Imaging Flashcards

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1
Q

what is an MRI scan

A

An imaging modality that uses a magnetic field and radiofrequency to generate an image of the patient

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2
Q

how do MRI’s work?

A
  1. Magnetic field - superconducting MRI scan - H+ align with the longitudinal magnectic vector - atom precession is out of phase
  2. RF pulse
    a. magnetic vector becomes transverse
    b. H+ precession changes to all in phase
  3. RF pusle stopped - vector and precession return to magnetic state - releases energy - image generated
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3
Q

what is the T1?

A
  • time to recover 63% of the longitudinal vector
  • fat appears bright
  • good for anatomy
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4
Q

what is the T2?

A
  • 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
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5
Q

What are X-rays?

A

A form of high energy electromagnetic radiation with a shorter wavelength than light

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6
Q

what are xrays used for?

A

simple bony anatomy - OA, fractures, deformity

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7
Q

How does xray work?

A

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
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8
Q

What is the difference with digital X-rays?

A

Uses a phosphorus detector plate

Conventional uses a photographic emulsion film

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9
Q

What are the risks associated with xray?

A

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

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10
Q

what precautions are required for xray usage?

A

Alara principle - as low as resonable able - minimise exposure

T- time
D- distance - from xray source
S - shielding - reproduction and thyroid

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11
Q

what is a sievert?

A

unit of effective dose of ionising radiation - takes into account the type or radiation and sensitivity of tissues/ organs

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12
Q

what is a dexa scan?

A

The use of xrays of different energy intensities to calculate bone mineral densities - kgm2

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13
Q

How does dexa scan work?

A

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

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14
Q

What is an ultrasound?

A

High frequency sound waves generated by a tranducer, reflected back and converted to an electrical signal

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15
Q

How does ultra sound work?

A
  • 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
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16
Q

What are the advantages and disadvantages of ultrasound?

A

advantages
* good for soft tissues, effusions, tendons
* cheap
* no ionising radiation

disadvantage
* user dependent
* difficult to interprete

17
Q

what is a CT scan?

A

computer tomography - the use of a computer to generate a cross-sectional image - using high-frequency energy from the electromagnetic spetrum

18
Q

How does Ct scan work?

A

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

19
Q

Risks and benefits of Ct?

A

Advantages
* complex bony anatomy - 3D appreciation
* complex fractures
* evidence of union
* bony deformity

Disadvantages
* not good for soft tissues
* CT = 6mSv (miliseverts) - ionising radiation

20
Q

what types of CT scan are you aware of?

A
  • 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
21
Q

what is a bone scan?

A

A form of gamma radiation produced by radioactive isotopes or nucleotides

22
Q

how are bone scans produced?

A
  1. unstable nuclei
  2. photoemission
  3. 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
23
Q

why is gel used for US scans?

A

stops air being trapped and reflecting the sound wave