Principles of Imaging

Question 1

What is the basis of all medical imaging?

Answer 1

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

Question 2

What are some types of energy source?

Answer 2

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

Question 3

What is spatial resolution?

Answer 3

How well the image is able to differentiate between fine structures

Question 4

What is contrast resolution?

Answer 4

Ability to make out tissue density using greyscale

Question 5

Describe plain radiography.

Answer 5

• 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

Question 6

What are the pros and cons of plain radiography?

Answer 6

Pros: Cheap, available, excellent spacial resolution

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

Question 7

What are the uses of plain radiography?

Answer 7

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

Question 8

Describe CT - computer tomography.

Answer 8

• 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

Question 9

What are the pros and cons of CT?

Answer 9

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

Cons: More ionising radiation, expensive

Question 10

What are the uses of CT?

Answer 10

• 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

Question 11

Describe ultrasounds.

Answer 11

• 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

Question 12

What are the pros and cons of ultrasound?

Answer 12

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

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

Question 13

What are the uses of Ultrasound?

Answer 13

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

Question 14

Describe Magnetic Resonance Imaging.

Answer 14

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

Question 15

What are the pros and cons of MRI?

Answer 15

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.

Question 16

What are the uses of MRI?

Answer 16

• 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

Question 17

Describe Nuclear medicine.

Answer 17

• 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

Question 18

What types of radio-isotopes exist?

Answer 18

• 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

Question 19

Describe the concept of signal to noise.

Answer 19

• 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

Question 20

What are the pros and cons of nuclear medicine?

Answer 20

Pros: Excellent signal to noise, extremely sensitive

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

Question 21

What are the uses of nuclear medicine?

Answer 21

• 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

Question 22

What are the safety risks associated with imaging?

Answer 22

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?

Question 23

What is ALARA?

Answer 23

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

Question 24

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

Answer 24

• 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

Question 25

What conclusions can we draw?

Answer 25

• 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