Imaging with X rays Flashcards
Define spatial resolution:
this is the ability to detect fine details.
T/F?
The degree of contrast between tissue type is extrinsic to imaging technique.
False
The degree of contrast between tissue type is EXTRINSIC to the imaging technique. It depends on the properties of tissues and the method of image formation.
In digital imaging it may be enhanced using post processing features.
Define contrast:
this is the ability to distinguish between adjacent structures in the image.
How would you test for SR?
equally spaced bars , the space between them is the same as the width.
SR as the highest occurring freq of lines that can be resolved
What is the unit of SR?
number of lines per mm or cm.
with a resolution of 101 lp/mm, you might expect to resolve details as small as ………mm
0.05mm
The smallest visible detail size is approximately half of the inverse of the resolution expressed in this way.
This type of noise is seen as graininess in fluoroscopy
Quantum mottle
T/F?
SNR increases as the number of photons increases.
True
once the x ray is passed through the patient, its captured on a large phosphor screen. this converts the invisible x ray into a visible image. What happens next: (3)
- recorded as a -ve image on film- film screen radiography
- recorded electronically prior to printing on a hard copy device- CR/DDR
- Displayed as a +ve image on a TV monitor- fluoroscopy
what does teh tissue contrast depend on?
The thickness of the structure
and the difference in linear attenuation coefficient of the tissues involved.
C ~(LAC1-LAC2)t
T/F?
The thicker the structure the greater is the contrast.
True
what does the attenuation depend on?
on tissue density and atomic number
What is the effect of high Kv on contarst
the higher the kv, the smaller the attenuation coefficient and the difference between them and therefore the less the contrast.
Why low KV increases contrast and vice versa?
The contrast is large at low kv because of the effect of photoelectric absorption in higher atomic number material (eg bone and muscle). It decreases notably as kv is increased and teh relative probability of compton increases
what is the effect of kv and contrast in low atomic number:
the contrast bwn the low atomic number tissues eg fat and muscle is small and does not decrease very much when the tube kv is increased.
The contarst bwn air and tissue (similar atomic number ) is due to what?
due to large difference is density
how could we increase contrast between soft tissue?
- lower the kv
2. use contrast medium with high atomic number to max photoelectric absorption.
what is Z and Ek of Iodine
Z= 53 EK= 33 KeV
what is Z and Ek of BARIUM?
Z= 56 EK= 37KeV
What is the minimum dose to produce satisfactory image in film screen radiography?
3 microGy
What is the minimum dose to produce satisfactory image in fluoroscopy?
0.2-0.5 microGy /s
T/F?
The entrance surface dose is usually lower than the exit dose.
False
The entrance surface dose -ESD- is usually much higher than the exit dose.
Compare the ESD with exit dose for the following.
- PA CXR
- skull
- AP pelvis
- Lateral view of lumbar spine
- 10x greater
- 100x
3.1000x
4.5000x
The averaged absorbed dose is within the region being imaged lie somewhere bwn entrance and exit dose.
what is the effect of increasing mA on dose and film density?
Increasing mA increases dose- both ESD and exiting dose.
It also produces darker image.
What is the effect of increasing kV on beam quality?
using a higher kV makes the beam more penetrating and increases the proportion of high energy photons that reach the screen film.
what is the effect of increasing kV on ESD and exit dose?
a lower entrance dose is needed for the same exit dose.
Increasing kV—> reduces ESD (skin dose)
what is the effect of filtration on skin dose?
increasing filtration reduces skin dose despite the fact that an increase in mA is needed.
T/F?
Increasing the focus to film distance-FFD- increases the dose to patient.
False
Increasing the focus to film distance-FFD- REDUCES the dose to patient.(SKIN DOSE),
although this necessitates increasing the mA needed to produce the desired image, it nevertheless reduces the skin dose.
The S/P depends on what?
The S(scattered radiation)/P (primary beam) depend on
- thickness of the patient’s body and
- the area of the beam.
What is the S/P for
- PA chest
- Lateral pelvis
- 4:1 for PA chest
- 9:1 for a lateral pelvis
What is the effect of volume of tissue irradiated and scatter?
The amount of scatter produced in the patient may be decreased by decreasing the volume of tissue irradiated.
How could you reduce the volume of tissue being irradiated?
By using collimation
By compression
Name three ways in which scatter is controlled by with penalty of increasing mA and hence patient’s dose:
- kV
- Grid
- Air gap
What is the effect of kV on contrast and why?
Reducing the kV does increase the contrast but primarily because of the increased differential photoelectric absorption.
What is a grid?
it consists of thin strips of heavy metal sandwiched between thicker strips of low attenuation interspace material.
What is the effect of air gap on contrast
use of air gap increases contrast but necessiates an increase in teh kV or mA.
it results in a magnified image.
Whatis the distance between the lead strips in the anti scatter grid?
0.05-0.07mm wide.
What is the number of strips per cm, the line density, in grid reconstruction?
30-80, with 40 strips/cm being typical.
The repition interval is 0.25 mm with the gap between strips being 0.19mm with 0.06mm wide lead strips.
what is the gap between lead strips filled with?
The gap is filled with an interspace material that may be Al, but is preferably a lower attenuation material such as carbon fiber.
What is grid ratio and what is it usually?
The grid ratio is the ratio of depth of interspace channel divided by its width.
Typically 8:1
What is the effect of larger grid on the contrast?
The larger the grid ratio the smaller is the angle of acceptance and the more efficient is the grid at absorbing scattered radiation, thus he greater is the contrast in the image.
T/F?
with very large field , esp at high kV more scatter is produced and a high ratio grid is preferrable.
True
T/F?
A grid is usually used in children or extremeties.
False
less scatter is produced in the patient.
