Imaging technology (pixel, voxel, resolution) 13/01/2023 Flashcards
What does radiographic density mean?
the degree of blackness on the image
What increases radiographic density?
Greater radiographic density may be produced by increasing
(1) mA the total number of x-rays that reach the film,
(2) s – the length of time the exposure takes place over,
3) kVp the penetrating power of the x-rays so how much energy you are giving the x-ray photons to reach the IR.
It can also be caused by post-processing data manipulation techniques such as windowing.
What is contrast? technology
Radiographic contrast, sometimes referred to as greyscale, is the difference in the displayed brightness between two areas of interest e.g. a lesion and background tissue.
This contrast is caused by the difference in absorption densities between different structures in the body, so in a chest x-ray bone and air for example.
What affects contrast? technology
Some ways to affect image contrast are by;
Exposure factors (kV only) – increase in kVp will decrease the contrast in the image because if you give the photons more energy, less will be absorbed by the tissue.
Windowing / post processing
How does analogue work? technology
In an analogue system, x-ray energy is converted to light, and the light waves are recorded just as they are. As a result we achieved continuous images, seeing various levels of brightness and colour – a complete representation of the information collected.
How does digital work? technology
When we acquire information using DR, the analogue signals are converted into numbers that are recorded and sampled (or counted) multiple times. These multiple numerical values are then divided into smaller elements that can be processed in different ways. A digital signal looks different to an analogue signal as you can see – more blocky in its nature as each numerical value is represented.
What is a matrix?
An arrangement of rows and columns on which pixels are arranged in order to create a displayed image . The pixels of information that we collect are displayed in a matrix.
Larger matrixes have better spatial resolution, but produce bigger quantities of data (and take longer to process).
What is Field of view?
The size of the displayed image
The amount of body part or patient included within this can vary
The larger the FOV, the larger the area that is imaged
What is a pixel?
Pixel
2-Dimensional
The basic component of an image matrix
Part of any digital picture we see (even non-radiographic).
The size of a pixel is determined via: pixel size = FOV/matrix
When keeping the field of view the same, if you had a larger matrix, you would have a greater number of smaller individual pixels.
What is a voxel?
3-Dimensional
Adds depth as a dimension
Used in volumetric scanning (such as in CT and sometimes MRI)
Why is pixel bit depth important?
The greyscales that cause differences in image contrast are determined by the Pixel bit depth. This affects the number of grey tones a pixel can produce.
When a pixel has a higher bit depth, the more grey scale the pixel can produce (i.e. black to white and all the greys in between). This means pixels with higher bit depths increase the number of different shades of grey that can be displayed on an image.
This allows us to differentiate different tissues
What is spatial resolution?
Spatial Resolution
The ability of an imaging modality to differentiate two adjacent structures as being distinct from one another (Radiopaedia, 2020)
How can we improve spatial resolution?
Keeping the same matrix size but reducing FOV results in increased spatial resolution.
More pixels are dedicated to a smaller physical area of your patient so more detail is possible.
This is why good collimation is important for image quality.
This is why “coned views” give better detail.
What is Contrast Resolution?
The ability of any imaging modality to distinguish between differences in image intensity (brightness)
What is temporal resolution?
A modalities ability to see fast moving objects in a rested or still state.
Poor temporal resolution can result in blurring and misregistered data.
Crucial in process where we image moving objects like the heart and blood.
Can be improved by reducing the time it takes to acquire a single slice/set of images.
Types of radiation?
Primary- from the target (useful)
Secondary- created when the primary beam interacts with matter (less penetrating, increases patient dose).
Scatter- multidirectional
Leakage- from tube housing.
How do we measure DAP?
Dose and area exposed
Ionization chamber
μGy/cm2 displayed on control panel/ separate unit
Integrated/ External to tube housing
An ionization chamber larger than the area of the x-ray beam is placed just beyond the xray collimators. The DAP ionization chamber must intercept the entire x-ray field for an accurate reading.
DAP value dependent on?
Filtration
kV
mA
time
Area exposed
Why is DAP used?
Correlates to total energy received by the patient
Can be converted from mGy/cm2 to mSv
Beam quality
Region of the body/ tissue sensitivity
Projection
Age
Body habitus and pathology
Information about grids?
