution will be diminished
as the image size increases.
Image size is therefore automatically predetermined if the size and number of
pixels are predetermined.
To double the resolution, both the number of pixels in a row and number of rows
must be doubled, i.e. the number of pixels is increased by a factor of 4.
All sliced images have a third dimension in addition to the X and Y axis, i.e. the Z-
axis.
The three dimensional unit of an image is the voxel unit, i.e. the volume element.

7. The number of voxels = the product of number of
 Elements along the y-axis – image row.
 Number of elements along the X–axis – image column and
 Number of elements along the Z – axis – the number of
slices.
For spatial resolution to be doubled, the amount of data
required is raised to third power; thus by a factor of 8.
There is therefore need for higher memory capacity for
storage and speed of information transmission.
The smallest unit of digital information can have a value of
either 0 or 1. This is called a bit. Eight bits constitute 1 byte
which is a unit of image storage capacity.
Digital imaging requires significantly high volume of data
which are stored in multiples of kilo, mega, giga, etc bytes.

8. 
Is a procedure which creates images of sections called
slices through the patients body. The images are created
by computer from digitized data obtained when the
patient is exposed to a sharp, narrow beam of radiation
(X-ray).
A single x-ray exposure, can produce many different
images using computer manipulation.
COMPUTED
TOMOGRAPHY

9. 
 There is an X-ray tube fitted with a collimating device
which shapes the X-ray beam to a narrow, flat fan beam.
 The beam angle covers about 30% to 50%. The beam is
accurately aligned to an array of small radiation detectors
in an arc form. Each detector is equidistant from the X-ray
tube’s focal spot. Each detector has a collimator that limits
its measurement of the approaching ray.
 The detectors accurately measure the X-ray intensity
which are digitized and fed to a computer. The
differential attenuation of the X-ray beams are
accordingly detected by the detectors. The differential
attenuation measurements are translated into data
indicating the relative opacities of structures lying in the
part of the X-ray beam.
THE PRINCIPLE OF CT

10. 
 CT was 1st introduced into Medical Imaging through the pioneering effort of Sir
Godfrey Hounsfield, as Emi scanner in 1973.
 A CT Scan Unit comprises the following parts.
 A Gantry: This is a frame which incorporates the X-ray tube, beam collimating
equipment and the detectors. The gantry centrally has a circular or elliptical aperture
through which the body part under investigation passes.
 An Operator’s Console: This is where most of the functions/operations of the unit
can be selected and executed.
 An X-ray Generator: This provides the high potential difference for the X-ray tube.
 A table or Cradle: For patient’s positioning and for accuracy with respect to the X-
ray beam and detectors. It is controlled by efficient motor devices.
 A Computer: This receives the intensity and by use of a programmed software,
provide processed images for interpretation, manipulation and display.
 A hard copy unit.
 A computer storage memory – for image storage with recall facility.
 These equipment component parts require an assured stabilized temperature,
relative humidity and power supply.
EQUIPMENT FOR CT

11. 
Computed Tomography (CT. Scan)

12. 
 The gantry has a central aperture through which the
patient/body part under investigations slide through. The
size of the aperture is significant. Too large an aperture
affects the image negatively, while too small an aperture
may not accommodate large sized patients especially in
the obese and abdominal distension.
 Apertures are designed to appear larger than its actual
size. Other factors that affect the aperture size are gantry
titling angle (about 20%) cranio-caudally and the oblique
angulations of the table (termed slewing), depending on
the model of the CT Unit.
 Current Spiral CT Units do not require gantry angulation
since all plane of cuts can be obtained during post-
processing of images.
THE SCANNING GANTRY:

13. 
 A high-powered, rotating anode X-ray tube with high anode thermal capacity is
used. The anode may be made of compound metallic disc with graphite balling. This
raises the thermal capacity and without proportional increase in the anode weight.
The focal spot size is made to be small e.g. 0.5mm.
 The X-ray tube and detectors can be arranged in the following ways:
1. The X-ray tube may be rigidly ganged to an arc of detectors.
 These detectors are directly aligned and confined to the X-ray beam.
 The tube and the detectors move simultaneously round a fixed point in space which
is equivalent to the centre of the gantry aperture.
2. The X-ray tube may move independently through a circular path, with the
beam aligned to a limited arc within a complete, permanently fixed ring of
detectors.
The X-ray exposure is continuous and the set of detectors aligned to the tube
changes as the tube moves.
THE X-RAY TUBE:

14. There are draw backs in this arrangement which are as follows:
1.There is increased number of detectors which increases the price of
purchase and maintenance.
2.There is increased poor geometry due to the required space for
passage of the tube, position of the detectors with respect to the
patient in the aperture.
3. There is increased penumbra. This implies increased magnification
and decreased spatial resolution.
4.The system is not cost effective; since only a set of detectors receive
radiation at a time.

