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Devices & Clinical Applications of CT
Transcript of Devices & Clinical Applications of CT
Attenuation /Absorption Coefficient
Low attenuation - dark (air)
High attenuation - light (bone)
discriminates densities of more than 2% difference in attenuation
1st scanner - 1971
80 * 80 pixels
20 minutes per ima
1st Oct 1971
up to 30 detectors (30 times more efficient)
still used rotate - traverse
160 acquisitions per rotation
decreased linear movement (from 160 to 6 steps)
increased angle of rotation (fan beam angle)
10 ~ 90 seconds per image
larger fan beam to cover whole fov
larger no. of detectors 288 ~ 800
no transverse motion
4~5 seconds per acquisition
Spiral / Helical scanning
1983, 11 years after Sir Godfrey Hounsfield introduced the CT,
Vannier wrote about their work with three-dimensional (3D) reconstructions from CT data.
very high cost of computers and specialised software
limited computer power and computer memory
very long reconstruction times
very crude 3D reconstructions
Principles of reconstruction
A 3D rendering technique represents a volume of data in one or more two-dimensional planes
conveys the spatial aspect of the data with use of visual depth cues.
Shaded Surface Display
first method to be introduced clinically and is still available on some equipment software.
SSD identifies surfaces between tissues of different attenuation and displays only those surfaces
each voxel is determined to be part of not part of the object of interest
example: if a threshold of 100 is selected, all voxels with a lower HU will be removed
•For a depth cue, surface contours are displayed with surface shading, having anatomy closer to the viewer appear brighter.
• Orthopaedics and maxillofacial surgeons and dentists found this post processing technique ideal and took advantage of this type of reconstruction.
Maximum Intensity Projection
only the voxel with the highest HU value along the z-axis is projected onto a 2d image
For each x y coordinate only the pixel with the highest HU along the z axis is represented
•This method tends to display bone and contrast material filled structures preferentially.
•The problem is that bone also has a high CT number and thus has to be removed by editing prior to MIP.
•Other low attenuating structures are not well visualised
•The clinical utility of MIP has been evaluated extensively and MIP has proved to be particularly useful in its original application: creating angiographic images form CT and MR data.
•A contrast enhanced vessel has a much higher CT number than its surrounding soft tissue and will be preferentially displayed with this technique.
• The displayed pixel intensity will represent only the material with the highest intensity along the projected ray.
• MIP images are not displayed with surface shading or other depth cues, which can make assessment of 3D orientation difficult
• A high intensity material such as calcification will obscure information from intravascular contrast material.
most advanced... most computer intensive 3-D rendering algorithm
overcomes many of the problems seen with surface rendering and MIP.
incorporates all of the relevant data into the 3-D image
in VR we assign an opacity level for every HU.
the colour of the voxel also depends on its HU.
each voxel will have an opacity (or level of transparency) depending on the attenuation of the tissue it represents
a view point is defined in space relative to the volume
this can be manipulated in real time
pixar image computer
originally developed in the 1980s
renders the entire volume of data rather than just surfaces or maximum intensity voxels, and so potentially conveys more information than a surface model.
the opacity is not linearly assigned to attenuation levels
does not rely on a single threshold but uses opacity curves
a simultaneous display of calcified plaques and vessel lumen is possible.
•The technology permits superficial structures to be peeled away and enables the spectacular "fly-through" of virtual endoscopy.
•Because the information from the entire data set is incorporated into the resulting image, powerful computers are necessary to do volume rendering at a reasonable speed.
Min Intensity Projection
displays voxels below a set threshold rather than above
opposite of MIP
mainly used for air filled structures such as lungs
Slip-ring tech (1987)
Helical / Spiral (1989)
continuous data acquisition
improved z axis resolution
faster scan time
heat capacity of x-ray tubes
speed of data acquisition
computer processing (speed and cost)
1991 - 1st dual slice scanner
run mainly in x y direction
longer acquisition time
still limited by z-direction resolution
1998 - 4 slice
longer scan lengths
improved z-axis resolution
2001 - 8 slice
2003 - 16 slice
...for the patient
make do without arterial catheterisation
still uses contrast / still uses radiation
...for referring physician
diagnostic and therapeutic interventions separated
allows referrer to be involved in reviewing therapeutic options and treatment planning
reduced risks allows for an increased referral rate
early diagnosis of vascular conditions
... for hospital / MID
no hospital stay
no expensive work-up
no modification of anticoagulation medications
no puncture site compression
no need for recovery area
less radiologist time needed
A CTA acquisition is volume rendered and can be viewed from any imaginable angle... no need for additional runs
DSA will only show contrast column
cta will show
calcified & soft plaque
extra vascular haemorrhage
CTA allows for accurate measurements of stenosis and aneurisms
allows for accurate planning and selection of most appropriately sized balloons, stents and endografts
besides 3d modes of viewing
most faithful to original dataset
when a number of axial images are used to extrapolate another image in another plane
can also be in a curved plane
ext iliac & superfical femoral
... with stents
superfical femoral with aneurism
standard method of analyzing coronaries and renal arteries
physiological information of bloodflow is missed
any delay in filling is not recorded as in DSA
Abdominal aortic aneurism
what is AAA?