Used to reduce scatter radiation
Stationary
Moving aka bucky
Parallel
Focused
Frequency and Ratio lines/cm2 : height d
Focal/Optimal source distance
Automatic Exposure Control Devices, how do they work?
AED / AEC / iontamats / chambers
Radiation that is transmitted through the body is converted into an electrical signal, which terminates the exposure time when the predetermined level of radiation (density) has been reached.
Photomultipliers- series of photocells or Ionisation Chambers
An AEC controls the TIME of an exposure
AEC what do you need to check
Still need to select:
Image Receptor
Grid
kVp
mA – (check if this is at its highest)
Density setting +/- 8
Position correctly
An AEC controls the TIME of an exposure
What is noise?
Insufficient energy – “grainy” image
Fluctuations in greyness
Insufficent mAs
One of the factors that effect sharpness, others are geometric unsharpness ( penumbra, magnification and distortion) and movement unsharpness
Too few photons reaching the detector
What is the aperture?
The hole the patient goes through in CT
Ultrasound attenuation information
The sound is attenuated as it travels through a medium
Attenuation occurs as distance travelled increases
materials have differing attenuation effects e.g. air is poor transmitter of sound
High frequency sound is attenuated more (less penetration) than low frequency sound
ultrasound, how does FREQUENCY affect IMAGE QUALITY?
As frequency is increased image quality is improved at the expense of penetration
With high frequencies sound is attenuated to a greater extent than low frequency
Attenuation is measured in Decibels (dB) and decreases exponentially with depth.
Limitations of ultrasound
Poor penetration of bone or gas as these structures are not transonic i.e. They reflect sound
Image quality is poor in obese patients
Visualisation of small structures requires high frequencies to be effective
Ultrasound applications
Ante-natal screening
Cardiac imaging
Vascular imaging
Abdominal imaging
Doppler blood flow
An alternative imaging method to using radiation where the risk is unjustified.
Obstetrics, foetal imaging, dating, abnormality screening
Gynaecological imaging – e.g. fibroids
Testicular & thyroid imaging
Breast lumps – Doppler U/S for imaging tumour vasculature
Heart Imaging
U/S Guided biopsy
Ultrasound probes and when used
CURVI-LINEAR used in neonatal imaging
PHASED ARRAY used in heart imaging
NTRA-CAVITY e.g. Rectal, vaginal or prostate imaging
A gel (coupling medium) would be used to improve the interface between tissue and Transducer to reduce reflections and eradicate air bubbles.
What colour is water on T1 sepeunce
Dark
What colour is water on T2 sep
Bright
How should window level and width be set?
Window level should be set near the average attenuation of the tissues of interest
Window width is narrowed or expanded (increase or decrease contrast)
What is window width
The range of Hounsfield Units displayed
Maximum WW c. 2000
Human eye can only distinguish 16 shades of grey
How does the kVp affect the image?
Increasing kVp = less contrast, (more grey)
Decreasing kVp = More contrast (more black & white)
How does the mAs affect the image?
Overall brightness
Increasing mAs = Darker image
Decreasing mAs = Brighter Image
Insufficient mAs = more noise/quantum mottle
How does the use of a grid improve the image?
Reduces scatter & Improves Image contrast
Reduces EI and DAP when measured at the detector as grid attenuates Xrays
mAs should be multiplied by 2/3/4/5 when using grid
With each exposure how do the changes affect the dose received to the patient. increasing kVp and mAs and decreasing them
Increasing kVp = Slightly increased dose
Increasing mAs = Dose increases proportionally
Decreasing kVp = Slightly decreased Dose
Decreasing mAs = Dose decreases proportionally
What happened to the DAP with each change that you made?
GRID use will reduce DAP but patient dose will remain the same as grid was placed after the patient!
What happened to the EI with each change that you made?
Increase kVp/mAs = Increased EI
Decrease kVp/mAs = Reduced DAP
Use grid = Reduced EI
Would a change to PA CXR make a difference to all these questions?
No, you are still passing your beam through the same tissue just in a different order
If SID is increased, mAs should be increased to compensate (Double distance 4x the mAs!)
kVp does not need to be adjusted when using a grid
DAP is measured at the tube for Mobile imaging so SID will not affect DAP
If SID is increased, mAs should be increased to compensate (Double distance 4x the mAs!)
kVp does not need to be adjusted when using a grid
DAP is measured at the tube for Mobile imaging so SID will not affect DAP
