5. Basic principles
• The internal structure of the object can be reconstructed from
multiple projections of the object.
• The ray projections are formed by scanning a thin cross section of the
body with narrow x-ray beam and measuring the transmitted
radiation with a sensitive radiation detector.
• Plain film imaging reduces the 3D patient anatomy to a 2D projection
image
6. Basic principles
• Limitation can be overcome, to some degree, by acquiring two images
at an angle of 90 degrees to one another
• For objects that can be identified in both images, the two films
provide location information
7. This is the basic idea of computer aided tomography. In a CT scan machine,
the X-ray beam moves all around the patient, scanning from hundreds of
different angles
8.
9.
10. CT Introduction
• Computed tomography (CT), originally known as computed axial tomography
(CAT or CT scan)
• The word "tomography" is derived from the Greek tomos (slice) and graphein (to
write).
• The tomographic image is a picture of a slab of the patient’s anatomy
• The 2D CT image corresponds to a 3D section of the patient
• It developed into a versatile 3D whole body imaging modality for a wide range of
applications in for example
• oncology, vascular radiology, cardiology, traumatology and interventional radiology.
• Computed tomography can be used for
• diagnosis and follow-up studies of patients
• planning of radiotherapy treatment
• screening of healthy subpopulations with specific risk factors
11. • CT has undergone several evolutions and nowadays multi- detectors
CT scanners have been evolved which have better application in
clinical field.
• CT scanning is perfectly suited for 3D imaging and used in, for
example, brain, cardiac, musculoskeletal, and whole body CT imaging
13. Gantry
• X ray tube
• Collimation
• Filters
• Detectors
• High voltage generator
14. X ray tube
•Earlier oil-cooled, fixed anode with large focal spot
was used.
•Now a days, Rotating anode type.
•More heat loading and heat dissipation
capabilities.
•Small focal spot size (0.6mm) to improve spatial
resolution.
•Anode heating capacity of 6.3 MHU and cooling
rate of 1MHU.
•Operated at 80-140kV tube voltages
15. Collimation
• X-ray beam collimated at two points, one
close to the x ray tube and the other at
the detector(s) with perfect alignment.
Each detector has its own collimator.
• Collimator at the detector controls scatter
radiation.
• The collimators also regulate the
thickness of the tomographic slice (i.e.,
the voxel length).
• Pixel size is determined by the computer
program and not by the collimator.
16. Filtration
• Beam shaping filters are being used to create a
gradient in the intensity of the X-ray beam
• They are sometimes called “bow-tie” filters
• They are mounted close to the X-ray tube.
• The purpose of the beam shaping filter is to
reduce the dynamic range of the signal recorded
by the CT detector
• Reduce the dose to the periphery of the patient
17. Detectors
• Small in size with good resolution
• High detection efficiency
• Fast response
• Negligible after glow
• Wide dynamic range
• Stable noise free response
• 800-1000 detector element along detector arc
• 1-320 detectors along z-axis
18. Xenon Detectors
• Photon enters detector and interacts with gas atom produces
electron-ion pair.
• Voltage between cathode and anode moves the e- towards anode
and positive ion towards cathode.
• When e- moves near anode, small current produced which is the
output signal from detector.
19. Solid state detectors
• Scintillators produce light when ionizing
radiation reacts with them.
• It is detected by photodiode , then gives
electric signal and digitized
• Electronic signal is proportional to Xray
intensity
• NAI, CdWO4
• Faster imaging rate, less patient dose
• Neglible afterglow.
