Imaging Flashcards
what is an MRI scan
An imaging modality that uses a magnetic field and radiofrequency to generate an image of the patient
how do MRI’s work?
- Magnetic field - superconducting MRI scan - H+ align with the longitudinal magnectic vector - atom precession is out of phase
- RF pulse
a. magnetic vector becomes transverse
b. H+ precession changes to all in phase - RF pusle stopped - vector and precession return to magnetic state - releases energy - image generated
what is the T1?
- time to recover 63% of the longitudinal vector
- fat appears bright
- good for anatomy
what is the T2?
- time to decay to 37% of transverse magnetic vector and the electrons come out of phase
- fluid is bright
- H20 for T2
- good for pathology - often has fluid
What are X-rays?
A form of high energy electromagnetic radiation with a shorter wavelength than light
what are xrays used for?
simple bony anatomy - OA, fractures, deformity
How does xray work?
Thermonic emission - tungsten cathode coil is heated in a vacuum to 2200deg and emits electrodes
Breaking radiation - Neg electrons hit a tungsten anode plate producing 1% X-rays and 99% heat
anode rotates to prevent it from melting
X-rays focused by a cone onto patient
Attenuation - Image produced by variable absorption by the patient - absorbed, reflected or transmitted
Phosphor crystal plate behind the patient - when exposed to xrays stains black
- bone - high beam attenuation - little xrays let through
- air/ fat - low beam attenuation - black
What is the difference with digital X-rays?
Uses a phosphorus detector plate
Conventional uses a photographic emulsion film
What are the risks associated with xray?
exposure to ionising radiation leading to a higher incidence of cancers and birth defects - damage of cell DNA
Birth defects - 8-25weeks - brain damage - doses of 100mSv-200mSv later
Higher risk in children as they are more sensitive to irradiation.
precautions required
Risk is dependent upon the dose and sensitivity of the tissue e.g breast/thyroid
10mSv - confers a 1 in 1000 increase in the lifetime risk of cancer (16 pelvic xrays)
Annual exposure background radiation - 2.4mSv
Pelvic xray dose - 0.6mSv
what precautions are required for xray usage?
Alara principle - as low as resonable able - minimise exposure
T- time
D- distance - from xray source
S - shielding - reproduction and thyroid
what is a sievert?
unit of effective dose of ionising radiation - takes into account the type or radiation and sensitivity of tissues/ organs
what is a dexa scan?
The use of xrays of different energy intensities to calculate bone mineral densities - kgm2
How does dexa scan work?
An AP pelvic and a lateral lumbar spine xray is used to calculated the BMD in kgm2.
BMD can be converted to a T score and compared to the peak BMD
What is an ultrasound?
High frequency sound waves generated by a tranducer, reflected back and converted to an electrical signal
How does ultra sound work?
- Voltage is applied to a piezoelectric crystal transducer causing a shape changes in the crystals producing US waves
- US waves hit the body tissue and either absorbed or reflected as an echo
- Returning echo hits piezoelectric crystals in transducer and changes shape generating an electrical signal
What are the advantages and disadvantages of ultrasound?
advantages
* good for soft tissues, effusions, tendons
* cheap
* no ionising radiation
disadvantage
* user dependent
* difficult to interprete
what is a CT scan?
computer tomography - the use of a computer to generate a cross-sectional image - using high-frequency energy from the electromagnetic spetrum
How does Ct scan work?
Buzz phrases:
* fan-shaped xray beam
* collinear detectors
* attenuation coefficient
components of CT scan
* Gantry - has an xray beam rotating around the patient generating xrays
* Stationary colinear detectors - detects the xrays
* couch
* control room - computer and monitor
how does it work
* each time gantry goes around patient it sends out xray beams
* xrays are absorbed, reflected or transmitted depending on the tissue involved = ATTENUATION
- Xrays detected by stationary colinear detectors
- Each rotation of the gantry creates an axial slice
Attenuation coefficients - expressed in housefield units
- water = 0
- bone = 1000
- to see bone window or soft tissue window etc, the attentuation window is adjusted
Risks and benefits of Ct?
Advantages
* complex bony anatomy - 3D appreciation
* complex fractures
* evidence of union
* bony deformity
Disadvantages
* not good for soft tissues
* CT = 6mSv (miliseverts) - ionising radiation
what types of CT scan are you aware of?
- helical CT - continuous rotation of gantry - allows multiple slices at the same time
- multidetector CT - single rotation of the gantry - multiple rows of detectors to take multi slices at same time
- PET CT - looks at metabolic activity - can be overlaid at same time as CT - co-registered image - gives more details
what is a bone scan?
A form of gamma radiation produced by radioactive isotopes or nucleotides
how are bone scans produced?
- unstable nuclei
- photoemission
- triple-phase bone scan
Technitium-99 - has an unstable nuclei releasing gamma radiation
- Technitium-99 is given intravenously with a carrier molecule methylene diphosphonate MDP
- travels in bloodstream
- binds to hydroxyapatite crystals, breaks up and releases gamma radiation ‘photoemission’
- detected by a gamma camera
- monitored using a triple phase bone scan in 3 distinct phases
- FLOW phase - 1-2mins - measures arterial blood flow
- BLOOD POOL - 3-5mins - measures extracellular fluid volume/ soft tissue activity
- STATIC phase- 4hrs - bone - skeletal activity
- good marker of osteoblastic activity and increased vascularity
why is gel used for US scans?
stops air being trapped and reflecting the sound wave
