The document serves as an introduction to computed tomography (CT) imaging, detailing its history, construction, operating principles, and applications across various medical fields. It covers essential topics including image quality, radiation doses, the role of contrast media, CT-guided interventions, and advanced techniques like dual-energy CT and perfusion imaging. Additionally, it discusses factors affecting image quality and the various types of artifacts encountered in CT imaging.

1. Introduction to CT imaging.
DR JACKSON BYAKIKA
LECTURER: DR BUGEZA SAM

2. OUTLINE.
• History.
• Indications for CT
• Construction of the CT Scan machine.
• Basic operating principles
• CT data acquisition and image reconstruction.
• Image quality
• CT image artifacts.
• Advantages over plain radiography.
• Radiation doses.

3. History.

4. Indications for CT
• Head: trauma, tumours, stroke, HIV complications, brain abscesses
• Chest: mediastinal disease, tumours, diffuse lung disease (high
resolution), pleural disease, early mets detection, aortic aneurism.
• Abdomen: liver lessions, pancreas- tumour, pancreatitis, trauma,
tumour staging, retroperitoneum
• Spine: vertebral fractures, disc prolapse, infection, tumours
• MSK: calcaneal fractures, recurrent shoulder disloacations, pelvic
fractures/ acetabulum, soft tissue tumours.

5. With (CT) imaging, no ordinary image receptor,
such as screen film or an image intensifier tube
is involved. A collimated x-ray beam is directed
on the patient, and the attenuated image-
forming x-radiation is measured by a detector
whose response is transmitted to a computer.
After the signal from the detector is analyzed,
the computer reconstructs the image and
displays the image on a monitor.

6. Helical CT, which has emerged as a new and
improved diagnostic tool, provides improved
imaging of anatomy compromised by
respiratory motion. Helical CT is particularly
good for the chest, abdomen, and pelvis, and
it has the capability to perform conventional
transverse imaging for regions of the body
where motion is not a problem, such as the
head, spine, and extremities.

7. When the abdomen is imaged with
conventional radiographic techniques, the
image created is low in contrast, principally
because of Compton scatter radiation. The
intensity of scatter radiation is high because of
the large area x-ray beam. The image is also
degraded because of superimposition of the
anatomical structures in the abdomen.

8. Conventional tomography is called axial
tomography bse the plane of the image is
parallel to the long axis of the body; this
results in sagittal and coronal images. A CT
image is a transaxial or transverse image that
is perpendicular to the long axis of the body.
Coronal and sagittal images can be
reconstructed from the transverse image set.

9. Introduction
• Computed tomography (CT) is a tomographic imaging
technique introduced in the early 1970s that generates
cross-sectional images in the axial plane.
• CT images are maps of the relative linear attenuation
values of tissues expressed in Hounsfield units (HU), also
known as CT numbers.
• CT as a modality generates 3D anatomical images using
axial scan projections. This offers the diagnostic
advantage of locating in depth pathologies.

10. First machine.

11. Construction of the CT Scan machine.
.
• Gantry and
table.
• The X ray tube
and generator.
• Collimation and
filtration.
• Detectors

13. • First-generation imaging system: translate and rotate, pencil
beam, single detector.
• Second-generation imaging system: translate and rotate, fan
beam, detector array.
Gantry
The gantry includes the x-ray tube, the detector array,
the high-voltage generator, the patient support couch,
and the mechanical support for each. These subsystems
receive electronic commands from the operating console
and transmit data to the computer for image production
and postprocessing tasks.

14. X-Ray Detectors
• The detectors used in CT typically comprise several components:
a scintillator absorbs the X-rays and converts them into visible
light. The light in turn stimulates a photodiode to produce an
electrical signal from which the image can be constructed.
• In order to improve spatial resolution, detectors have become
smaller (typically around 0.5 mm in the direction of the
acquisition plane and 1 mm in the z-direction), more sensitive
(allowing some dose reduction) and react faster (allowing
increased gantry rotation speeds). Better construction of modern
detectors also reduces the length of wiring and, thus, electrical
noise, a factor that is essential for low-dose imaging.

15. Helical CT
• Helical CT involves moving the patient through
the CT gantry aperture during data acquisition
and has become standard for nearly all CT
body acquisitions.

