Differential Absorption

• Maximal differential absorption results from lower kVp values
• But as the kVp is lowered patient dose increases
• AS you drop the kVp the film becomes much lighter due to weaker beam, this is compensated by increasing the mA and the time.
• At lower energies the % of photoelectric interactions increases (relative to compton), above 40 kVp compton is the predominant form of radiation.
• To image small differences in soft tissue use low kVp technique ( for maximum differential absorption)
• Four things that can be seen on an X-ray is air, fat, water, metal
• The interaction of X-rays and tissue is proportional to the density (specific gravity) of the tissue.
• Attenuation is the reduction in the number of X-rays as they penetrate matter. X-rays are attenuated exponentially.

X-Ray Quantity- The numbber of x-ray photons in a beam

• Quantity=output intensity=radiation exposure
• Roentgens measure of photons often stated in milliroentgns (mR)
• International community uses gray (Gy) and Sievert (Sv)
• Gray is patient dose
• Sievert is occupational exposure both are the same numerically

• In diagnostic radiology 1 rad = 1 rem = 1 roentgen
• The final measure is in terms of mR/mAs
• The high frequency generators produce double the mR/mAs over the single phase machine when the kV is the same on both.
• At 70 kVp there is usually 3-6 mR/mAs
• As the time goes up the film gets darker, that’s why the high frequency generators require less time.
• MAs
• Quantity is directly proportional
• If the mAs doubles the film density double or gets darker
• If the mAs is halved the film density is halved, gets lighter.

• KVp
• 15% rule not in book
• an increase in the kVp by 15% must be balanced by a 50% reduction in mAs
• 80kVp X 15% = 92
• Conversely a decrease in kVp by 15% must be balanced by a doubling of the mAs
• The result in either case are two films of equivalent average density but a change in contrast.

• Distance
• Inverse square law
• To maintain the density of the film if the FFD is increased 100% (e.g 40"-80") you need to quadruple the mAs.
• Conversely if you cut the FFD by 50% (e.g 80"-40") you would need one 1/4th the mAs to create films of equal density.
• If you increase the distance by 50% (40"-60") you would double the mAs to create films of equal density.
• If the FFD is increased from 40"-72" the mAs is tripled
• Inverse Square Law = I1/I2= (d2/d1)^2
• Example: A technique at 40" is exposed at 300mr. What would the exposure be if the tube was moved to 72" w/out changing the kVp or mAs.à I1= 300mr(40"/72")^2 I1= 93mr
• Example 2: A technique at 72" is exposed to 300mr . What would the exposure dose be if the tube was moved to 40".à I1= 300mr(72"/40")^2 I1=972mr
• With tube tilt you move the tube closer by 1 inch for every 5 degrees of tilt. I1= 15mr(43"/40")^2 I1= 13mr which is a 14% decrease, therefore a tube tilt of 15 degrees does not need to be moved however, above 15 degrees the tube must be moved closer.

X-Ray Quality

• Refers to the penetrating power of the beam and evaluating the kVp
• Hard vs. Soft X-rays
• Hard x-rays have higher kV values
• Soft x-rays have lower kV values

• Measured by half valued layer (HVL)
• HVL is the thickness of an absorbing material necessary to reduce the intensity to ½ of its original value.
• As kVp goes up so does the HVL thickness

• Factors affecting beam quality
• KVp
• Filtration
• Types of filtration
• Inherent- oil and glass and plastic plug built into tube
• Added- Government stipulated thickness of aluminum needed to a filter
• Compensating- Slabs of aluminum and reduces the beam at the top of the thorasic spine.

Beam Restricting Devices

• Two types of photons interact with the film to form image: those which don’t interact and Compton scatter x-rays
• Remenant x-rays- photons that exit the patient and interact with the film
• Collimation reduces patient dose by:
• Limiting the volume of the tissue irradiated
• Secondarily reducing the amount of scatter (also reduces fogging)
• Failure to collimate is the greatest contributor to needless patient exposure.
• Factors affecting scatter radiation
• As kVp increases the relative number of compton interactions increases.
• In general high (optimal) kVp technique is preferred to low kVp technique due to patient dose considerations.
• Field size- as the area of the beam enlarges, scatter increases. Seen more on small film sizes.
• For a spot film, the mAs must be increased to keep the density the same due to the scatter.
• Patient thickness- the thicker the body part the more scatter.
• Patient thickness is under your control
• Deciding to perform lumbar spine and abdominal films in the recumbent versus upright positions
• If you decide to use patient compression

Control of scatter radiation

• Use beam restriction and grids
• Beam restriction
• Aperture diaphragm
• Cones or cylinders
• Variable aperture collimator

• Aperture diaphragms are generally not suitable for general practice
• Cones are primarily used by dentists
• General practice requires variable aperture collimator.
• Variable aperture collimators (positive beam limitation)
• Off-focus radiation may be reduced by a first stage entrance shuttering device
• Second stage collimator shutter leaves are made of lead (at least 3mm thick), work in pairs, independently, yielding square or rectangular fields, which compliment the film size being used.
• A light localization by lamp and mirror.
• Place rheostat on x-ray room lights
• Collimator bulbs are expensive, handle with tissue when installing or their life will be shortened and crack.
• Light must coincide with the x-ray beam.

• If the equipment is properly installed and maintained the resulting film will show evidence of four sides of collimation.
• Collimator adds about 1mm of aluminum filtration equivalent