3. Pam Cherry MSc TDCR, Angela M. Duxbury MSc FCR TDCR, PRACTICAL
RADIOTHERAPY PHYSICS AND EQUIPMENT, 2009, Second Edition, publish by
John Wiley & Sons.
Computed tomography is more commonly known as ‘CT’. It was also
known as ‘CAT’ which stands for Computed Axial Tomography.
CT is a way of using X-rays to take detailed pictures or images in
very fine slices through the part of the body that the doctor has asked to be
investigated - similar to looking at one slice of bread within the whole
loaf.
Tomography is actually originated from the Greek words which are
‘tomos’ means slice, and the Greek word ‘graphein’ means write. These multiple slices of images which are two-dimensional (2D) X-ray images that are generated during a CT scan can be reformatted in multiple planes, and reconstructed into three‐dimensional (3D) images - in other words, many pictures of the same area are taken from many angles and then placed together to produce a 3D image.
2D images of CT Scan of heart
3D images for CT Scan of heart
When CT scanners were first invented, they took one slice at a
time and were quite slow when compared to today’s machines. Most modern
scanners now take more than one slice at a time.
This may range from 4 to 64 slices and up to 320 slices for the
most recent machines. This is referred to as “multislice” or “multidetector”
technology and may be abbreviated as MSCT or MDCT.
What CT Scan is used for?
CT images of internal organs, bones, soft tissue and blood vessels
typically provide greater detail than traditional x‐rays, particularly of soft tissues and
blood vessels.
Using specialized equipment and expertise to create and interpret
CT scans of the body, radiologists can more easily diagnose problems such as
cancer, cardiovascular disease, infectious disease, appendicitis, trauma and
musculoskeletal disorders.
CT can be a life-saving tool for diagnosing illness and
injury in both children and adults.
It is also often used to look inside the body before another
procedure takes place, such as radiotherapy treatment or a biopsy (where a
small sample of tissue is taken so that it can be examined under a
microscope).
A radiographer is performing a CT Scan
procedure for her patient
How does it work?
CT Scanner produces more detailed image rather than X-ray machine
as CT Scanner emits a series of narrow beam compare to diagnostic X-ray machine
only emit one way radiation beam.
The X-rays from the CT Scanner will be received by a detector that
is located on the opposite side of your body which can see hundreds of
different levels of density and also tissues inside a solid organ.
Then, images of the scan will be produced by a computer and can be
viewed on a computer monitor, printed on film or transferred to a CD or DVD.
Sometimes a contrast dye is used because it shows up much more
clearly on the screen.
If a 3D image of the abdomen is required the patient may have to
drink a barium meal. The barium appears white on the scan as it travels through
the digestive system. If images lower down the body are required, such as the
rectum, the patient may be given a barium enema. If blood vessel images are the
target, the barium will be injected.
Bottles of barium that patients
need to drink before undergoes CT procedure
Who does Computed Tomography?
Radiographers or radiation technologists will perform the CT Scan procedure for
the patient. The CT scans are interpreted by radiologists who will examine the
images taken by the radiographers in great detail to know any pathological
conditions of the patient.
-to
create pictures of the head including the
skull, brain,
eye sockets, and sinuses
-to diagnosebrain tumors, bleeds, injuries to the brain and other major
brain diseases
Lumbosacral Spine CT
scan
-to
diagnose lower spine and surrounding tissues
-to access the spine for a herniated disk, tumors, and other lesions, the
extent of injuries and blood vessel malformations
-to evaluate the effects of treatment of the spine such as surgery or therapy
Chest/Thoracic CT scan
-to create cross-sectional pictures of the
chest and upper abdomen
-to
visualize organs and tissues
-provides images of multiple tissues such as lungs, heart, bones,
muscles, blood vessels and soft tissues
- to detect acute and chronic changes in lung parenchyma, diagnose tumors,
emphysema, inflammations
CT Angiography
-helps
in the visualization of blood flow in the arteries throughout the body
-used in the diagnosis of aneurysms (bulging), stenosis
(narrowing) of the arteries, dissection of the aorta
Abdominal
CT scan
-to create cross-sectional pictures of the
belly area
-to visualize organs such as stomach, gall bladder,
liver, spleen, pancreas, kidneys, lower gastrointestinal (GI) tract, the
colon and rectum
Lumbosacral Spine CT scan
Thoracic CT scan
Angiography CT scan
Abdominal CT scan
Cranial CT scan
Benefits
CT scanning is painless, noninvasive and accurate.
A major advantage of CT is its ability to image bone,
soft tissue and blood vessels all at the same time.
Unlike conventional x‐rays, CT scanning provides very detailed images
of many types of tissue as well as the lungs, bones, and blood
vessels.
CT examinations are fast and simple; in emergency
cases, they can reveal internal injuries and bleeding quickly enough
to help save lives.
CT has been shown to be a cost‐effective imaging tool for a wide range of clinical problems.