16. IMAGE QUALITY AND RADIATION DOSE
• Image quality in CT is mainly determined by image
noise, spatial resolution and (lack of) artefacts.
Increasing image noise decreases first the detection
of low-contrast structures and, as it gets larger, also
the detection of small high-contrast structures. Image
noise is determined by the following factors: radiation
dose, X-ray attenuation within the patient, efficiency
of the detection chain (scintillator, photodiode and
detector electronics), section thickness and image
reconstruction filter and technique.

17. • Image noise is inversely related to the square
root of the exposure dose used for constructing
a particular image, which in turn is proportional
to tube current, section acquisition time and
section thickness. Put more simply, if all other
factors remain constant, the dose must be
increased fourfold to reduce the image noise by
half, whereas using a collimation of 1.25 mm
rather than 5 mm will double the image noise.

18. CONTRAST MEDIA
• Intravenous contrast medium is essential for
optimising most CT examinations because the
resulting differential contrast enhancement of
the various tissues improves delineation of
normal and abnormal structures.

19. Preparation
• Before imaging, the patient record should be checked for a
history of known allergies, previous reactions to contrast
agents, renal function, thyroid disease and medication with
metformin.
• Overt hyperthyroidism is an absolute contraindication to
iodinated contrast medium. IV contrast injection in patients
due to have radioiodine therapy or scintigraphy should only be
performed after discussion, as the iodine will block uptake of
radioactive iodine for several weeks. Pregnant patients require
no particular considerations, but the newborn child should
have their thyroid function checked within the first week of life.

20. Contrast Medium Injection
• For injection of contrast agents, an 18-20G
intravenous cannula should ideally be sited in
an antecubital vein. If the chest or neck are to
be examined a right-sided approach is
preferred because the horizontal course of the
left brachiocephalic vein may lead to more
artefacts.

21. Potential Complications
• Extravasation of contrast medium at the
injection site is often painful. If this occurs, the
injection should be stopped immediately.
• Nonionic low osmolar contrast media have
replaced ionic high osmolar contrast agents,
as they have a significantly lower rate of
adverse events including allergy.

22. contn
Reactions may be divided into early (<60 min) or
delayed (>60 min). Early reactions are most usefully
categorised as mild-to-moderate (nausea, vomiting,
urticaria, bronchospasm, vasovagal reaction) or
severe (laryngeal or pulmonary oedema,
hypotension, anaphylactoid shock, respiratory or
cardiac arrest). Delayed reactions include skin
rashes, pruritus, headache, dizziness, nausea,
diarrhoea, chills, rigors, flulike symptoms and arm
pain.

23. OPTIMISING IMAGE ACQUISITION
• The patient should be positioned
appropriately, e.g. with arms down for cervical
imaging and raised for thoracic imaging, to
reduce artefacts, with the area to be imaged
positioned in the centre of the gantry. A
digitally acquired radiograph (scout view,
scanogram, topogram) should be obtained to
plan the anatomical length and field of view to
be examined.

24. CT-GUIDED INTERVENTIONS
• Biopsy, drainage, aspiration, injection and
ablation procedures under CT guidance are
well established and, in general, have a
relatively low complication rate. The use of CT
to guide procedures is complementary to US
and fluoroscopy, and best used where the
latter techniques would not allow safe access.

25. ADVANCED TECHNIQUES
Cardiac CT
• Modern CT techniques can quantify coronary calcium and
evaluate cardiac morphology, coronary arteries, cardiac function
and perfusion.
Dual-Energy CT
• Dual-energy CT imaging provides images in which one material
is eliminated. Elimination of soft tissue creates so-called iodine
maps; elimination of calcium removes bones and calcified
plaques; and elimination of iodine creates a virtual unenhanced
image. Dual-energy CT can characterise renal calculi and
distinguish between gout and other crystal arthropathies.

26. Dual-Energy CT
• Subtraction imaging is helpful for eliminating
bony structures in CT angiography, for
example the skull base, neck or for run-off
studies. It can also be used for creating iodine
maps in various organs and allows the
detection of regions with reduced perfusion in
the lungs or the differentiation of small cysts
from enhancing tumours.

