Basic Radiation Science/Principle of Digital Imaging Flashcards
Define voxel
Volume Element
A voxel is a unit of graphic information that defines a point in three-dimensional space.
Since a CT section has a finite thickness, each pixel actually represents a small volume element, or voxel
Define Pixel
the smallest discrete two-dimensional unit of a digital image
What is the structure of any atom
Atoms are made up of protons and neutron located within the nucleus, with electrons in orbitals surrounding the nucleus.
The number of protons in the nucleus
Atomic number
The total number of nucleons (neutrons + protons) in the nucleus
Atomic mass number
Any nucleus which contains same number of protons as the given nucleus but has a different mass number
Isotope
An atom with an unstable nucleus, characterised by excess energy available which undergoes radioactive decay and creates: gamma rays, alpha or beta particles
Radioisotope
o A radiation which removes an electron from the outer shell of an atom
Ionising radiation
Non-ionising radiation
Electromagnetic radiation which does not cause the breakage of atoms- can be waves or particles
Negative effects of ionising radiation
Produces highly reactive ions which can cause damage to DNA etc.
Negative effects of non-ionising radiation
Can agitate body tissues as they produce heat, but not long term biological effects
How does: Increases in wavelength = decreases in frequency Effects energy
decreases energy
——— are extremely high energy, they are used in radiotherapy. They can cause severe ionisation and can pass through the body.
Gamma Rays
——– uses sound waves, and produces heat in the body but no radiation
Ultrasound
How is EMR refracted
High energy electromagnetic radiations are not refracted.
What does the transmission of radiations depends on (tissue)
Different materials transmit different amount of radiations- depends on the density and type of tissue. The higher the atomic number, the higher the absorption of x-rays.
The higher the atomic number, the ——- the absorption of x-rays.
Higher
Define attenuation
Attenuation is the reduction of the intensity of an electromagnetic beam as it traverses matter. It can be due to photoelectric absorption, Compton scattering and pair production
Define luminescence
Fluorescence and phosphorescence ( electron transitions within the material being irradiated cause the emission of photons that have less energy than the incident photons).
What is Newton’s Law on the conservation of energy
Energy can neither be created nor destroyed but can be changed from one form to another. The amount of energy in system is constant.
What is Newton’s Law on Conservation of Momentum
The total linear or rotational momentum in a given system is constant (when we consider collision of two bodies)
What are Newton’s 3 laws of motion
- A body will be at rest or will travel with a constant velocity unless acted upon by a net external force 2. The rate of change of momentum of a body is proportional to the applied force 3. The action has equal and opposite reaction
What is the purpose of image processing?
This influences the image appearance, and is affected by the amount of collimation, and whether the field was centred correctly (by anatomical location or not)- If the object in the centre of the film it will appear correct in orientation (no elongation etc).
Describe ‘DDR’ image receptors
- DDR is cassetteless.
- The x-rays are recorded by a digital image capture device which records the x-ray image.
- There are two types, flat panel detectors and solid-state detectors.
- The x-rays are converted into a charge which forms an analogue signal which is then converted into a digital signal, giving a number for each pixel representing its level of exposure.
- The thin film transistors reflect pixels of an image.
- The crystals are ionised by the photons which forms a pre-electron current.
- The lighter the pixel, the less current present, the more grey the colour.
Describe the ‘computed’ image receptors
Computed radiography employs the process of photon stimulated luminescence. A phosphor coated imaging plate is exposed to x-rays. The energy is stored in the phosphor layer. This is then scanned by a laser. This stimulates the phosphor layer, which then emits light of a brightness related to the original exposure.
Describe the ‘Film-Screen’ Image receptors
Film-screen fluoresces in the presence of x-rays, it emits visible light whose brightness depends on the level of exposure. Photographic film is placed in contact with the screen to produce an image.
Describe the ‘Film’ image receptors
Film involves the direct exposure of photographic film to x-radiation, this is then chemically processed and becomes permanently darkened to a degree which depends on the level of exposure.
