Radiography and CT Flashcards
How was the X-ray discovered?
- By accident
- Trying to improve the cathode ray tube by making stronger electric fields to cause electron beams
- Accidently exposed a photographic film that was in the line of the cathode
- Some non-visable radiation more penetrating more than visable light
- Wilhelm Conrad Rontgen (1901)
What did the discovers of X-rays find out that the new radiation could do?
- Penetrate tissues but the body absorbs to varying degrees
- Different levels of exposure on film revealed the internal structure of the body
Where do X-rays sit on the electromagnetic spectrum?
- Very small wavelengths- the shorter the wavelength, the higher the energy it carries
- The ionising radiation has enough energy to harm tissue
What are the components of electromagnetic waves?
Partly electric and partly magnetic
What is the range of a radio wave?
M
What is the range of a microwave?
cm
Outline the components of the cathode ray tube
- Electron beam
- Cathode
- X-rays
- Tungsten anode
How does the cathode ray tube work?
- Pump out the internal air to create a vacuum
- Apply a voltage between the negative anode and positive cathode
- The voltage (when high enough) causes electrons to be dislodged from the cathode and then accelerates toward the anode
- As it accelerates, it suddenly stops as it hits the high density metal material
- All the energy gathered over the path of acceleration is released in an instant
- This energy dissipates as heat and x-rays
What determines the different strengths of the x-ray generated?
The voltage between the anode and the cathode
What are the two ways that x-rays can be produced?
First
- Electron collides with a tungsten atom
- Tungsten atom loses electron from lower from lower orbit
- Electron from higher orbit falls to lower orbit
- Excess energy released as an X-ray
Second
- Nucleus may attract speeding electron
- Electron slows and changes course
- Braking action slows electron which releases excess energy
- Excess energy released as an X-ray
How is the x-ray beam focused?
Tungsten anode geometry is such that the angle creates the x-ray going out the window
The light determines the field of exposure
How is heat produced by the x-ray dealt with?
- Is normally suspended in an oil bath (high density medium to dissipate the heat)
- The tungsten filament is a plate which slowly rotates presenting a different area and preventing overheating
What is penetration in terms of x-rays?
The higher the energy, the easier penetration of the tissue is
There is less interaction with the tissue
What is attenuation in terms of x-rays?
Inverse to penetration
Good penetration means harder to interact with the tissue
Whereas interaction with the tissue means it is absorbed
Attenuation increases with atomic number of exposed material
- Attenuation falls with increasing energy of photons
What happens to penetration and attenuation when there is low energy (soft)?
Allow the visualisation of fatty tissue
What happens to penetration and attenuation when there is high energy (hard)?
Can visualise bones and not tissue
How does analogue imaging work?
- Through film
- The light exposes crystals on the film
- Gets developed and can see areas where crystals have been exposed
How does digital imaging work?
- Have individual pixels
- Sharper image is smaller pixels
- Independent of the size of the pixels (within the pixel is only a single grey (or colour) value)
How does film radiography work?
- X-rays are converted to visible light by an intensifier
- Visible light oxidises radio-sensitive crystals producing a latent
Why did origional film x-rays need longer exposure?
- The images that are generated to visualise film are high energy, high penetration and low attenuation
- X-rays are poor at interacting with thin film so need longer exposure- however this increases an individual’s radiation dose
What was invented to reduce exposure time and radiation in film X-rays?
An intensifier- converts the x-rays into visable light which is good for film exposure and safer
How is film radiography visualised?
- Exposed radiosensitive crystals are converted to black metalic silver
- Unexposed crystals are washed out of the emulsion
- Uses a developer, a fixer and then a washer
What do the light and dark areas represent in film radiography?
- Dark indicate low tissue density (muscle)
- Light indicate high tissue density (bone)
How does digital radiography work?
- A layer of phospher (encased in plastic) is exposed to X-rays
- The phosphor absorbs energy and releases it over time (latent image)
- The intensity is read by a light detector of certain locations and the computer puts the image back together again
How does the phosphor layer work in digital x-rays?
