Diagnostic imaging Flashcards
how does computed x-ray tomography work?
Uses x-rays to make cross sectional images through the body.
Can be used to rebuild transverse section/3D images
What do CT images show?
Bone=white
Variations in physical density
What is Scintigraphy
Using radioactive gamma rays that become incorporated into bone metabolism and can be detected with a gamma camera
Why is scintigraphy useful?
Can detect bone cancer earlier than with x-rays as bone metabolism alters before bone damage
What can you see with Scintigraphy?
Metabolic activity
darker = more activity
What is MRI?
making images by applying pulsed magnetic energy to the patient and collecting the radiowaves that are emitted between pulses
What is is MRI good?
More soft tissue detail, good for neurological exams.
Bone = black
How does ultrasonography work?
Making images by firing pulses of high frequency sound and collecting the returning echos
What can ultrasonography be used for?
Organs etc near the surface
What do you need to do before ultrasound exam?
Clip hair and use jelly to get good skin contact
What else can diagnostic imaging be used for?
To guide biopsy and facilitate treatment
What is radiography?
making images representing the shadow of an object placed in an xray beam
What is tomography?
making images that represent a section through the body
What are x-rays?
Short wavelength, high energy electromagnetic radiation
What type of imaging do we normally use with x-rays?
Negative imaging - bones white where few xrays reach the plate
How do we make x-rays?
Fast electrons released from a filment in an aluminium filter when its heated by a low voltage curent. The tungsten target attached to copper (to deal with the heat) then has a high voltage passed through it so the electrons are drawn to the tungsten. The electrons are slowed by the positive tungsten nucleus and the loss of energy causes the x-ray beam. This takes place in a vacuum.
What is the point of the aluminium filter?
absorbs low energy non-penetrable x-rays to reduce exposure to patient
(also non intentionally reduces contrast)
What do you need to make x-rays?
A source of electrons
way to accelerate them
target to collide with
What does x-ray absorption depend on?
tissue density
atomic no of elements in tissue (big atoms absorb more)
thickness of tissue
How does something appear that x-rays pass through easily?
Black/lucent e.g air
How does something apper that absorbs x-rays?
Pale/opaque e.g metal
Three types of contrast media?
-ve : lucent (gas) - to examine organ wall
+ve: opaque (high atomic no) - barium (GI) / iodine(angio)
doube: distend using gas, add small vol of positive to coat inner surface
What is mAs?
Milliampere seconds - current passing through tube and duration of current
mAs = mA x s (so can trade off)
What does high mAs do?
more electrons so more x-rays = darker radiograph
What is kVp?
Kilovoltage peak - voltage applied across the x-ray tube
What does high kVp do?
High kVp = more xrays and higher energy = more penetrating (for thicker parts)
What does film look like if kVp too high/low?
too low - all white (exaggerated contrast)
too high - all black (poor contrast)
What does too high mAs do?
Thin parts appear black as overexposed
What is scattered radiation?
x-rays that have been deflected from the primary beam by interacting with the patient/table etc
Problem with scattered radiation?
- make radiographs look indistinct and grey
- safety of handlers
How do we minimise effects of scattered radiation?
- minimise vol of tissue irradiated (collimate primary beam with the diaphragm)
- lead-rubber between casette and table top
- high kVp so need less mAs so less scatter
What is an x-ray grid?
A series of fine lead strips parallel to the direction of the primary beam, between patient and film so absorbs any scattered radiation.
When to use an x-ray grid?
With bigger animals as more scattered radiation
What is the unit for measuring the ability of radiation to ionise air?
Roentgen = 2.58x10-4 C/Kg (electrical charge/ionisation energy per kg of air)
What unit is used for measuring radioactive decay?
Becquerel = 1 radiation disintergration /second
Unit for measuring absorbed radiation?
Gray = 1joule/kg
used for radiation doses
unit for measuring dose of effective radiation?
Sievert = gray x q.f (quality factor - varies with type of radiation, for x-rays q.f.=1)
What does ALARA mean?
As low as reasonably achievable
What is the maximum permissable dose (MPD) at work? and how is it detected?
20 mSV/year. Use a film badge or thermoluminescent detector (TLD)
Three way to protect form radiation?
1) Time - minimise time in radiation if possible
2) Distance - stay away from patient (inverse square rule)
3) Barriers - absorb scatter - lead aprons + gloves, mobile barriers, walls
Define the controlled area
2m radius of primary beam
Define the designated peron
Named in local rules as permitted to take x-rays
Radiation protection advisor (RDA) ?
Doesnt work at practice but inspects and advises. provides the local rules.
Radiation protection supervisor (RPS)?
normally a partner of the practice, notifies health and safety executive that radiography is performed, administers local rules, monitors exposure, informed about broken equipment.