Give 2 generic EMR properties
- Travel in straight lines (Rectilinear Propagation)
- Travel at the speed of light (3x108ms-1) in a vacuum
- Carry energy and momentum
- Unaffected by electrical or magnetic fields
- Travel explained by ‘Wave’ or ‘Quantum’ Theory
What controls the penetrating power of the x-ray beam (Quality)
KvP
What controls the quantity of x-rays
mAs
‘if you increase the KVp by 15% and halve the mAs the image will seemingly appear to have received a ‘similar’ exposure.’
what is this an example of
Rule of constant intensity
An image can be defined as a two-dimensional function f(x, y)
What are x and y?
x and y are spatial (plane) coordinates
What constitutes BETTER SPATIAL RESOLUTION
More pixels = finer detail seen
Define Bit Depth
The number of grey shades avaliable for image display
If 2n is the number of grey shades. What is the usual clinical number?
210/12 most common for clinical use
- cassette with a plate, where the image is formed on the cassette
- Takes longer time
- One reader but in theory can have multiple rooms imaging, so more cost-effective
- Quality of image not as good
Which aquisition system is being described?
Computed
- Almost instantaneous
- Better quality
- Only get the information you irradiated
- Only the exposed pixels contribute to image database
Which aquisition system is being described here?
Direct/Indirect
In order to examine a specific body part, the radiographer normally selects it via the programme on the generator console.
‘———’ exposure factors are automatically selected.
‘Recommended’ exposure factors are automatically selected.
What should the histogram ‘signature’ tell you about the image
The histogram ‘signature’ should tell you how the exposure of the image will appear to the eye
If this were the histogram for the image, would it be under or over exposed?
The image on the right will be underexposed, as the pixels are at the white end of the spectrum
What does “white to the right” refer to on a histogram
Histogram is pushed to the RIGHT if under-exposed
What is the purpose of gamma for image optimisation
Gamma controls the contrast (via a look-up-table (LUT) curve in Radiography practice)
LUT is always automatically directly proportional, but can be edited
What is this line?
look-up-table (LUT) curve
Controls the contrast of the image
LUT is always automatically directly proportional, but can be edited
What is windowing of an image
The input range is reduced to only include the ‘useful’ data, and the output range remains unchanged
What is wrong with the top image, and why?
as exposure factors weren’t correct, lots of the image was lacking ‘full blackness’.
The left cursor of the histogram was moved to the right, to the edge of the active pixels.
How can contrast be corrected?
A LUT curve is applied to create the optimal image.
An infinite array of LUT’s could be applied to suit the potential desires of every clinical situation
Purpose of inverting an image
Once optimised, the image can also be inverted by flipping the LUT. This allows detection of fractures etc which weren’t previously spotted.
Why Originally was x-ray film tinted blue.
Colour makes it easier to see pathologies on images, as we see in colour better than monochrome:
Why aren’t all iimages in colour if we can see pathologies better with colour?
An RGB colour file is 3 times bigger than monochrome
- requires 3 times more image storage space
- takes 3 times longer to transmit over a network
- visual benefits cannot be justified
Low Pass filters create what effects
Low Pass filters create relatively gentle smoothing effects e.g.
- ‘Blur’
- ‘Blur More’
The ‘Gaussian’ low pass filter is very fierce.
Purpose of high pass filters
High Pass filters sharpen the edges e.g.
- ‘Laplacian’ or ‘Laplacian-like’
- ‘Sobel Gradient’
How are smoothing masks achieved
Blue ‘Smoothing’ Mask
The smoothing is achieved by attenuating high spatial frequency details and therefore reducing ‘noise’.
What does the Gaussian mask do
The Gaussian mask utilises a ‘bell shaped’ distribution and is typically applied using masks up to 25 x 25 in size but is often circular.
The blurring (smoothing) effect is very strong, even at low settings!
What is the golden rule for smoothing and sharpening an image
Golden Rule – ‘Smooth then Sharpen’
If you ‘sharpen’ before ‘smoothing’ you will also ‘sharpen the noise’ and the image will look speckled
Diagnostic Images are created and stored in DICOM:
what does that stand for
_D_igital _I_maging and _Co_mmunications in _M_edicine
Images may be converted into other formats in order to reduce their file size e.g. TIFF or JPEG
The greater the compression the……?
- the smaller the image file size
- the less storage space required
- the faster it will transmit across a network
- the lesser the image quality
DICOM file contains?
both a header and the image data
- Header: Stores patient information, the type of scan, image dimensions, etc..
- Image Data: Image Matrix as an array of binary numbers.
Can be multiple slices that form a 3D representation
What is PACS data storage
Linked to radiology information system.
Images are stored in a server off-site and after a predetermined number of years may be jpged to store for a longer time.