50% increase of normal diameter
degeneration of tunica media
results in SLOW and continuous dilation of lumen
affects mainly elderly white men
smoking a strong risk factor
body surface area
most AAA are asymptomatic and incidental findings
US is the standard imaging tool
bedside in emergency
intima - endothelial layer
adventitia - collagen
media - smooth muscle surrounded by connective tissue
gives structural and elastic properties to artery
most occur below renal arteries and above iliac arteries
unilateral pouch like bulge
bulge involves entire diameter
haematoma that forms as the result of a leaking hole in an artery.
forms outside the arterial wall
is contained by the surrounding tissues.
Also it must continue to communicate with the artery to be considered a pseudoaneurysm
involvement of renals, visceral and iliacs dictates management
infrarenal aortic neck also determines surgical approach.
rupture outside hospital
- <50% chance of surviving to hospital
- of those who make it to hospital.... survival rate falls by 1% per minute
mortality from elective AAA repair is drastially lower than that associated with rupture
importance of early detection [and repair free from from complications]
until they expand or rupture
unimpressive back pain
symptoms from local compression
back pain (from erosion of aneurism into adjacent vertebrae
venous thrombosis (from compression)
symptoms of rupture
65% die from sudden cardiovascular collapse before hospital
altered mental status
sensitivity and specificity
when performed by trained personel
detection of infrarenal AAA
detects free peritoneal blood
ideal screening tool and to follow AAA over time
inability to detect
branch artery involvement
presence of bowel gas and obesity render it useless
it is only seen on half of all AAA
usually performed for abdominal symptoms
aortic wall calcification is the only specific finding
Advantages over US
accurate size measurements
detects involvement of visceral arteries
not limited by obesity and bowel gas
concomittal evaluation of other abdo organs
longer study time
NEED to TRANSFER patient out of A&E
better imaging of branch vessels
not so good for assessing suprenal vessels
not for the unstable patient
may have a role when there is contrast allergies
superseded by CT
open or endovascular
well established... used for over 50 years
8% mortality rate... mainly myocardial infarction
an endograft [cloth graft with metallic exoskeleton]
introduced into the lumen via the femoral vessels
placed within the diseased lumen
contains aortic flow
decrease pressure on aortic wall
over time AAA size will decrease as will the risk of rupture
less per-operative mortality
higher long-term morbidity
leakage into aneurism which results in a sustained pressurisation of the sac [endotension]
up to 30% of endovascular patients will need 2nd intervention to repair endoleak in the 6 years after initial procedure
4 types of endoleaks.
Type IV endoleak
Blood flow due to porosity of the graft fabric itself.
Type III endoleak
Inadequate or ineffective sealing of overlapping graft joints or rupture of graft fabric.
Type II endoleak
Blood flow into sac via backflow from collateral vessels
Type I endoleak
Incomplete/ineffective seal at the end of the graft.
linear attenuation coefficient of each pixel (μ )
each value μρis assigned a CT Number
on each image, CT numbers are represented by a shade of grey depending on the WW/WL selected
ct numbers based on a magnification factor of 1000 are called Hounsfield Units (H)
partial volume effect
CM at a programmed volume and rate of injection.
A saline flush eliminates the wastage of CM in the dead space.
1 and 5 ml per second
depending on the type of study and the quality of the venous access
A dual head injector
• Improved patient throughput,
• Reduced exposure of hospital staff to radiation exposure
• Improved image quality which is also reproducible
Advantages of a power injector include;
reduce the volume of CM
saline after CM.
Hand Injection - less safe
A saline flush injected immediately after the CM and injected at the same rate keeps pushing the CM and prolongs the aortic enhancement plateau.
volume from the syringe to the arterial circulation
• Antecubital vein
• Subclavian vein
• Brachiocephalic vein
• Superior venacava
Total dead space varies 15-30cc.
Normal blood flow in the veins is normally much slower than the rate at which we are injecting the CM
Since during the injection, blood mixed with CM is pushed at a faster rate than normal flow rate, the rate will fall back to normal rate once the injection stops.
This leads to a premature end to the aortic enhancement plateau and does not take full effect of the injected CM.
All components used such as connection tubes, central lines etc. are rated according to the ability to withstand pressure.
a set max pressure which will automatically stop injection if exceeded
typically set at around 300 psi
the pressure on the syringe plunger varies to maintain the programmed injection rate.
pressure on the plunger varies depending on many parameters such as;
Length of tubing
Size of cannula
Details of their pressure rating are included on their packaging and if no reference to pressure rating is listed, it has to be assumed that they are not safe for use with power injection.
Allan McLeod Cormack
had a theory that it must be possible to display differences in absorption.
conceived the idea of using attenuation measurements to reconstruct an image of the brain.