27. CT Perfusion
• CT perfusion (CTP) imaging is a technique that uses the
contrast attenuation curves within tissues and their
afferent and efferent vessels to provide measures of
blood volume, blood flow, transit of blood through the
tissue (mean transit time) and leakiness of blood
vessels (permeability × surface area).13 CTP requires
rapid intravenous bolus injection (4–10 mL/s) and
repeated CT data acquisition of the same anatomical
area. CTP is now well-established in the brain for the
diagnosis of stroke.

29. Image generation

30. Detectors.

31. Diagram of the first-generation CT scanner, which used a parallel x-ray beam with translate-
rotate motion to acquire data.

32. Diagram of the second-generation CT scanner, which used translate-rotate motion to acquire
data.

33. Diagram of the third-generation CT scanner, which acquires data by rotating both the x-ray
source with a wide fan beam geometry and the detectors around the patient. Hence, the
geometry is called rotate-rotate motion.

34. Diagram of the fourth-generation CT scanner, which uses a stationary ring of detectors
positioned around the patient. Only the x-ray source rotates with a wide fan beam geometry,
while the detectors are stationary. Hence, the geometry is called rotate-stationary motion.

35. Principles of helical CT. As the patient is transported through the gantry, the x-ray tube traces a
spiral or helical path around the patient, acquiring data as it rotates. t = time in seconds.

36. PRINCIPLES OF COMPUTED
TOMOGRAPHY
• CT involves the imaging of thin axial sections
of the body with X-rays. The X-rays pass
through the body and are detected by a
detector positioned on the opposite side of
the body. The projection data must be
acquired from multiple angles around the
body. From these raw data, a computer
reconstructs a map of the local attenuation
within the examined section.

37. How is a CT image produced?
CT Numbers and Image Matrix
• Each image in the transverse (= axial) plane consists of a
matrix of data points (picture elements, pixels) that are
assigned a number that represents the X-ray attenuation
(μ) at that particular point in the body. The resulting
numbers are called ‘Hounsfield units’ (HU). Air is assigned
a CT number of −1000 HU and water is assigned 0 HU.
The CT numbers of solid soft tissues are centred around 0
HU, the lungs around −1000 HU and bone usually well
above 200 HU, with dense cortical bone often nearing
+1000 HU.

38. • The size of the image matrix used in modern
multidetector CT systems is usually 512 × 512
pixels. However, some manufacturers provide
768 and 1024 matrices for high-resolution
applications. For body applications, the pixel
size is usually in the range of 0.6–0.8 mm, for
the brain it is approximately 0.5 mm and for
extremities it varies between 0.3 and 0.5 mm.

39. • Each pixel collects attenuation data from a
corresponding small volume (voxel) within the
body region being examined by CT. The body is
represented in a CT data set as a 3D matrix.
Each matrix element contains the CT number
of its respective voxel in the human body.

40. Relation among field of view, matrix size, voxel, and pixels
FIG.

41. Sample CT image. A CT image is composed of pixels (picture elements). Each pixel on the
image represents the average x-ray attenuation in a small volume (voxel) that extends through
the tissue section. (In this example, the pixel size is exaggerated. In addition, in a real CT
image, all tissues within a single pixel would be the same shade of gray.) w = width.

42. Hounsfield Values (UNITS).
Definition CT-value:
• CT-values contain the linear absorption
coefficients of the underlying tissue in every
volume element with respect to the μ-value of
water. Using this definition the CT-values of
different organs are relatively stable and
independent of the X-ray spectrum.
• CT-value = (μ - μwater) / μwater x 1000 HU

43. Cont’n
• Each pixel is assigned a numerical value (CT number), which is the average
of all the attenuation values contained within the corresponding voxel.
• This number is compared to the attenuation value of water and displayed
on a scale of arbitrary units named Hounsfield units (HU) after Sir Godfrey
Hounsfield.
• This scale assigns water as an attenuation value (HU) of zero.
• The range of CT numbers is 2000 HU wide although some modern
scanners have a greater range of HU up to 4000HU.
• Each number represents a shade of grey with +1000 (white – most xray
absorbing i.e bone) and –1000 (black – least xray absorbing i.e air) at
either end of the spectrum.