It is not required when the air gap technique is used.
define contrast improvement factor:
contrast with a grid divided by contrast without a grid
Usually between 2-4
depending on grid ratio and the various factors affecting the relative amount of scatter produced.
How could you reduce magnification using FFD and object to film distance.
By using a longer FFD or by decreasing the object to film distance
What is a distortion:
How could you minimise it?
this refers to a difference between the shape of a structure in the image and in the subject.
This could be reduced by using a longer FFD.
what is an unfocused grid?
if lead strips are parallel to each other and to the central ray of xray beam.
The rays obliquely hit the grid- due to divergence .they will therefore be increasingly attenuated until at an angle that is half the acceptance angle there would be complete cut off
How could you reduce the effect of unfocused grid?
by using a longer FFD or a grid with lower grid ratio.
what is a focused grid?
the strips are tilted progressively from the centre to the edges of the grid so that they al point towards the tube focus.
what are the 2 functions of grid?
to transmit primary radiation and to absorb scattered radiation.
This may be judged by its selectivity (usu in the range of 6-12)
how do you measure selectivity of a grid?
fraction of primary radiation transmitted/ fraction of scattered radiation transmitted
The fraction of scattered radiation transmitted depend on what?
depends on grid ratio and kV
T/F?
Using antiscatter grid increases patients’ dose.
True
This is because for same film density mA has to be increased
What is a grid factor?
exposure necessary with a grid/ exposure necessary without a grid
typically in teh range of 3-5
What does the grid factor depend on?
grid ratio
patient thickness
kV
what alternative is there for grid?
collimator- usu 5mm wide.
usually used in CT.
What are the 3 different types of unsharpness:
geometrical
movement
absorption
How could you reduce the geometrical unsharpness/ blurring/ penumbra
small focal sot size
decreasing the object-film distance
using a longer FFD
How could you reduce the movement unsharpness
by immobilisation - eg in mammo
by holding breath
short exposure time- req use of high mA and a sensitive imaging system)
Give one example of absorption unsharpess and where you are more likely to see this
a gradual change in the absorption near the edge of a tapered or rounded structure- eg blood vessel.
what is the target made of in X-ray tube?
tungsten
The target is a disk that is 10-15mm in diameter and about 1mm thick.
It is set into a cylindrical block of copper
The heat is transferred to the copper by…..
conduction
the X-ray tube within its housing is surrounded with….
oil
What detrmnes the tube rating?
The actual focal spot.
What is the target angle?
7-20 degrees
What is the effect of steeper target on tube rating and the field covered?
Use of steeper target serves to increase the target heat rating for a given effective focal spot or to reduce the effective focal spot size for a given target heat rating.
The steeper the target the narrower the useful x ray beam and the smaller the field covered.
Give an example where steep angle is used
mammography and in cardiac angiography
How many focal spot are there and why?
there are 2,
The smaller is used for improved resolution and the larger one for thicker part of the body where a greater intensity of x rays is needed.
What focal spot sizes are used in mammo, general radiography, Fluoroscopy and CT scanning?
- 1 and 0.3 mm for mammo
- 6-1.2 mm for general radiography
- 6 mm for fluoroscopy
- 6-1.0 mm for CT
What is blooming?
As mA increases the focusing of the electron beam becomes less effective, this is referred to as blooming and it becomes more significant at lower settings of kV.
go through focal spot size
on page 58/59 of farrs
how could you spread the heat in X-ray tube
rotating anode which is made up of Tungsten.
What is anode disk made of?how big is it in diameter?
it is 7-10cm or more in diameter.
usually made up of Molybdenum. alternative is graphite
Why would you use molybdenum in anode disk?
because of its high melting point and low density.
what is the target made of?
it is usually an alloy of tungsten and rhenium which has better thermal characteristic than pure tungsten and does not roughen with use as quickly.
what is the stem made of?
what is the rotor made of?
molybdenum
a blackened copper rotor
what lubricates the bearings?
soft metal such as silver, which allows the rotor to rotate freely within a vacuum.
What are the good thermal properties that are used in anode X-ray tube?
high melting point and relatively low conduction rate in its stem.
describe the heat loss process in X-ray tube.
Heat is taken up by the oil and transferred by convection to the housing from which it is lost by radiation and in some design , by fan assisted convection through the surrounding air
What is the benefit of oil?
it transfers heat from the tube to the housing, it is also electronically insulating.
The housing itself is lead lined , to prevent radiation being emitted in directions other than that of useful bea.
what is the unit of heat loading and what is the formulae?
Kv x mAx constant potential
measured in joules or heat units.
what is the effect of increasing exposure time on mA?
The allowable mA at a particular kV increases as the exposure time is lengthened.
The effect of increased exposure on heat rating?
Rating decreases as the exposure time is increased.
The effect of increasing kV on Rating.
Rating is decreased in inverse proportion as the kV is increased
The effect of increasing the effective focal spot size on Rating.
increases with the effective focal spot size and for a given effective focal spot that is greater for smaller target angles.
The effect of rotating vs stationary anode on Rating.
The speed of anode rotation
rating is greater for a rotating anode
it is greater for a high speed anode because the heat is spread more evely
The effect exposures on Rating.
for exposure shorter than 1 s, is greater for a constant potential , than for a single phase pulsating potential because the former produces heat more evenly throughout the exposure.
What is the heat capacity in the following:
Standard Rx
Angiography
CT
standard radiography - 250 KJ
angiography- 1 MJ
CT scanner- 5 MJ
What rate can teh X-ray tube run at usually?
they can run continuously at 350 J/s - 4mA at 90 kV
What is a space charge effect?
The max mA is smaller at a low kV than at a high kV
in general X-ray tubes for diagnostic radiology do not operate above 150 kv and the max mA for radiography is no more than 700mA.