15. X-RAY TUBES & DETECTORS
X-ray Tubes and detectors are motor-driven for smoother
speed, synchrony and efficiency.
The detectors are judged by its efficiency in detecting X-ray
photon energy; undetected X-ray photons do not contribute to
image quality but increase radiation doses to the patients.
Detectors should also have wide dynamic range.
This means that it should be able to detect wide range of beam
intensities, from very low to very high intensities and convert
them to proportional output signals.
Artifacts are produced when the beam intensity exceeds the
dynamic range; such that errors occur in the acquisition of
radiation data.
Detectors in the CT Scanners may be either crystal scintillation
detectors with photo-multiplier or gas ionization detectors.

16. X-RAY TUBE DETECTORS
X-ray Tube detectors may be of two types:
1. Solid state detectors ie the Crystal deteors
2. Gas ionization Chambers, using Xenon Gas.
Some modern CT units use crystal photodiodes
containing materials such as cadmium tungstate.
Some others use detectors containing Xenon
gas under pressure.

17. Other crystal detectors include sodium
iodide, cesium iodide or bismuth germinate.
Crystal detectors are coupled to photo-
multipliers which detect the emitted light and
convert them to an amplified electrical
signals.

18. An ideal detector has the following characteristics:
 It should be small in particle size, to enable for
large numbers to be used.
 Should combine good sensitivity with wide
electronic dynamic range.
 Electronic dynamic range is the ratio of the
largest signal that does not saturate the system
to the smallest detector signal.
 It should have no after-glow effect.
 Indifferent to temperature changes & relative
humidity.
 Rapid Response

19. Xenon detectors have the following
advantages:
 Constant sensitivity in both short and long
terms.
 Indifference to changes in ambient
temperature and humidity.
Wide dynamic range.
 Rapid response.
 No after-glow effect.
 Relative small size which favours its use in
the construction of large detector array.

20. Requires a constant potential supply.
The KV must be stable. Fluctuation cause
image artifacts and affects the tube/detector
component parts.
Three phase, 12-pulse outputs of typical
range of 100 – 150kv is ideal.

21. Demands high tube voltages of as much as
130kv on 300mAs.
The heat storage capacity may be of the order
of 1million heat units.
Most have dual focal spots of 0.6 – 1mm and
broad focus of up to 1.8mm.
The CT tubes have a warranty life of 250, 000 –
300, 000 exposures, depending on the model,
manufacturer and generation.

22. The computer memory is the recipient of the series of dose measurements.
In these measurements, the computer has to note the position of the X-ray
tube, detectors and the voxels through which the beam has passed through.
It therefore captures the raw data or the unprocessed measurements. The
computer applies an algorithm which changes the raw data into a scan data
by the assignment of CT numbers to each pixel.
The process used in many CT computers is the back-projection algorithm.
The back-projection algorithm is also modified by the use of convolusion
process to compensate for excessive high or low dose readings.
These processes ensure optimum image quality of the parts under
investigation.
Image reconstruction is also one of the major functions carried out by the
computer storage memory.
Images can be recalled from the raw or processed data for reconstruction
and manipulation; depending on the clinical requirements of the procedure.
The computer memory is also a storage facility.
Scanned data are stored on short-term basis. The volume of the data
storable depends on the capacity of the disc, and the type or model of the
scanner.
It is from this memory that images are transmitted to PACS system. Magnetic
optical disc, floppy disc or hard copy photography CT films.

23. The 1st generation of CT scanner is the translate/rotate
type.
Here, there is an X-ray tube and a single detector which
move parallel on each side of the object under
investigation.
This is translation movement. After each movement a
rotation of 1˚ is made by the tube and the detector and
the movement is repeated. This is rotation movement.
Due to the slow mechanism of this process, the 1st
generation scanners were generally slow.