20. • Xenon filled ionization detectors used earlier
• Fewer ring artifacts
• Lesser detection efficiency - 70% efficient
• Solid state detectors
• High detection efficiency - Approaching 100% efficient
• Improved image quality
• Small in size
21. • Anti scatter grid is used to prevent detector cross talk
• Septa and strips of anti scatter grids should be small, since they
reduce the effective area of the detector and thus reduce the
detection of X-rays
22. • Detector sizes are effective at the iso-center
• The minimum number of detector elements should be
approximately (2 FOV)/d to achieve a spatial resolution
of d in the reconstructed image
→ ~ 800 detector elements are required to achieve a spatial
resolution of 1 mm within a reconstructed image at a field of
view of 400 mm
• Spatial resolution can be improved by use of
• the quarter detector shift
• dynamic focal spot
23. What are we measuring?
• The purpose of a computed tomography acquisition is to measure x
ray transmission through a patient for a large number of views.
24. • The average linear attenuation coefficient (µ), between tube and
detectors.
• Attenuation coefficient reflects the degree to which the X-ray
intensity is reduced after passing through the material and getting
absorbed by the detectors.
• Depends composition of the material, the density of the material, and
the photon energy
• Beer’s Law
• where
- I(x) is the intensity of the attenuated X ray beam,
- I0 the unattenuated X ray beam,
- x the thickness of the material
25. • As an X ray beam is transmitted through the patient, different tissues
are encountered with different linear attenuation coefficients.
• The intensity of the attenuated X ray beam, transmitted a distance d,
can be expressed as:
26. • The CT image is represented as the Matrix of the number.
• A two dimensional array of numbers arranged in rows and
columns is called Matrix.
• Each number represent the value of the image at that location.
• Pixel – each square in the matrix ( picture element)
• Voxel – Volume element
27. CT Number
• Each pixel element in the matrix represents
the linear attenuation coefficient of the
corresponding voxel of the object.
• From the CT matrix, the reconstructed linear
attenuation coefficient(µ) is transformed
into Hounsfield units (HU)
29. Pitch
• The relationship between patient and tube motion is called Pitch.
• It is defined as table movement during each revolution of x-ray
tube divided by collimation width.
• Increasing pitch reduces the scan time and patient dose.
• For a 5mm section, if patient moves 10mm during the time it takes
for the x-ray tube to rotate through 360˚, the pitch is 2.
• If pitch is 0.75, slow table motion and increases radiation dose
• If pitch is 1.5 , fast table motion and decreased image quality
30. Image Display
• CT image is displayed on TV monitor for immediate viewing
• Display matrix has on average 512x512 some may have 1024x1024
• But one display has only 256 shades of gray
• Thus, we are going to image CT no from -1000 to 1000 with 256
shades of gray
• Pixel size on average 0.1 mm with scan field view of 40cm
31. • With pixel size of 0.1 mm,40 cm scan field contain 4000 pixels
• These 4000 pixels displayed on 512x 512 TV monitor will result in 8 pixel per
matrix element
• Now the challenge is how to display CT no from -1000 to 1000 with only 256
shades of gray.
• Thus, assign 8 CT no.s to the same shade of gray and display entire range of
information in compressed scale.
• And select a CT no that will be average of body examined. Computer instructed to
assign one shade of gray to each of 128 CT no below and 128 CT no above
baseline.
32. • Centre CT no is window level
• Range of CT no's above and below are called window width
• This leads to the concept of Windowing, also known as
• grey level mapping,
• contrast stretching,
• histogram modification
• contrast enhancement
33.
34. Tomographic acquisition
• Single transmission measurement through the patient made by a
single detector at a given moment in time is called a ray
• A series of rays that pass through the patient at the same orientation
is called a projection or view
• Two projection geometries have been used in CT imaging:
• Parallel beam geometry with all rays in a projection parallel to one another
• Fan beam geometry, in which the rays at a given projection angle diverge
35.
36. • Purpose of CT scanner hardware is to acquire a large number of
transmission measurements through the patient at different positions
• Single CT image may involve approximately 800 rays taken at 1,000
different projection angles
• Before the acquisition of the next slice, the table that the patient lies
on is moved slightly in the cranial-caudal direction (the “z-axis” of the
scanner)
37. Scan projection radiograph
• The SPRs are used for planning the start and end position of the CT
acquisition
• The technician selects from the SPR the optimal scan range for the
actual CT scan. Also the field of view (marked in yellow), and gantry
angle of scan (brain).
