medical imaging exam

function of xray table and bucky

• supports patient and permit xray transmission

• robust

• radiolucent

• easily cleaned

• hard to damage - ie scratch

• height adjustable and floating

• include time for detector, grid and aec (automatic exposure control)

tube supports

• mechanical framework which holds the xray tube in

• permits easy manipulation flexibility and stability of position

• ceiling/overhead - allows longitudinal, transverse and vertical movement

• floor to ceilding (or wall) - allows longitudinal and vertical movement

exposure chart details are specific

• name of projection

• kVp

• mAs or mA and s

• SID - source to image distance

• grid or non grid

what happens when you press the xray exposure button - first pressure

• anode begins to rotate

• cathode heats to release electrons

what happens when you press the xray exposure button - second exposure

• electrons bombard anode

• xrays produced

• timer circuit “breaks”

cephalad

 towards head

caudad

towards feet

what is the issue with the image

low density

what is the issue with the image

high density

• higher object density leads to high radiographic density on an image

• this is because if the object is more dense it attenuates the beam more - then we say that something that appears whiter on a film has a lower radiographic density

• air and fat is radiolucent and less dense - attenuates the xray beam less so comes up on the xray as black and is radiographically dense

contrast

the difference in density on adjacent areas of a radiographic image

high contrast (short scale)

• greater difference between adjacent densities = more black and white

low contrast (longer scale)

• lesser difference between adjacent densities = more shades of gray (not beneficial)

spatial resolution

• the ability of an imaging system to differentiate between two near-by objects

  • sharpness, detail, definition

differences between DR and CR system

• DR is sharper and has improved spatial resolution

kVp

• kVp controls the penetrating strength or the quality

• the higher the kVp, the more likely the xray beam will be able to penetrate through thicker or more dense material

• a good kVp results in sufficient penetration and contrast

• to high kVp = loss of contrast

• too low kVp = lack of penetration

mAs

• controls the amount of xray photons or the quantity

  • the product of tube current, mA, and exposure time, s

  • the control pannel provides the highest safe mA for the shortest exposure time

• too low mAs = insufficient detail and too bright (low density)

• too high mAs = increased dose and too dark (high density

lattitude

• the range of an xray exposure that delivers a density in the useful range for diagnostic radiography

  • wider range allows for errors, but leads to does creep

  • 50% below ideal exposure = quantum mottle

  • for CR 200% above ideal exposure = contrast lost or burnout

  • for DR 400% above ideal exposure = contrast loss or burnout

  • with DR even if you are incorrect with the kVp and the mAs the image is still quite good - more room for error

  • new machines also have post image abilities to fix an image if it is bad

  • this can sometimes be a disadvantage as once you take an image, realise it is faulty, you cannot figure out if you over exposed the patient or under exposed them as the image seems normal

quantum mottle

• if we double the exposure we get contrast loss and burnout

digital exposure manipulation

• the computer algorythm will undergo automatic rescaling to produce the best possible image

• however, once again

  • quantum mottle/noise if exposure is too low ~ 50%

  • loss of contrast/burn out if exposure too high ~ 200-400%

15% rule for contrast

ncrease kVp by 15% and halve mAs

• lower contrast

• same exposure

• lower dose

decrease kVp by 15% and double mAs

• higher contrast

• same exposure

• higher dose

15% rule for penetration

• a 15 percent increase in kVp will result in doubling the exposure

grids

• transmit direct xrays, absorb scattered xrays

  • increase resolution and contrast

  • require an increase in mAs

body habitus

• thicker tissue requires greater exposure

  • muscle requires greater exposure than fat

  • need a higher exposure in kVp and mAs for xraying people with more fat or muscle over the bone

paediatric exposure

• lower mAs to reduce exposure and motion - kids move around so lower the time of exposure