24. 2nd Generation
Same as the 1st generation, i.e. translate/rotate movement.
However, more than one detector is used.

25. 3rd Generation
Are of the rotate/rotate type i.e. the tube rotates with the set
of detectors. The tube produces a fan-beam of radiation
which is mechanically connected to the row of detectors.
The detectors cannot be individually calibrated and hence
failure of one has a more noticeable effect on the images.
The third generation scanners have limited set of detectors.
Detector failure and other faults produce ring artifacts in
third generation machines.

26. 4th Generation
4th generation scanners are characterized by:
A static ring of detectors around the gantry.
An X-ray tube which rotate round the ring of detectors.
The X-ray tube is positioned within the ring of detectors.
There is still controversy over choice of which generation is
preferred among the third and fourth generation scanners.
The preference differs from one manufacturer, vendor or
user to the other.

27. COMPARISON
3rd Generation
• 1. Focus - object
distance = Object - detector
distance.
• 2. A single detector failure causes
noticeable image deterioration.
• 3. Individual detectors cannot be
calibrated as they are always under the
radiation beam.
• 4. Control of scattered radiation is
easier.
• 5. Radiation dose to the patient is
less.
• 6. Extra technology required since
the detectors have to rotate along with the
X-ray tube during exposing.
• 4th Generation
• 1. Shorter object – focus distance
than object – detector distance. This
requires smaller focal spot to improve
image un-sharpness.
• 2. A single detector failure does not
have significant image deterioration.
• 3. Individual detectors can be
calibrated.
• 4. Control of a scattered radiation is
less effective.
• 5. Radiation dose to the patient is
more
• 6. Extra technology not required to
safeguard the electronic amplifiers
employed to reduce image noise; since
the detectors do not rotate round.

28.  SCANOGRAM
This is a digital radiograph of the part under investigation used to
Provide reference for the subsequent lateral axial sections to be produced, ie for
Planning of procedure.
Other uses include:
To estimate the degree of tilt required for angled projections e.g. in coronals for
brain [depending on the model of the CT Unit]
To make preliminary diagnosis of certain disease conditions.
To indicate reference level of axial images on processed radiographs.
To define the regional scope of parts for inclusion.
Scanograms are produced by a continuous exposure by a stationary tube while the
part under investigation is slowly moved by the cradle through the gantry aperture.
Scanograms are possible with third and fourth generation CT scanners; including the
current spiral/helical versions.

29. The CT image is a display of discrete series or
dots or pixels which represent the smallest
image volume called voxel.
The smallest voxel element is represented by
the CT number; which is the measure of the X-
ray attenuation of the material in each voxel.
The CT numbers of structures range from -
1000 for air to up to 3000 for dense bones;
based on arbitrary allocation of zero to water.

30.  Window Width is the range of CT numbers of
certain range of structures at which their
shades of gray display can be appreciated.
 The window Level is a more precise CT value
which most approximately demonstrates a
certain structure.

31.  Introduced clinically in 1991
 Has resulted in a revolution for diagnostic imaging.
There has been increased diagnostic yield such as in
CT angiography, renal colic, detection of lung and liver
lesions and introduction of CT colonoscopy.
High resolution imaging e.g. temporal bone, virtual
endoscopy etc.
For the past 10years, helical CT has improved in the
following aspects:
 Faster gantry rotation.
 More powerful X-ray tubes.
 Improved interpolation algorithm.

32. In helical CT, the X-ray beam rotates through the part
under investigation in a helical format, while the part is
gently advanced through the aperture by the cradle.
The most recent advancement in helical CT technology
is the Multi-slice/Volumetric imaging. This technology
is capable of acquiring different channels [slices] of
helical data simultaneously. There are such models as
2-slice, 4-, 8-, 64-, 164- slices etc.
The salient advantages of multi-slice CT (MSCT) are:
 Simultaneous, shorter imaging/data acquisition time.
 Retrospective creation of thinner of thicker sections
from the same raw data.
 Improved three dimensional rendering with reduced
helical artifacts.

33. APPRECIATION
THANK YOU FOR
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