38. • Automatic exposure control systems in computed tomography derive
information on the X-ray transmission through the patient from the
scan projection radiographs.
• This can be used to adjust the tube current according to
• The overall size of the patient
• Longitudinal variations in attenuation – called z-axis modulation
• Rotational variations in attenuation
39. • Z-axis modulation
• Adaptation of the tube current (mAs) is
only shown at four levels
• However during the helical acquisition the
tube current continuously optimized at
each level within the scanned range
• The tube current is increased in areas with
high attenuation and decreased in areas
with low attenuation of X-rays
40. Axial CT
• X-ray tube and detector rotate 360°
• Patient table is stationary
– With X-ray’s “on”
• Produces one cross-sectional image
• Once this is complete patient is moved
to next position
42. First Generation
• Electric and Musical Industries Ltd. EMI Scanner
• Pencil beam X ray
• Source and detector coupled rigidly
• Rotate and translate
• Rotate at 1° interval for 180 projections, 24cm FOV
• 160 rays measured per translation, total – 28800 rays
• 4.5 minutes for single slice scan, 1.5 min to
reconstruct
44. • Efficient scatter reduction
• Contrast resolution of internal structures was unprecedented,
• More scan time
45. Second Generation
• Narrow fan beam with angle of 10°
• Multiple detectors – linear array of 30 detectors
• Rotate and translate geometry
• Acquisition time reduced
• Detectors measured more scattered radiation
• Limited to head CT
46. Third Generation
• Rotate and rotate geometry
• A wide aperture fan beam of 40° to 60°, which covers entire patient,
so that translational movement was eliminated
• Linear detector array of 400 – 1000 detector elements
• Xray tube and detectors were joined to have synchronous rotation
• Imaging process is significantly faster than 1st and 2nd generations
47. • Nowadays 3rd generation CT offers scan time
less than 0.5s
• Large number of detectors and lack of
calibration present in detectors gives
characteristic image artifact known as ring
artifacts.
48. Fourth Generation
• Fourth generation scanners were developed specifically to alleviate
the ring artifacts produced by the third generation.
• Specifically, the impossibility to have such a large array of rotating
detector elements (>400) are perfectly synced and calibrated to one
another.
• By removing the detectors from the rotating gantry and putting them
in a stationary ring around the patient, detectors were able to
maintain calibration
• Less efficient use of detectors, lessthan1/4 are usedat anypoint during
scanning
49. • This stationary 360 degree ring of detectors
required an increased number of detector
elements (~5000 total)
• The fan-shaped x-ray beams are processed (to
construct the image) with individual detectors
as the vertex of a fan.
• This fan beam data is acquired using one
detector over the time period it takes for the x-
ray tube to rotate from side to side of the fan
arc angle
• Rotate and Stationary geometry
50. Fifth generation
• Design: x-ray tube is a large ring that circles patient, opposed to
detector ring
• Use : for cardiac tomographic imaging “cine CT”
• X - rays produced = high - energy electron beam
• No moving parts to this scanner gantry
• It is capable of 50 - millisecond scan times and can produce 17 CT
slices/second
• stationary/stationary geometry
51. • Sweeps an intense electron
beam across a large, stationary
anode target which surrounds
the patient
• X-rays are emitted from the
point where electrons strike
target
• X-rays transmitted through
object are measured by a
stationary array of detectors
52. • Very fast scanner ,data collection for 1slice is 50-100 ms
• Requires no mechanical motion to acquire data
• Cine CT systems, have higher noise level and lower spatial
resolution but are ideal for some clinical application
• Cardiac imaging with and without the use of contrast
agents, lung imaging, and paediatric studies
53. Helical CT
• X-ray tube rotates as patient is moved smoothly into x-ray scan field
• Simultaneous source rotation, table translation and data acquisition
• Produces one continuous volume set of data for entire region
• Data for multiple slices from patient acquired at 1sec/slice
• Slip ring technology – 1989 kalender
• Very high power x ray tubes
54. • Scan speed
• Improved Contrast
• Improved detection – BH differences,
small lesions
• Reconstruction and manipulation –
volume of data collected
• Disadvantages of helical CT scans -
introduction of artefacts such as
windmill artefacts
55. Multi slice / detector CT
• multiple detector array
• The collimator spacing is wider and more of the x-rays that are
produced by the tube are used in producing image data
• Opening up the collimator in a single array scanner increases slice
thickness, reducing spatial resolution in the slice thickness
dimension
• With multiple detector array scanners, slice thickness is
determined by detector size, not by the collimator
56. • with fast multislice CT scanners it is possible to scan almost the entire
body of an adult within one breathhold at a slice thickness well below
1 mm.