CT is less sensitive to patient movement than MRI.
CT can be performed if you have an implanted medical
device of any kind, unlike MRI.
CT imaging provides real‐time imaging, making it a good tool for
guiding minimally invasive procedures such as needle
biopsies and needle aspirations of many areas of the
body, particularly the lungs, abdomen, pelvis and bones.
A diagnosis determined by CT scanning may eliminate
the need for exploratory surgery and surgical biopsy.
No radiation remains in a patient's body after a CT
examination.
X‐rays used in CT scans should have no immediate side
effects.
Risk
CT Scan might risk the patients’
health through two factors which are ‘radiation exposure’ and ‘contrast
medium’.
Radiation exposure
A CT scanner uses X-rays to obtain
the pictures required for the radiologist to make a diagnosis. As is commonly
known, X-rays are a form of radiation and must be used carefully by
trained professionals to decrease the risks involved. The risks of radiation
exposure are explained fully in the item entitled Radiation Risk of Medical
Imaging in Adults and Children; but in summary these are:
A
very small increase in the risk of developing cancer later in life. This
low risk is considered to be outweighed by the benefits provided
by the scan.
Risk
to an unborn child if you are pregnant. This risk could take the form of a
very small increase in the risk of cancer or a malformation if you are
exposed to radiation during the first months of your pregnancy.
Contrast Medium
There is also a small risk of
allergic reaction to iodinated contrast when it is injected. It is important to
make the radiographer or nurse aware of any other allergies that you may have
prior to having the injection. If you are allergic to other foods or drugs, it
increases the chance that you will have an allergic reaction to iodinated
contrast.
People who are allergic to the
iodinated contrast used in CT may get some of the following symptoms:
CT imaging system is made up of two main components located within
the simulator which are :
1) CT Scanner
2) Laser System
Scanner 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.
The scanner acquire the
image data that will be constructed by the virtual simulation
application into 3D virtual model of the patient.
A) X-ray Tube
The x-ray tube use in
multislice helical CT imaging have special requirements. The x-ray
tube can be energized up to 60s continuously.
Some x-ray tube operate
at relatively low tube current, however for many,
the instantaneous power capacity must be high.
B) Scanner Bore
The traditional 70 cm
scanner aperture with 50 cm maximum field of view is too
limiting for many radiotherapy technique. Therefore, the manufacturer now
produce ‘large bore’ versions providing apertures up to 85 cm which enable
most therapy patient to be scanned in an optional position without gantry
restriction.
However, the use of wide-bore
scanner will results in low contrast resolution and will increase image
noise compared standard scanner.
C) High-speed Rotor
High-speed rotors are used in
most for the best heat dissipation. Experience has shown that x-ray tube
failure is a principal cause of CT imaging system malfunction and is the
principal limitation on sequential imaging frequency.
D) Focal Spot Size
CT imaging systems
designed for high spatial resolution imaging incorporate x-ray tubes with
a small focal spot.
E) Detector Array
Multislice helical CT
imaging systems have multiple detectors in an array up to tens of
thousands. Previously, gas-filled detectors were used, but now, all are
scintillation, solid state detectors.
Scintillation detectors have
high detection efficiency. Approximately 90% of the x-rays incident on the
detectors are absorbed.
F) Scanner Couch
A flat, stable couch is
essential for radiotherapy planning.
Usually made up of carbon fibre
which can be be adapted to fit any scanner cradle.
The couch must be level
and orthogonal in its movement through the scanner aperture with maximum
deflection of 2mm along its range.
Figure of basic components of CT Scanner
Laser System
There are 3 types of lasers
used in the CT Scan which are :
Internal laser
Wall-mounted
laser
Overhead
saggital laser
When lasers are position at zero
setting, their intersection point is coincident with the center of the scan
plane.
A) Internal
laser
All
scanners contain an internal laser to identify the scan plane.
This
internal laser is mounted at the scanner bore of the CT scan machine.
This
type of laser use to mark the patient during the simulation
process as well as treatment process.
Other
than that, it is also used during the QA test to determine the alignment
of the laser with the center of the water phantom.
B) Wall-mounted Laser
These
types of laser usually situated at right and left of the room .
These
laser is used to mark a reference within the image data from which beam
isocenter coordinate are defined.
Other
than that, it is also used to aligned the patient during simulation
and treatment process as well as aligned the couch.
If
non-moving laser is provided, these are mounted at a set height in
relation to the scan aperture, requiring vertical movement to be
affected through adjustment of couch height.
C) Overhead Saggital
Laser
These
laser also have the same function as wall-mounted laser which is used to
mark (on the patient) coordinates defined during simulation and that may
represent isocenter, field corners or markers.
It
is projecting at the same fixed distance as lateral laser bot
orthogonal to the scan plane.
These
lasers are always capable of lateral movement because the
scanner couch may not.