44. Hounsfield units for representative materials with
values of physical and electron densities
A CT scan through the chest showing the
detail in the lungs. The mediastinum is
white and there is no detail. This is a lung
window setting.

45. Hounsfield Scale

46. CONT’N

47. Window level (WL) and window width (WW)
• Whilst the range of CT numbers recognized by the computer is
2000, the human eye cannot accurately distinguish between
2000 different shades of grey.
• Therefore to allow the observer to interpret the image, only a
limited number of HU are displayed.
• A clinically useful grey scale is achieved by setting the WL and
WW on the computer console to a suitable range of Hounsfield
units, depending on the tissue being studied.
• The term ‘window level’ represents the central Hounsfield unit
of all the numbers within the window width.

48. Cont’n
• The window width covers the HU of all the tissues of interest and
these are displayed as various shades of grey.
• Tissues with CT numbers outside this range are displayed as either
black or white.
• Both the WL and WW can be set independently on the computer
console and their respective settings affect the final displayed image.
• For example, when performing a CT examination of the chest, a WW
of 350 and WL of +40 are chosen to image the mediastinum (soft
tissue) (Fig. 1.5a), whilst an optimal WW of 1500 and WL of –600 are
used to assess the lung fields (mostly air)

49. Windowing of Hounsfield values at different
evaluations of CT images
• Human eye can
distinguish only 60-
80 grey values !

50. Image reconstruction
• The acquisition of volumetric data using spiral CT
means that the images can be post processed in
ways appropriate to the clinical situation.
• Multiplanar reformatting (MPR) – by taking a
section through the three- dimensional array of CT
numbers acquired with a series of contiguous slices,
sagittal, coronal and oblique planes can be viewed
along with the standard trans-axial plane

51. Image Quality Factors
• The factors used to evaluate digital image quality
include the following:
• Brightness
• Contrast resolution
• Spatial resolution
• Distortion
• Exposure indicator
• Noise

52. Image quality
• Image quality connotes how clearly the image displays information about
the anatomy, physiology, and functional capacity of the patient, including
alterations in these characteristics caused by disease or injury.
N.B. Image quality is referred to as the clarity

53. Factors that affect Image Sharpness.

54. Factors that affect Image Contrast.

55. Factors that affect Image Noise.

56. CT image artifacts.
• Is a feature or appearance that is seen on an image, which
does not actually exist.
• They occur in all imaging modalities and are often
unavoidable.
• Understanding artefacts helps prevent confusion for
pathology.
• However, with the increasing speed of image acquisition in
a single breath hold by the most modern scanners, many
artefacts are being minimized or eliminated.

57. Types of Artefacts:
• Motion.
• Noise.
• Ring.
• Metal.
• Out of beam.
• Cone beam(multi-detector)
• Windmill (helical).
• Beam hardening.
• Partial voluming.

58. Motion artifact
• Motion (patient, cardiac, respiratory, bowel)
causes blurring and double images, as well as long
range streaks which occur btn high contrast edges
and the X-ray tube position when the motion
occurs.
• Faster scanners reduce motion artifact b’se pt has
less time to move during the acquisition. i.e,
I. Faster gantry rotation or more X-ray sources
II. More detector rows allows a greater volume to
be imaged in a single gantry rotation,

59. Cont’n.
• Rigid body motion artifacts - mainly a problem with head
CT, can be reduced using special reconstruction techniques.
• Respiratory motion in cone-beam CT with slow gantry
rotation can be estimated and corrected, thus reducing
artifacts.
• With a very fast scanner, the heart can be scanned during
diastole within a single heartbeat, significantly reducing
cardiac motion, thus allowing evaluation of the coronary
arteries.

60. Motion artefact
Motion causing blurring and double images ( left), as well as long range streaks (right).

61. Ring artifacts.
• Caused by a mis-calibrated or defective detector
element, which results in rings centered on the
center of rotation.
• This can often be fixed by recalibrating the
detector.