• Acquisitions with multi-detector row CT scanners are usually
operated in a helical mode.
57.
58. • Multiscanning therefore reduces motion artifacts and consequently
improves image quality
• Development in software and computer capacity lead to processing
and reconstruction in a short time
60. Algebraic Reconstruction
• Algebraic reconstruction technique (ART) is an iterative approach that
solves many equations to find attenuation values of each pixel in the
image matrix.
• This technique requires a computer to solve a large number of
simultaneous equations to reconstruct the image
61. • This method accurately solves the problem, it is
a highly impractical technique. For an image slice
of N by N voxels, it requires the solution of a
system of at least N2 equations.
• Because this process is extremely
computationally taxing, other methods with
shorter reconstruction techniques provide a
better alternative.
62. Iterative recontruction
• It start with assumption that all point in matrix have same value and it was
compared with measured value and make correction until Values come with in
acceptable range
• Iterative (statistical) reconstructions are sometimes used
• These are routinely used in nuclear medicine.
• available for commercial CT scanners
63. • Potential benefits of iterative reconstructions
• the removal of streak artefacts (particularly when fewer projection angles are
used)
• better performance in low-dose CT acquisitions
• However, images may be affected by other artefacts
• aliasing patterns
• overshoot in the areas of sharp intensity transitions
64. Simple back projection
• The image is created by reflecting the attenuation profiles back in
same direction they were obtained
• The figure below shows
(a) the X-ray projection under a certain angle
(b) leading to one transmission profile
65. • The backprojection distributes the measured signal evenly over the
area
(c) under the same angle as the projection
(d) yielding a strongly blurred image
• Transmission profiles are taken from a large number of angles and
backprojected
• Mathematics shows that simple backprojection is not sufficient for
accurate image reconstruction in CT
66. Filtered back projection
• Most popular algorithm
• Raw data is mathematically filtered by a convolution
kernel (filter) and back projected
• Compensates sudden density changes, that cause image blurring
• Reverses image blurring and restores true image of the object
• Types of convolution kernel
• Ram-Lak filter (Ramp Filter)
• Sheep-Logan filter
• Hamming filter
• Soft tissue filter
x
y
68. Image quality
• Image quality is the visibility of diagnostically
important structures in CT image
• The factors that affect Image quality are
• Quantum mottle (noise)
• Resolution (Spatial and Contrast)
• Patient radiation dose
69. Quantum mottle
• It is the statistical fluctuations of x ray photons absorbed by the
detector
• Quantity and quality of x ray beam, number and effieciency of
detector, scan time
• Mottle becomes more visible when reconstruction accuracy improves
• Noise can be reduced by increasing Tube current (mA) at the cost of
patient exposure, or by increasing the reconstructed slice thickness,
at the cost of spatial resolution
70. Spatial resolution
• It is the ability of CT scanner to display separate image of two objects
placed close together
• Depens on xray focal spot size, detector size, reconstruction
algorithms, and display
• Usually measured in lp/cm (5-15lp/cm)
71. Contrast resolution
• It is the ability of CT scanner to display an image of relatively large
object (2-3mm) that is slightly different in density from its
surroundings
• It depens on tube current, beam filtration and reconstruction
algorithm
72. • Temporal resolution is the ability to resolve fast moving objects in the
displayed CT image.