A bit like fluorescence but stores the light for longer
What is the phosphor layer scanned by in digital radiography?
- Scanned by a laser which causes the screen to emit light
- Then energy is proportional to the magnitude of x-ray exposure
What is the digital radiograph made up of?
- Pixels
- Pixels are segments each assigned a different grey (or colour value)
What are 4 benefits of conventional film radiography?
- Radiologists and doctors must have adequete access to computers for digital
- Computer equiptment is expensive in digital
- No problems associated with virus or hacker software
- Better spatial resolution than digital images
What are 4 benefits of digital radiography?
- No film or film jackets to purchase or a processor to maintain
- Lower dosage as decreased retakes (can manipulate the exposure)
- No time spent filing/searching for old films
- Increased efficiency
What is lifetime exposure of radiation made up of?
- 50% from radon in the earth- makes up 84% natural
- 15% medical imaging- makes up 16%
What is radiation dosage measured in?
millisieverts (mSv)
What is angiography?
- Inject a liquid dye into the blood stream
- Contains metal particles
- These show up on angiographs of radiography and CT scans
What does tomography do?
Capture three-dimentional data on the internall structure of an object by using serial sections
What is computed tomography (CT)?
Tomographic medical imaging method where a three-dimensional image of the internal structure of an onject is generated using a large series of radiographic sections (x-rays)
What length are CT waves?
- Short (10^-10m)
- Try and optimise frequencies of the x-rays we use
What is the path of each photon called?
A ray
What is the quantity of photons reaching the plate called?
- Detector measures the degree of attenuation (photons that are absorbed or scattered)
- Called the ray sum
What is an issue with using X-rays to visualise structures?
- Only provides us with a 2D image of a 3D structure
- Difficult to interpret
WHat did Godfrey Hounsfiled do?
- Was an electrical engineer in the 60s
- Realised there must be a wat to capture information from many viewpoints using x-rays
- Then process together
What is the view of a CT scan?
- Detector measures attenuation of every beam (ray sum) from different viewpoints
- Then get a complete set of ray sums
- Attenuation of each ray sum is correlated with ray position (attenuation profile) for each view
What is a back projection?
Collates data from attenuation profiles to produce an image
What is a phantom structure?
A structure with a known density used to see how accurate a scan is
Who was Allan MacLeod Cormack and what did he do?
- South African physicist
- Worked with Hounsfield (made the machine to produce x-ray souces and detect attenuation profile) to do the mathematics for CT reconstruction
Who created the computed axial tomography (CAT)?
Hounsfield and Cormack
What is the general structure of a CAT scanner?
- X- ray source on one side
- Detector on the other side
- They move aroynd the head to gather different views
What is slice construction in CT?
Computer generation of a large series of two-dimensional x-ray slices based on x-ray attenuation coefficients (i.e back projections)
What is the 3D reconstruction in CT scans?
Computer reconstruction of a large series of 2D x-ray slices to generate a 3D image of an object’s internal structure (i.e stacking slices)
What is windowing in CT?
The window is the range of CT numbers that will be displayed with the different shades of grey, ranging from black to white
What is segmentation in CT?
Method to select part of a 3D reconstruction using CT numbers
What does a collimator do?
- Is a dense metal to focus the x-ray beam and avoid scattering inside the scanner (which would subject the patient to non-diagnostic ionising radiation)
- Also, the x-ray source produces different energies, the low ones are unhelpful as they are absorbed by all the patients tissues, the collimator removes these low energy rays
What does the x-ray tube do in CT?
- Emits x-rays
- Which pass through a collimator and produces and x-ray beam in a fan or a cone shape (modern), older produce a beam
What is the fan or beam directed at in CT?
The object, e.g. patient skull
What does the attenuation after the beam has passed through the object allow a computer to do?
