Chapter 7 Radiography Flashcards
electron source in x-ray tube
filament
negative cathode
positive anode in x-ray tube
target
usually tungsten
line focus principle
target/anode is angled at 15 degrees
-although a large area of the target is irradiated, this will appear small when viewed from the patient’s perspective
focal spot
area generating x-rays
evacuated envelope
offers electrical insulation and shielding
where do primary x-rays go through from xray tube?
x-ray tube window
leakage radiation from x-ray tubes
transmitted through x-ray tube housing
leakage Kair < 1 mGy/h at 1 m per regulation
what is secondary radiation in x-ray tube
sum of leakage and scattered radiation
where do energetic electrons strike?
the target
produce heat and x-rays
anode material stores heat energy
what is tube loading
heat energy deposited in focal spot
what does tube loading depend on
tube voltage
tube current
exposure time
total energy also depends on number of exposures
how do modern anodes spread heat loading over large area?
circular and rotate at high speed (10,000 rpm)
what are stationary anodes embedded in and where are they used
embedded in copper block
portable x-ray units
heat capacity of anode
several hundred thousand joules
why have the anode angle?
permits large area to be irradiated, helping to reduce heat problems, while maintaining a a small focal spot (less blur)
what is power rating
max kW that a focal spot can tolerate in a specified exposure time
power loading for large focal spot
100 kW
power loading for small focal spot
25 kW
how to achieve the required x-ray tube output if power loading is limited?
may have to increase exposure time
how much of the electrical energy supplied to x-ray tubes is converted to heat?
99%
how is heat transferred from focal spot to anode?
conduction
then anode radiates (via light) to tube housing
how are x-ray tube housings cooled
immersed in oil which aids heat dissipation by convection
what happens when anode heat capacity is reached?
anode must cool down before additional exposure is allowed
how long does it take a hot x-ray tube to cool?
several minutes
heel effect
-x-rays are produced within the tagert; thus they are attenuated as they travel through it
-attenuation is greater in the anode direction than in the cathode direction because of differences in path length within target
-thus get higher x-ray intensity at cathode end and lower at anode end
-i.e. anode end of image will be underexposed and cathode end overexposed
put cathode end at thicker side of patient to prevent this
how to reduce magnitude of heel effect
increase anode angle
increase source to image distance
decrease field size
where are low tube voltages used?
imaging extremities (bone)
thin body parts
infants
50-65 kV
intermediate voltages
70-90 kV
high voltages
120 kV
used in chest x-rays
-high kV reduce patient dose when Kair at image receptor is kept constant
-high kV reduces latitude- results in narrower ranged of detected signals that is easier to capture and process
what is tube output proprtional to at a given kV?
tube current
exposure time
tube currents in radiography
a few hundred mA
exposure time in radiography
very short < 10 ms
short < 100 ms
small focal spot size
0.6 mm
-sharp images, better spatial resolution
large focal spot size
1.2 mm
-tolerate high heat loading, therefore reducing exposure times
mA and exposure time of chest xray
200 mA
5 ms
mA and exposure time of abdominal x-ray
400 mA
50 ms
HVLs for 60, 80, 100, 120 kV in mmAl
2,3,4,5
tube output at 1 m (mGy/100 mAs) for 60, 80, 100, 120 kV
3.5, 6.5, 10, 14
what is source to image distance
distance from x-ray tube source to image receptor
what is source to skin distance
distance from x-ray tube source to front surface of patient
what is source to object distance
as it sounds
geometric magnification
SID/SOD
why does geometric magnification create distortion?
because it varies with patient depth
what increases with longer SID?
More radiation is required = more exposure time and more motion blur
most common SID in radiography
100 cm
why are air gaps not commonly used in radiography?
air gap = magnification imaging
-increases focal spot blur
scatter to primary ratio in radiography
> 1
typical grid ratio for scatter removal grid
10:1
grid lines are aligned along what?
anode cathode axis
grid ratio vs voltage and body thickness
lower voltage- lower grid ratio (8:1)
thicker body part- higher grid ratio (12:1)
when are grids optional?
body parts thinner than 12 cm
what does grid do?
increase contrast at the cost of more dose
digital x-ray material
scintillator- data is read out and transmitted electronically. More expensive
photostimulable phosphors- use a reader
how does scatter/primary change with thickness and FOV?