• Good temporal resolution avoids motion artefacts and motion
induced blurring of the image.
• A good temporal resolution in CT is realized by fast data acquisition
(fast rotation of the X-ray tube).
73. Artifacts
• In computed tomography (CT), the term artifact is applied to any
systematic discrepancy between the CT numbers in the reconstructed
image and the true attenuation co-efficients of the object .
• Artifacts can seriously degrade the quality of computed tomographic
(CT) images, sometimes to the point of making them diagnostically
unusable.
• To optimize image quality, it is necessary to understand why artifacts
occur and how they can be prevented or suppressed.
74. On basis of origin
1. Physics-based artifacts.( result from the physical processes involved
in the acquisition of CT data)
2. Patient-based artifacts .(caused by such factors as patient
movement or the presence of metallic materials in or on the
patient)
3. Scanner-based artifacts.( result from imperfections in scanner
function)
4. Helical and multi-section technique artifacts.(produced by the
image reconstruction process)
75. • Phyisics based
• Beam Hardening ( Cupping Artifacts & Streaks and Dark Bands)
• Partial Volume
• Photon Starvation
• Undersampling
79. Cardiac CT
• Cardiac CT is based on the
synchronization of image
reconstruction with the ECG and
selection of the best cardiac rest
phase
• Reconstructions of the heart at
different cardiac phases
demonstrating the difference in
blurring of the coronary arteries at
different cardiac phases
80. • Cardiac scanning requires the cardiac motion to be minimised.
Therefore to “freeze” the motion
• Image during phase of least cardiac motion (generally diastole, or end systole)
• Cardiac reconstruction can be retrospective ECG-gated
reconstructions and prospective ECG-triggered reconstructions.
• Retrospective ECG-gated reconstructions
• A helical scan is performed with an overlapping pitch
• The cardiac phase selection data is selected retrospectively based on
registration of the raw data and the ECG during one or more entire cardiac
cycles.
• Prospective ECG-triggered reconstructions are “step-andshoot” (i.e
“axial”) acquisitions. An advantage of such acquisitions is the
reduction of patient dose.
82. CT Fluoroscpy
• Dynamic CT can be used for image guided interventions, this
technique is referred to as CT fluoroscopy.
• Technical developments in CT that have provided the technical
preconditions for CT fluoroscopy
• continuous rotating X-ray tube, short rotation time
• hardware fast enough for real- time image reconstructions.
• CT fluoroscopy is routinely used for taking difficult biopsies.
• Relatively new clinical applications are guidance of RF ablations,
vertebroplasty, kyphoplasty and alcohol ablation of tumors.
83. • The noise is much higher in the image of the CT fluoroscopy guided
puncture compared to the diagnostic plan scan.