• check exposure chart

• consider non-grid technique

  • adult AP abdomen 81/10

  • baby AP abdomen 65/2

casts

• if you are imaging someone wearing a cast increased exposure is needed

• fibreglass

  • increase mAs by 20-30 mAs

• medium plaster

  • increase mAs by 50%

• larger plaster

  • increase mAs by 100%

additive diseases

• increase the attenuation of the beam

• anatomically radiodense

  • atelectasis

  • pneumonia

  • metastases

destructive diseases

• decrease attenuation of the beam

• more xrays getting through to the detector

  • osteoporosis

  • emphysema

  • multiple myeloma

post processing

manipulation of the radiograph after aquisition

• windowing - brightness and contrast

• image annotation

• image orientation

• magnification

• post processing collimation

• image stitching

• measurement

• printing

• usually DR and CR can do alot of these

• however we should do them to make sure

post processing collimation - masking or shuttering

• eliminate the ambient light around the image improving resolution and contrast

• does not change dose

edge enhancement or high pass filtering

increases contrast, enhances edges

smoothing or low pass filtering

averages out neighbouring pixels, reduces noise and contrast

finger series

• PA

• oblique

• Lateral

PA finger critique

• no rotation of the digit

• fingernail centered over distal phalanx

• no soft tissue overlap

• soft tissue and boney trabeculation

• open IP and MCP joint spaces

• distal phalanx distal MC

oblique finger critique

• 45 degree digit rotation

• no soft tissue overlap

• distal phalanx to distal MC

• open IP and MCP joint spaces

• soft tissue and boney trabeculation

lateral finger critique

• distal phalanx to distal MC

• condyles superimposed

• clear of other fingers

• soft tissue and boney trabeculae

• lateral second digit we turn the hand around to reduce magnification from the plate

thumb series

• AP/PA

• oblique

• Lateral

AP thumb critique

• no rotation

• equal soft tissue each side

• from distal phalanx to trapezium

• open joint spaces

• no overlap

• soft tissue and boney trabeculae

lateral thumb critique

• condyles of phalanx super imposed

• from distal phalanx to trapezium

• open joint spaces

• no overlap

• soft tissue and boney trabeculae

hand series

• PA

• oblique

• lateral

PA hand critique

• no rotation

• open joints

• slightly separated digits

• all anatomy distal to radius and ulna

• soft tissue and boney trabeculation

oblique hand critique

• minimal overlap of metacarpals 3, 4, and 5

• separation of metacarpals 2 and 3

• open interphalangeal and MCP joints

• digits separate

• whole hand

• distal radius and ulna

• soft tissue and boney trabeculation

lateral hand critique

• true lateral position

• superimposed metacarpals, phalanges, radius and ulna

• phalanges extended

• thumb slightly abducted from hand

ball catchers critique

• both hands to carpal region

• metacarpal heads to separate

• MCPs JTs demonstrated

critique vs interpretation

critique

• refers to radiographic quality

• ie. positioning exposure marker on or off

interpretation

• refers to pathology and abnormal appearances

critique 3 step process

• classify the image - is it diagnostic or not

• determine the cause - is it technical equipment related pathological

• recommend corrective action

• paceman acronym - positioning, anatomy, collimation, exposures, markers aesthetics, and names

structure of a long bone

• most long bones have a diaphasis in the shaft

• also an epiphysis at the bottom and a metaphysis

• fibrous membrane covering its surface

• there is trabeculae in the middle which is quite hollow - in medullary cavity

open fractures

penetrated the skin surface

complete fractures

 all the way through the bone

incomplete fractures

not all the way through the bone

complete fractures may include

• transverse = straight across the bone

• oblique = an oblique line across the bone

• spiral = looks like a corkscrew

• comminuted = more than two parts to the fracture

incomplete fractures may include

• greenstick and fissure

where is the fracture on the bone

• proximal, distal, midshaft, etc

• diaphysis, metaphysis, epiphysis

• intraarticular, extra-articular

• land marks? surgical neck of femur, medial malleolus, etc

is the fracture displaced

• re the fragments displaced, ie out of alignment

• if they are, describe the displacement

  • the displacement of the distal component in relation to the proximal component

rheumatoid arthritis

subluxations, dislocations, joint space narrowing, cysts, erosions

osteoarthritis

• joint space narrowing

• sclerotic margins

• osteophytes

• bony cysts

Scatter radiation

• Radiation that has changed direction due to interaction with material

  • Detrimental to the contrast of an image

  • Increases the patient dose

  • Factors effecting the rate of scatter

    • KVp

    • Irradiated material

Other sources of scatter include materials beyond the image receptor

• Table

• Floor

• walls

Effects of kVp increased

• Increased penetration

• Decreased attenuation

• Increased scatter

• Decreased contrast

with a bigger attenuation material

there is more scatter

Beam restriction

• Aperture diaphragms

• Cones/cyliners

• Collimators

• All limit the final beam to a more refined focal point

Adjustable lead shutters - on collimator

• The mirror reflects a light to represent the intended irradiated area

• The edges of the shutters are bevelled to prevent leakage

Air gap technique

A gap between part and detector

• Atleast 15cm

• Increased SID to decrease magnification - have to do this

• Means we have to use a higher exposure

• Useful for horizontal ray hips

• Some of the scatter is detoured away from hitting that detector

• Good for horizontal ray views of the hip

Post processing collimation

• Performed after image acquisition

• No patient dose reduction

Why a grid?

• To decrease scatter and increase image contrast/quality

•  better contrast and resolution

Radiographic grids

• Transmit direct xrays, absorb scattered xrays

  • Thin strips of lead (grid strips) attenuate the beam, separated by:

  • An interspace material - aluminium - which do not

Parallel grids

• Linear or crossed

• Lead and interspace run parallel

focussed grids

• Linear or crossed

• Lead and interspace angled from centre