Intro to Radiology Flashcards
Why is it important to limit use of radiology to necessity?
The amount of radiation for medical purposes is several times higher than environmental exposure
Risk of x-ray of fetus
- increases probability of developing a malignancy in the first 10 yrs of life by about 40%
- always ask, if unsure obtain urine beta-HCG
If a patient is pregnant is it possible to xray ?
Yes. Can xray areas away from abdomen w/ proper shielding
How are xrays produced?
-ionizing radiation:
Inside the xray machine, heat is applied to a cathode. The positively charged anode draws electrons from the cathode. This is a very high voltage difference so electrons are sent w/ high force, and w/ colliding atoms eventually energy is released in the form of an xray photon.
Charges of cathodes/anodes.
Cathode: -
Anode: +
What is the relationship between voltage and energy?
The degree of energy of the beam is proportional to voltage.
Higher voltage = more penetration.
Differential absorption of xrays
- Gas (air) = black (blocked no xrays so will be the darkest)
- Fat = slightly more dense than air
- Soft tissues (muscle, solid viscera)
- Calcified = white (blocked the most xrays)
What properties allow you to visualize things on xrays?
- contrasting tissues
- between black and white, everything is gray. The human eye can recognize ~20 shades of gray.
Ways to enhance xrays
- change kilavoltage: higher kV = more penetration = more black
- use of grids (Bucky grid) to reduce scatter
- Use of fluorescent screens to reduce exposure
How are plain films viewed?
as if the patient is facing you
If you want and xray to capture the heart and lungs, what view would you select?
- PA
- beam penetrates posterior to anterior
If you want to capture the spine, what xray view would you select?
-AP
Explain why you always need at least 2 views in an xray
Sine the image is going from 3D to 2D, you need a view 90 degrees from the first view to be able to tell any depth of anything
Other options of xray views
- PA
- AP
- lateral
- oblique
- L or R lat decubitus
What are other additions to plain films to give more information on the image?
- R/L markers
- bubbles w/ beads
- arrow
Magnification on an xray
- greater the distance from the film, the greater the magnification
- this is why you want to be as close to film as possible
- otherwise will get false widening
What point should be considered in horizontal films w/ an air:fluid interface?
If shot en face, the air:fluid interface is not visible.
What is significant about the lateral decubitis position in xrays?
This positioning allows for gravity to pool fluids, like in a pleural effusion
Silhouette sign
When 2 similar densities are side by side, their boundaries will be lost; ex: heart border could be obscured by pneumonia
air bronchogram
air-filled bronchus surrounded by greater density; pneumonia allows lung and bronchus to be ditinguished; so it is always pathologic
What points should be considered in the use of portable films?
- can’t generate as high of energy level
- Can’t get PA view
- Can’t get down to 10th rib
- Pt will be poorly inflated, improperly exposed, malpositioned and falsely magnified
What can be used in xray for things that don’t normally show up?
- contrast material: barium, iodine, etc.
- swallowed, enema, or IV
- not as common d/t use of CT and reactions
How do structures show up on xray when contrast is used?
- it will be a cast of the lumen of the structure
- if a tumor is located w/i a muscle layer, it won’t be picked up
- colon cancer would show up as a filling defect
- a kidney stone would show up where contrast stopped
When would you avoid using contrast?
if they have the possibility of stroke or an area that the contrast could leak out
Renal calculi on xray
- 75% calcium oxalate/phosphate will show up on plain xray
- ~10% triple phosphate stones
- ~15% uric acid stones
- ~1% cystine stones (radiolucent)
All show up on CT
Gallsones on xray
- 75% are cholesterol (radiolucent)
- 25% pigmented w/ calcium (radio-opaque)
- since only 25% chance of seeing them on xray, send to get other modality
Computerized Axial Tomography aka CAT, CT
- instead of film uses gas or crystals
- increases gray scale to ~2,000 shades of grey
- this improves gradation w/i soft tissues
The measurement window used in CT
- hounsfield units (HU)
- 1000 HU (air) is black and +1000 (bone) is white
- wide window = increased gray
- narrow window = increased contrast (b/c anything less than window will be black and anything higher will be white)
What is the ideal window of HU in CT?
You want to narrow the window to no more than 400 HU to get the ideal amount of contrast in CT
How are CTs viewed?
as if you’re standing at the patients feet looking up towards their head
CT is good for what?
bones, head, neck, chest, abd, pelvis
CT is not detailed enough for what?
- fine image of brain, soft tissues of joints or spine (MRI better)
- not good for same densities
How can you enhance CT?
- w/ IV contrast (same in MRI)
- only used if need to distinguish it from another area and it will pick it up
Magnetic Resonance Imaging (MRI)
- Does not utilize ionizing radiation
* Exploits the magnetic properties of elements and for medical purposes, i.e. hydrogen in water and lipids
What does the quality of image in MRI depend on?
-amount of water/lipid in tissues
MRIs are good for what?
- soft tissues
- brain
- SC
MRIs are contraindicated in who?
- people w/ aneurysm clips
- pacemakers (not always)
- foreign body in eye, etc
- however, nothing these days that’s implanted is magnetic
What is the principle mechanism of MRI?
- random nature of water molecules under magnetic field align positive H nuclei toward negative pole
- a radiofrequency pulse is applied perpendicular to the alignment, causing tilt
- when the rf is turned off, the H rearranges, releasing radio wave
- the wave is picked up, amplified, digitized and projected
“T” weights
T1: pure fluid is black (CSF, blood)
T2: pure fluid is white
if you want to capture the vertebra what modality would you choose?
CT
if you want to capture the spine what modality would you choose?
MRI
-but know that insurance can’t go to an MRI first
MRA
combination of MRI and angiography
Ultrasound
• Very high frequency, converted to digital signal • Air does not provide a medium for transmission, bone blocks transmission • No fixed projections • No harm to fetus at energy level used
Ultrasound is best for what?
- best on soft tissues and soft tissue interfaces
- e.g: fluid filled cyst
- never use contrast
Ultrasound is useful in what areas?
- OB/GYN
- cardiology
- GI
- GU
Doppler Ultrasonography
• Moving structures produce a shift in
frequency which corresponds to speed of movement
• US converted to color-enhanced visual
signal and/or audible signal – doppler shift
Doppler Ultrasonography is useful for what?
- fetal heart sounds
- DVT
- atherosclerosis
Radionuclide imaging
• Takes advantage that certain chemicals concentrate in
certain structures, i.e., iodine in thyroid, phosphates for bone, etc.
• Tag the organ-specific chemical with a radioactive compound
What is the most common radioactive compound used in radionuclide imaging
Technetium (99mTc)
-other examples: thallium scan for heart and xenon or vent/perf scan
Why is there poor spatial resolution in radionuclide imaging?
- Gamma rays = decay of nucleus = gamma counter
- Short half life; hours to days
