Principles of Imaging Flashcards
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
What are some types of energy source?
- Ionising: detach electrons from atoms - x-rays, gamma-rays, positrons
- Non-ionising - sound, FM radio, magnetism
What is spatial resolution?
How well the image is able to differentiate between fine structures
What is contrast resolution?
Ability to make out tissue density using greyscale
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
What are the pros and cons of plain radiography?
Pros: Cheap, available, excellent spacial resolution
Cons: Ionizing radiation, relatively poor contrast resolution, planar technique
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)
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
What are the pros and cons of CT?
Pros: Cross-sectional, better contrast than radiography, goos spatial resolution
Cons: More ionising radiation, expensive
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
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
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)
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
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
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.
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
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
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
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
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
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
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?
What is ALARA?
- It is the principle of ionising radiation exposure
- Dose - As Low As Reasonable Achievable
- Cannot remove radiation completely - “background” radiation
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
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
