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Flashcards in Principles of Imaging Deck (25):
1

What is the basis of all medical imaging?

- Energy source - usually EMR, acoustic
- Interaction of that energy with the body
- Detection device for that energy
- Image display

2

What are some types of energy source?

- Ionising: detach electrons from atoms - x-rays, gamma-rays, positrons
- Non-ionising - sound, FM radio, magnetism

3

What is spatial resolution?

How well the image is able to differentiate between fine structures

4

What is contrast resolution?

Ability to make out tissue density using greyscale

5

Describe plain radiography.

- Uses Crookes tube (X rays): Vacuum tube with current between a filament and target
- electron density of tissues i.e atomic number and concentration - transmission of X-rays
- Photographic film (negative) - more recently an X-ray detector
- Light box - more recently a computer monitor

6

What are the pros and cons of plain radiography?

Pros: Cheap, available, excellent spacial resolution

Cons: Ionizing radiation, relatively poor contrast resolution, planar technique

7

What are the uses of plain radiography?

- Initial examination
- Bones for trauma
- Chest for all symptoms
- Abdomen for pain
- Breast for detection of cancer (mammography)

8

Describe CT - computer tomography.

- Crookes tube (X-rays)
- electron density of tissues -- transmission of x-rays
- X-ray detector: Solid-state scintillation crystal; gas ionization chamber
- Computer monitor
- 360 degree imaging to produce 3D image on the computer from individual slices

9

What are the pros and cons of CT?

Pros: Cross-sectional, better contrast than radiography, goos spatial resolution

Cons: More ionising radiation, expensive

10

What are the uses of CT?

- Head for acute symptoms e.g trauma, stroke
- Chest/abdomen advanced imaging - malignancy, pain investigation
- Bones for subtle lesions - trauma, tumour, infection
- Non-invasis angiography

11

Describe ultrasounds.

- Piezoelectric crystal - produces sound waves
- Sound velocity and tissue reflectivity - transmission of high frequency sound
- Piezoelectric crystal - converts sound to an electric signal
- Computer monitor needed
- Essentially sonar technology

12

What are the pros and cons of ultrasound?

Pros: No ionising radiation, cheaper than CT, freely available, solic v cystic

Cons: Operator dependent, sound penetration problems (bone, fat, gas)

13

What are the uses of Ultrasound?

- Children and pregnancy
- Screening for abdominal symptoms
- Pelvic pathology in women
- Small parts
- Musculoskeletal, especially muscles, tendons and ligaments
- Blood vessels

14

Describe Magnetic Resonance Imaging.

- Very strong magnet and radio wave generator
- Proton density and microenvironment of those protons - emission of FM radio waves
- Radio receiver
- Computer monitor/film

15

What are the pros and cons of MRI?

Pros: No ionising radiation, excellent contrast resolution, reasonable spatial resolution

Cons: Expensive, strong magnet, long scan times, claustrophobia, air/calcification/metal distorts the magnetic field.

16

What are the uses of MRI?

- Brain except acute trauma
- Spine for pain or neurological deficit
- Joints for pain, bones where X-ray not helpful
- Breast in young women for detection of cancer
- Pelvic malignancy
- Non-invasive angiography

17

Describe Nuclear medicine.

- Gamma emitting radioscope and chemical - radiopharmaceutical tag
- Accumulation of radiopharmaceutical chemical interaction with organ/tissue - emission of gamma rays
- Gamma camera - scintillation crystal
- Computer monitor/film

18

What types of radio-isotopes exist?

- Alpha particles = nuclei of helium atoms
- Beta particles = electrons
- Gamma rays = photons

- Gamma rays are penetrating - very similar to X-rays but of slightly higher frequency

19

Describe the concept of signal to noise.

- Slightly different concept to image contrast
- Accumulation of radiopharmaceutical in the organ of interest compared to rest of the body
- Higher the accumulation, the better

20

What are the pros and cons of nuclear medicine?

Pros: Excellent signal to noise, extremely sensitive

Cons: Persistent ionising radiation, very poor spatial resolution, poor specificity for pathology, expensive

21

What are the uses of nuclear medicine?

- Functional cardiac imaging
- Bones for occult trauma or tumor
- Functional renal imaging
- Functional thyroid imaging
- V/Q scan for PE
- Detection of LN spread of tumor

22

What are the safety risks associated with imaging?

Ionising radiation is present in the form of X- or gamma-rays.
- Can move an electron from its orbit
- Breaks chemical bonds
- Harmful to tissue especially DNA
- Rapidly dividing cells cannot repair the DNA

Non-ionising radiation is used in ultrasounds and MRIs
- Non specific interaction with DNA
- Can heat tissue (like a microwave)
- May affect the developing foetal brain?

23

What is ALARA?

- It is the principle of ionising radiation exposure
- Dose - As Low As Reasonable Achievable
- Cannot remove radiation completely - "background" radiation

24

What are the ways in which we can reduce the harmful effects?

- Fewer procedures - reduce radiation dose
- Prolonged intervals between exposures - Allow DNA to repair
- Not children/pregnant - Dividing cells; lifetime "cumulative" risk if there is DNA damage to these cells

25

What conclusions can we draw?

- X-ray: Good spatial resolution
-CT: Good spatial resolution and contrast
- US: Good contrast, good function
- MRI: V. good contrast and good function
- NM: V. good signal to noise, good function