62. Ring artefact

63. Beam hardening and scatter artefacts
• Beam hardening and scatter both produce dark streaks between
two high attenuatation objects (eg metal or bone), with
surrounding bright streaks.
• These artifacts are a particular problem in the posterior cranial
fossa, and with metal implants.
• These can be reduced by:
I. Using iterative (trial and error) reconstruction.
II. Dual energy CT reduces beam hardening, but not scatter.
• Beam hardening and scatter also cause Pseudo-enhancement of
renal cysts.
• i.e. Areas that are surrounded by a ring of high density material
become brighter due to beam hardening and scatter

64. Metal artifact
• Metal streak artifacts are caused by multiple
mechanisms, including beam hardening, scatter,
Poisson noise, motion, and edge effects.
• The Metal Deletion Technique (MDT) is an iterative
technique that reduces artifacts due to all of these
mechanisms.
• In some cases, the improved image quality can
change the diagnosis.

66. Noise
• Poisson noise is due to the statistical error of low photon counts, and results in
random thin bright and dark streaks that appear preferentially along the
direction of greatest attenuation.
• This can be reduced using iterative reconstruction, or by combining data from
multiple scans.
• Poisson noise can be decreased by increasing the mAs & also be reduced by
increasing the slice thickness.
• Hence, a tradeoff between noise and resolution
• Model-based iterative reconstruction (MBIR), for example, attempts to smooth
out the noise while preserving edges, resulting in a plastic appearance, where
there are small clusters of pixels with similar Hounsfield units.

67. Poisson Noise artefact

68. Cone-beam (multi-detector row) and
Windmill (helical) artifacts
• In helical CT, the table is continuously advanced as the X-ray tube rotates
around the patient.
• As the detector rows pass by the axial plane of interest, the reconstruction
oscillates between taking measurements from a single detector row, and
interpolating between two detector rows.
• If there is a high contrast edge between the two detector rows, then the
interpolated value may not be accurate.
• This creates smooth periodic dark and light streaks originating from high
contrast edges, which are called windmill artifacts
• These artifacts can be reduced with Adaptive Multiple Plane Reconstruction
(AMPR), which uses tilted planes for reconstruction

70. Out of field “artifact”
• Due to a suboptimal reconstruction algorithm, and
can be fixed using a better algorithm.
• Images can then be acquired using a field of view that
is much smaller than the object being scanned, thus
reducing the radiation dose.
• Higher resolution scanners will likely require iterative
reconstruction or limited field of view scans to reduce
the radiation dose required to achieve an acceptable
level of noise.

71. Partial voluming artefact
• Different tissue densities within a single voxel
lead to ‘averaging’ of data. E.g , a small black
object within a larger white space would look
like a shade of grey.

73. Advantages over plain radiography.
Advantages:
• The data acquired in one scan can subsequently be
manipulated to provide multiplanar and 3D reconstructions.
• no superposition → higher contrast
• three-dimensional localization
Disadvantages:
● Radiation side effects due to the much larger dose rates.
o I head CT scan gives a dose equivalent to 115 CXRs
o I chest CT scan gives a dose equivalent to 400 CXRs
o I abdominal CT gives a dose equivalent to 500CXRs

74. Radiation doses.
• Radiation doses in CT are relatively high.
• For example, the effective dose of a head scan is 2 mSv, of the
thorax 10 mSv and of the abdomen 15 mSv.
• This is a factor 10 to 100 higher than radiographic images of the
same region, but the diagnostic content of the CT images is typically
much higher.
• Using a lower tube current and a higher voltage can reduce the dose
in some CT scanners.
• However, there is still some risk to a developing fetus. CT scans are
not recommended during pregnancy.

75. General xray side effects
• Stochastic effects whose probability increases with the radiation dose:
carcinogenic and genetic effects.
• Deterministic effects associated with a threshold radiation dose.
1. skin erythema
2. loss of hair
3. skin desquamation
4. cataracts
5. fibrosis
6. depression of the bone marrow- anaemia
• The number of chest X-rays which would be needed to reach the erythema
skin dose threshold (2-3Gy) is approx. 10,000 The number of CT studies to
produce the same effect is only 100, but note fluoroscopy time only 30mins.

76. Food for thought
• Contrast CT
• Interventional CT
• Cardiac CT
• Fifth generation CT
• Dual energy CT and subtraction imaging
• CT Perfusion technique

77. REFERENCES:
• GRAINGER AND ALLISONS DIAGNOSTIC
RADIOLOGY, 6TH
EDITION.
• REVIEW OF RADIOLOGICAL PHYSICS

78. Thank you.