Treatment Couch for CT Scanner in Diagnostic Imaging
In 1960's, after so many research, the
world's first CT scanner was invented. The inventor was Godfrey Newbold
Hounsfield, who was born in England 1919. He and Alan Cormack, a medical
physicist, together developed and placed the first brain scanner into operation
in 1971 for a company called EMI Ltd. Hounsfield conceived his idea in
1967 In 1979, they were awarded the Nobel Prize in medicine and
physiology. The first EMI-Scanner was installed in Atkinson Morley Hospital in Wimbledon,
England, and the first patient brain-scan was done on 1 October 1971. It was
publicly announced in 1972.
Left; Godfrey Newbold Hounsfield
Right; Alan McLeod Cormack
The original 1971 prototype took 160 parallel readings through 180
angles, each 1° apart, with each scan taking a little over 5 minutes. The
images from these scans took 2.5 hours to be processed by algebraic
reconstruction techniques on a large computer. The scanner had
a single photomultiplier detector, and operated on the Translate/Rotate
principle.
The first production X-ray CT machine (in fact called the
"EMI-Scanner") was limited to making tomographic sections of the
brain, but acquired the image data in about 4 minutes (scanning two adjacent
slices), and the computation time (using a Data General Nova minicomputer) was about 7
minutes per picture. This scanner required the use of a water-filled Perspex tank with a pre-shaped rubber
"head-cap" at the front, which enclosed the patient's head. The
water-tank was used to reduce the dynamic range of the radiation reaching the
detectors (between scanning outside the head compared with scanning through the
bone of the skull). The images were relatively low resolution, being composed
of a matrix of only 80 × 80 pixels.
The first CT system that could make images of any part of the body and
did not require the "water tank" was the ACTA (Automatic Computerized
Transverse Axial) scanner designed by Robert S. Ledley, DDS, at Georgetown University.
This machine had 30 photomultiplier tubes as detectors and completed a scan in
only nine translate/rotate cycles, much faster than the EMI-Scanner. It used
a DECPDP11/34 minicomputer both to operate the
servo-mechanisms and to acquire and process the images. The Pfizer drug company acquired the prototype from the
university, along with rights to manufacture it. Pfizer then began making
copies of the prototype, calling it the "200FS" (FS meaning Fast
Scan), which were selling as fast as they could make them. This unit produced
images in a 256×256 matrix, with much better definition than the EMI-Scanner's
80×80.
Since the first CT scanner, CT technology has vastly improved.
Improvements in speed, slice count, and image quality have been the major focus
primarily for cardiac imaging. Scanners now produce images much faster and with
higher resolution enabling doctors to diagnose patients more accurately and
perform medical procedures with greater precision. In the late 1990s CT
scanners broke into two major groups, "Fixed CT" and "Portable
CT". "Fixed CT Scanners" are large, require a dedicated power
supply, electrical closet, HVAC system, a separate workstation room, and a
large lead lined room. "Fixed CT Scanners" can also be mounted inside
large tractor trailers and driven from site to site and are known as
"Mobile CT Scanners". "Portable CT Scanners" are
lightweight, small, and mounted on wheels. These scanners often have built-in
lead shielding and run off of batteries or standard wall power.
In 2008 Siemens introduced a new generation of scanner that was able to
take an image in less than 1 second, fast enough to produce clear images of
beating hearts and coronary arteries.
Generations of CT Scanner
There
are four generation of CT Scanner. In the first and second generation
designs, the X-ray beam was not wide enough to cover the entire width of the
'slice' of interest.
A
mechanical arrangement was required to move the X-ray source and detector
horizontally across the field of view.
After a
sweep, the source/detector assembly would be rotated a few degrees, and
another sweep performed.
This
process would be repeated until 360 degrees (or 180 degrees) had been
covered. The complex motion placed a limit on the minimum scan time at
approximately 20 seconds per image.
In the
3rd and 4th generation designs, the X-ray beam is able to cover the entire
field of view of the scanner. This avoids the need for any horizontal
motion; an entire 'line' can be captured in an instant. This allowed
simplification of the motion to rotation of the X-ray source.
The a, b, c and d images are the x-ray beam
distribution of 1st, 2nd, 3rd and 4th generation of CT scanner respectively.
First generation
detectors:
one
type
of beam: pencil-like X-ray beam
tube-detector
movements: translate-rotate
duration
of scan (average): 25-30 mins
Second generation
detectors:
multiple (up to 30)
type
of beam: fan shaped x-ray beam
tube-detector
movements: translate-rotate
duration
of scan (average): less than 90 s
Third generation
detectors:
multiple, originally 288; newer ones used over 700 arranged in an arc
type
of beam: fan shaped x-ray beam
tube-detector
movements: rotate-rotate
duration
of scan (average): approximately 5s
Fourth generation
detectors:
multiple (more than 2000) arranged in an outer ring which is fixed