• During CT fluoroscopy modest image quality is usually sufficient
• CT fluoroscopy should be performed using a relatively low tube
current in order to reduce exposure of the patient and medical staff
84. • Drainage of fluid collections such as cysts, abscesses (pus),
lymphoceles (lymph fluid), bilioma (bile), hematomas (blood), for
example, to remove fluid from an infection or wound
• Diagnostic biopsy to remove a tissue sample for pathologic or
cytological lab testing
• Pain therapy, for example, the injection of therapeutic agents into a
spinal disk space to alleviate pain (see above images)
• Minimally invasive operation, for example, cyst removal or ablation
(cutting away) of tumors (such as brain tumors)
85. • Dynamic study of knee or elbow motion, swallowing or study of the
larynx
• CT arthrogram: injection of contrast into joint space for easier
diagnosis of injury
• Guidance of embolization to stop bleeding, for example, in liver and
spleen trauma
• Monitor difficult endoscope placement, for example in the
gastrointestinal tract
86. CT Angiography
• The goal of CT angiography is to obtain 3-
dimensional information regarding the anatomy
of the vascular system
• widely used in imaging the coronary arteries to
locate blockages and/or stenosis
• very important in areas where the vasculature is
highly convoluted, such as the Circle of Willis in
the head
87. • Diagnosis of pulmonary embolism,
• Aneurysm in major blood vessels,
• Aortic dissection, atrial fibrillation,
• Visualization of the renal arteries,
• Arteriovenous malformation, and
• Coronary artery disease
• Stent placement and coronary bypass surgeries
• DSA
88. CT Perfusion
• Perfusion refers to the supply of blood to a region of tissue
• It is particularly relevant in conditions of hypo-perfusion, including
stroke and stenosis.
• In the context of CT, it is mainly applied to the brain.
89. • The goal of this application is to obtain quantitative images
representing the rate of blood flow to a region and volume of blood in
that region at any given moment
• Perfusion CT is unique among CT applications in that it provides
functional information, whereas most applications provide
anatomical information
90. Dual Energy CT
• Dual energy CT, also known as spectral CT, is a computed
tomography technique that uses two separate x-ray photon energy
spectra, allowing the interrogation of materials that have different
attenuation properties at different energies.
91. • material decomposition images (mapping or removing substances of
known attenuation characteristics, such as iodine, calcium, or uric
acid)
• virtual non-contrast images (iodine removed)
• iodine concentration (iodine maps)
• calcium suppression (calcium removed)
• uric acid suppression (uric acid removed)
• electron density maps
• effective atomic number (Zeff) maps
93. MPR
• uses the 3-D data to show other
planes that were not acquired
directly during the acquisition,
including sagittal and coronal
cross-sections reconstructed
from the axial images.
• Since the entire volume data is
available, it is possible to achieve
any required plane
94. Curved MPR
• used for the analysis of vessels
where the plane of the cut is
parallel to the vessel, thus showing
the anatomical details of the
vessel.
• When cutting perpendicular to the
vessel, the real dimensions of the
vessel can be measured.
95. Maximum Intensity Projection
• MIP is a reconstruction whereby in the view
angle selected, the maximal intensity value
along the line perpendicular to the view
represents this line of pixels in a 2- D
presentation of the reconstructed body.
• MIP reconstruction is mainly used to show
vessels with contrast material in CT
Angiography (CTA) and MR Angiography (MRA),
but is also used in PET examinations to provide
clear views of lesions
96. Minimum Intensity Projection
• A variation of MIP is the Minimal Intensity Projection, added at a later
stage. Here the minimal value along the view line is representing the
line
• This type of reconstruction is used to demonstrate organs filled with
air in CT examinations, such as airways and sinuses
MIP Min IP
97. 3D SSD
• 3-D Surface Shaded Display (3-DSSD) that recognizes tissue by its
density or manually by drawing the contour of the organ
• This method actually shows only the surface of the organs as an
opaque object. Slicing through a surfaced rendered object will not
reveal internal objects
3-DSSD reconstruction of Abdominal Aneurysm
(left) and of Cranial (MCA) Aneurysm
98. VRT
• Volume rendering reconstruction (VR)
takes the entire volume of data,
calculates the contributions of each
voxel (volume pixel) along a line from
the viewer’s eye through the data set,
and displays the resulting composite
for each pixel of the display.
99. • Virtual Endoscopy
• With virtual colonoscopy (VC),
internal vessels or organs are
seen as if a virtual endoscope is
penetrating the body and viewing
the organ from a virtual
viewpoint
• colon, small intestine or the
stomach
100. • Vessel Analysis
• allows easy identification and
display (panoramic and cross
sectional) of vessels such as
carotids, aorta and vessels of the
extremities