-increases with FOV
-increases with body thickness
explain automatic exposure control
-ionization detector is between patient being x-rayed and image receptor
-when signal from detector matches a pre-set treshold of Kair, the exposure is terminated
-yields consistent image quality despite variations in body shape
-prosthetic device- AEC may overexpose the image
exposure index
-measure of Kair at image receptor
-1 uGy of Kair is EI of 100
deviation index
-quantifies how closely Kair at receptor matches target value
- plus or minus 3 means the exposure is double or half the target Kair
(god forbid they made it + 2 and -2)
SID for chest x-ray
180 cm
-large SID minimizes cardiac magnification, gives more reliable estimate of size of heart
entrance Kair in PA chest x-ray
0.1 mGy
easy to transmit through the lungs
chest imaging
easy to transmit through lungs
therefore 120 kV, to reduce image dynamic range and patient dose
-large focal spot size to reduce exposure to < 5 ms
-10: 1 grid ratio
-uses AEC
-Kair at receptor is 3 uGy, EI of 300- requires about 1 mAs
SID in bedside radiography
100 cm
bedside imaging
-manual techniques- technologist monitors EI
(too much EI = too much dose, low EI = suboptimal image)
-large focal spot to reduce exposure time <10 ms and minimize motion blur
-usually not practical to use gird- use 80 kV to reduce photon energy and reduce scatter
-processing of ICU chest x-rays using unsharp mask enhancement improves visibility of tubes, lines, catheters
-grids must be used for adult bedside abdominal imaging- grid ratio 6:1 is used as these are easier to align than standard grids
abdomen/pelvis imaging SID
100 cm
abdomen/pelvis imaging
-entrance Kair 3 mGy
-80 kV
-large focal spot reduces exposure time < 100 ms
-grid ratio 10:1
-AEC gives Kair at image receptor of 3 uGy- EI of 300
-requires 20 mAs
extremity SID
100 cm
extremity imaging
-50-65 kV
-small focal spot (0.6 mm) to improve sharpness (ex detect hairline fracture)
-reduce max power to 25 kW to reduce focal spot
-60 kV and 400 mA = 24 kW
-grids not necessary but can be used
-Kair at receptor 10 uGy, EI = 1000 (because photon energies are lower). At the same Kair, lower energy x-rays deposit less energy in detectors
pediatric imaging
-large focal spot to minimize exposure time and blur (100 kW power)
-because children are small, magnification is low and focal spot blur is minimal
-optional grid for patients < 12 cm
-kV and use of grid increases as patient is bigger
-EI values ~ 300
when is lesion contrast maximized?
-when voltage is set to just penetrate a patient
-lower voltage will not penetrate
-higher voltage reduce contrast
-penetration decreases with tissue thickness and increases with kV
what does lesion CNR depend on?
-choice of kV and mAs
-when lesion Z is similar to background, reductions in lesion contrast with increasing kV will be modest. But when lesion Z is different from background, reducing kV will improve visibility of the lesion
-mAs impacts mottle. If mAs quadruples, image mottle is halved
is noise fixed with AEC?
Yes
lesion CNR depends on kV
-use of high voltage reduces CNR and patient dose
effect of time on resolution
reducing exposure time reduces motion blur and thus improves resolution
impact of scintillator detector thickness on resolution
thicker = more light diffusion = less sharp
impact of detector thickness for photostimulable phosphors
-thicker = readout laser light scatters more = less sharp
pros and cons of Se-based photoconductors
-good resolution
-rarely used because they absorb little of incident x-rays
limiting resolution of 35x43 cm detector (2kx 2.5 k matrix size)
-pixels are 175 um - 6 pixels/mm, limiting res is 3 lp/mm
limiting resolution of 20x24 cm detector (2kx2.5 k matrix size)
-pixels are 100 um - 10 pixels/mm, limiting res is 5 lp/mm
limiting resolution of digital detectors vs screen films
3 lp/mm vs 6 lp/mm (i.e. half)
in digital radiography, what is usually more important, resolution or CNR?
CNR
examples of artifacts
-false teeth, hairpins, jewellery
-spills of barium and iodine on bedclothes appear as “all white” artifacts
when does grid cutoff occur
grid not aligned properly
-if grid is upside down, central region will have normal appearance but appear white towards edges
-grid cutoff can also occur when SID doesn’t match that of grid
“ghost” image
when a region of an image has received an unexpected high or low exposure, this region can have a different sensitivity for a short period after the exposure
-can give ghost image when subsequent exposure is obtained
-ex: ghost of metallic implant can appear in images obtained after a radiograph on a patient with this implant
-minimize by waiting for digital detector to recover
kerma area product for adult PA chest x-ray
-entrance Kair is 0.1 mGy, areas is 1000 cm^2, KAP is 0.1 Gy-cm^2
lateral projection is double (0.2 Gy-cm^2)
kerma area product for skull x-ray
lateral = 0.5 Gycm^2, AP = 1 Gy cm^2
kerma area product for AP abdominal radiograph
3 Gycm^2
benchmark for KAP for adult body and head ( not chest)
1 Gy cm^2
lower for chest (0.1- 0.2 Gy cm^2) and for kids
skin dose in radiography
<10 mGy
-no clinical importance
embryo dose with abdominal radiography
1 mGy
very low risk
extemety and chest patient effective doses
< 0.1 mSv
skull, cervical spine, abdomen, and pelvis/hip effective dose
0.1- 1 mSv
thoracic and lumbar spine effective dose
1 mSv
anode tube dissipation rate
10 kW
Are SIDs< 100 cm used?
Not in radiography
65 cm used in mammo
limiting spatial res of human eye at 25 cm distance
5 lp/mm
typical xray tube anode heat dissipation rate
10 kW
average kerma-area product for complete xray examination
1 Gy-cm2
