X RAY/CT Flashcards
what is x ray
cathode towards metal target plate anode
sheet of electrons
thermionic emission
in vacuum towards metal plate anode
CT mechanism
gun fires electron beam focus coil deflection coil target rings and detector rings ring rotates round patient to fire sheet of electrons with no superimposition of structures like in x ray
In computed tomography, a transverse slice of the patient is imaged, avoiding the superimposition of adjacent structures that occurs in conventional radiography.
The slice is defined by a “sheet of x-rays” produced by a narrow, well-collimated, x-ray fan beam rotated around the patient.
The x-ray beam is attenuated by absorption and scatters as it passes through the patient.
Detectors on the other side of the patient measure the x-ray transmission through the patient. These measurements are repeated many times from different directions, whilst the tube is pulsed as it rotates 360° around the patient.
GE CT scanner
64 detectors
detector and tube move around patient at the same time
4th generation x ray tube alone rotates with thousands of stationary detectors in a tube formation
dual bolus CTU
initial bolus 50-75ml is administered at T0
5-7 mins later second bolus
patient scanned at 60-90 seconds
initial bolus in pyelographic phase
poportion in nephrogenic phase
second bolus is in vascular phase
2 scans - non contrast and then second scan 60-90 seconds after second bolus
ultra low dose CT KUB
ultra low dose
lower current
wavelength x ray
10-8 to 10-12 m
or 0.1 to 10nm
1nm = 10-9m
visible light wavelength
380-700nm
em spectrum
from <0.1nm 10 to -19 m
to 1000m
Gamma 10-19 x ray 10-9to12 UV 100nm light 300-700nm Infra red 10um microwaves 1cm radio 1m long waves 1km
what happens to x rays
X-rays may be:
Transmitted: pass through unaffected
Absorbed: transfer to the matter some or all of their energy
Scattered: diverted in a new direction, with or without loss of energy
what is attenuation
Attenuation is the fractional reduction in the intensity of the primary x-ray beam as it passes through a medium.
Attenuation = absorption + scatter
The linear attenuation coefficient is related to the attenuating property of a material, i.e., how well it absorbs, scatters, or transmits x-rays.
Tissues of different density cause attenuation of x-rays at different rates.
For example, a higher proportion of x-rays will pass through a similar volume of lung than bone, and hence, a corresponding higher proportion of x-rays will reach the detector. This allows different tissue types to be differentiated in the final image
how is image produced in x ray
screen composed of phosphors
x ray beams collides with this
light is emitted causing a reaction in x ray film
latent image produced
morden use a resuable plate
photostimulable phosphor
following exposure plate scanned laser beam and light emitted is measured to form a digital image
how to minimise radiation with c arm
screening time, pulsed vs continuous fluro, low frame rate
use collimation for reagion of interest
reduce contrast of image
position patient as close to detector as possible
omnipaque dose
300mg/mL
Excretory Urography
Omnipaque 300/350: 200-350 mgI/kg body weight
i.e. 50mls
omnipaque dose
osmolality
ionic/monomer
300mg/mL
Excretory Urography
Omnipaque 300/350: 200-350 mgI/kg body weight
i.e. 50mls
LOCM - still higher than plasma 2-3 x i.e. 884 mOSM/kg
non ionic monomer
HU units water air fat tissue bone stone
CT pixel numbers are proportional to the difference in average x-ray attenuation of the tissue within the voxel and that of water. A Hounsfield unit (HU) scale is used; water is assigned a value of 0; the scale extends from -1,000 HU for air to +3,000 HU for dense bone.
Soft tissue +100 to +300 HU
Fat -120 to -90 HU
Kidney stones 300 – over 1000HU
split bolus technique
split bolus technique where an initial bolus of contrast (50–75 mL) is administered and a second bolus (50–75 mL) is administered 5–7 min later.
The patient is then scanned after a further 60–90 s. When this single post-contrast scan is obtained, the majority of the initial bolus is in the pyelographic phase with a proportion in the nephrogenic phase, and the second bolus is in the vascular phase; the maximal anatomical detail of the kidney and its collecting system can, thereby, be obtained
cortico medullary vs nephrogenic phase
coricomedullary 25-40sec
diff tumour from pseudotumour based on enhancement, but not good if in renal medulla
nephrogenic 60-100s
most important for tumour detection
tumour thrombus
contrast agents
based on iodine
classifications
Group of drugs used to improve visibility of internal organs in XR based imaging techniques due to high atomic weight of iodine = radiodensity
Based on the chemical modification of a 2,4,6-tri-iodinated benzene ring
Different types: ionic/nonionic and monomeric/dimeric
They do not differ greatly in their imaging capabilities
They do differ in their SE profile
iodine is water soluble
LOCM vs HOCM toxicity profile
less nephrotoxic
LOCM still higher than plasma 2-3 x
visipaque is iso osmolar 290mosmol/kg but expensive
side effect contrast agent
<3% mild hypersensitivity flushing uritcaria retching coughing dizziness
mod severe <0.004% vomiting, headache, facial oedema, anaphylaxis
delayed allergic reaction >1 hr, rash, skin redness, swelling
CIN 2-5% peaking day 3-5 return to baseline 10-14days
CIN peak incidence, when resolve
CIN 2-5% peaking day 3-5 return to baseline 10-14days
mechanism of CIN
due to hypoxia
Despite extensive speculation, the actual occurrence of contrast-induced nephropathy has not been demonstrated in the literature.[4] The mechanism of contrast-induced nephropathy is not entirely understood, but is thought to include direct damage from reactive oxygen species, contrast-induced increase in urine output, increased oxygen consumption, changes in dilation and narrowing of the blood vessels to the kidneys, and changes in urine viscosity.[citation needed]
voxel vs pixel
The slice is subdivided into a matrix of 512 × 512 volume elements (voxels). The image is reconstructed by a computer as a corresponding matrix of 512 × 512 picture elements (pixels).
A pixel is a representation of the average linear attenuation coefficient of a voxel. The image is displayed as a matrix of pixels; the brightness or greyscale value of each pixel represents the average linear attenuation coefficient of the contents of the corresponding voxel.
voxel vs pixel
The slice is subdivided into a matrix of 512 × 512 volume elements (voxels). The image is reconstructed by a computer as a corresponding matrix of 512 × 512 picture elements (pixels).
A pixel is a representation of the average linear attenuation coefficient of a voxel. The image is displayed as a matrix of pixels; the brightness or greyscale value of each pixel represents the average linear attenuation coefficient of the contents of the corresponding voxel.
