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.
linear attenuation coefficient
The linear attenuation coefficient is related to the attenuating property of a material, i.e., how well it absorbs, scatters, or transmits x-rays.
mosmol/kg what is it
milliosmoles per kilogram
Some medical tests report results in milliosmoles per kilogram (mOsm/kg) of water. An osmole is an amount of a substance that contributes to the osmotic pressure of a solution. A milliosmole is one-thousandth of an osmole
what are contrast agents
heterogenous group of radiopharmaceuticals used to enhance tissue definition
iodinated agents
based on organic salts of iodine
high atomic / molecular weight of iodine 127
making it radiodense
sievert vs gy
sievert is the measure of the health effect of low levels of ionizing radiation
a measure of the stochastic health risk
grey is the absorped dose of energy deposition of 1j/kg and is a measure of the deterministic effect, used in theraputic radiotherapy
risk of anaphylaxis, risk of death
<0.001%
1 in 100,000 disk of death
american college radiology defined post contrast AKI as
1.5 x or more increase in creatinine from baseline in 48-72 hours
arterial phase
corticomedullary phase
nephrogenic phase
timings
15-20 sec
renal arteries
30-40sec
intense enhancement renal cortex, preferential arterial flow to cortex , glomerular filtration contrast, medulla less well enhanced, renal veins
80-120 sec
tubular filtration of contrast , subtle parenchymal lesions
density on ct of renal masses
haemorrhagic cyst on unehanced
fat
equivocal enhancement
> 70 on unenhanced is haem cyst
fat is less than -20 HU
equivocal enhancement 10-20HU - seen in cyst due to beam hardening, or poorly enhancing papillary rcc
oncocytoma characteristis
sharply demarcated
uniform enhancement
central scar
clear cell rcc on ct
most strongly ehancing in CM phase
best seen in nephrogenic phase
papillary rcc on ct
homogenous hypovascular can mimic cysts enhancement can be 10-20 more likely bilateral and multifocal
chromophobe rcc
sharply demarcted
slightly lobulated
can have central scar and spoke wheel pattern contrast similar to onc
often ehance less than ccrcc and more homogenous
XGP
chronic granulomatous infection with accumulation of lipid laden macrophages diffuse renal destruction can be segmental renal enlargement can see macroscopic fat
dual energy ct
two ct data sets acquired correspond to x ray attenuation from lower and higher energy x rays two data sets manipulated to extract information renal stone types renal and adrenal masses
what is attenuation
is the reduction in the energy of the primary x ray beam as it passes through a medium
structure of hyperosmolar ionic monomer
has COO- as well as three iodine and two side units in heaxagonal ring
anaphylaxis protocl
500mcg 0.5ml IM adrenaline 1:1000, repeat 5 min if not better chlorphenamine 10mg, IM or slow IV hydrocoritsone 200mg IV 500ml to 1l fluid challenge child 20ml/kg crystalloid
CIN and IV contrast royal college radilogists
risk of IV contrast non existant for patients GFR more than 45
no precautions needed
risk of CI AKI low or non existant 30-45
may benefit hydration if acutely deteriorating, not recommended for all
GFR less than 30, or acutely deteriating, weigh risk vs benefit, can give periperocedural IV hdyration
but is not absolute CI to medically indicated iodinated contrast media administration
x ray safety in theatres
lead gown thickness 8
IRMER trained surgeon understanding equipment all equipment tested and functioning correctly tube close to patient staff stand far away lead aprons reduce dose 90% 0.35mm thickness lead apron radiation dose to staff and patient justified pregnancy test screening - doors closed and signs ALARA principle