Can be used by a computer to work out the relative density of the tissues examined (profile generated)
How is a 360 image of the object gathered in CT?
- The object can be rotated incrementally (e.g. steps of 1 degrees) within the beam
- More commonly, the beam can be rotated incrementally around the object (steps of 1 degrees)
In CT what does a thinner slice provide?
Thinner slice is more information but is slower to take and patient will habe more ionising radiation.
What happens in CT once a scan of a slice has been taken?
The x-ray set up will be moved down/up by tthe depth we want to capture another 360 attenuation profile until the entire area is sectioned
This information can be used by a computer to work out the relative density of the tissues examined
What is important to consider in terms of angles of sectioning in a CT?
- Need to consider what information we want to capture and what is the best view to section
- Tend to have better refistration between the slices in the plane where the information was collected
- Using a transverse plane to see sagittal for example won’t be as accurate as seeing transverse
What are the usual thickness of body CT slices?
0.05-30mm
Why does a CT need to be calibrated?
Need to work out the density of surrounding materials so it is accurate
Outline what a pencil beam scanner (1st generation) is
- X-ray beam
- Uses a single source and detector
- Rotate by small amount and then repeat
- 180 decree arc covered
- Then gantry moved to next location for following slice
- Step and shoot
What is a gantry in CT?
The setup of the x-ray source and detector
What are 2 advantages of the 1st generation CT scanner?
- Easy calibration and little noise
- Small steps mean that there is sufficient data
What are 2 disadvantages with the 1st generation scanner?
- Slow, so not good for lungs or heart where they change shape rapidly
- Is no longer used
Why are 1st and 2nd generaiton CT scanners limited to a 180 degree rotation?
- The x-ray source and detector array were powered by cables
- Thats why the gantry could only be rotated a certain amount
How are 3rd generation onwards CT scanners able to perform 360 degree rotation?
- Slip-rings were invented
- They are contacts that could slide apart from one another and could move 360
Outline the fan beam incremental CT scanner (2nd generation)
- X-ray fan
- Array of detectors
What are three disadvantages with the 2nd generation CT scanner?
- Did not cover full field of view
- Faster than 1st generaiton but still slow
- No longer used
Outline the rotate-rotate scanners (3rd) generation
- X-ray fan
- Array of detectors
- Full field of view covered
- Source and detector array able to move in a circle within the scanner
- All readings of view collected simultaneously
What are 3 advantages of the 3rd generation CT scanner?
- Fast
- Reduces artefacts from patient motion
- Still used
What is a disadvantage of the 3rd generation scanner?
Ring artefacts if detector array is misaligned in relation to the source
Outline the rotate only scanners (4th generation)
- Fixed 360 degree detector array
- Rather than moving the array, the source is rotated
- Is fast
What are 2 disadvantages of 4th generation scanners?
- Motion artefacts are problematic
- Can be addressed but increases radiation dose
Outline ultrafast CT (5th generation)
- 1980s
- Neither source nor detectors move
- Suitable for cardiac imaging
What are three disadvantages of 5th generation CT?
- Poorer spatial resolution
- High cost
- Not used in routine imaging
Outline dual source CT scanners (6th generation)
- Two sources and detector arrays at 90-95 degrees to each other
- Often one whole field of view and the other a partial field of view
- X-rays from each source can be at different energy levels- see different densities therefore views of the same structure
Outline 7th generation CT scanners
- Similar to 3rd generation
- Cone rather than fan of x-rays
- Flat plate rather than multirow detector
- Used for dental imaging
- Still in development
Outline helical CT (spiral/overlap CT)
- Fast scan
- Slow processing
- Standard for medical diagnostic CT
- Single source
- Detectors in array
- Source and detectors rotate whilst bed moves patient- therefore helical scan
What is a voxel?
Volume element
Rather than pixel ‘squares’, more like 3D cubes
What is the CT number proportional to?
Proportional to the mean relative linear attenuation in a voxel
What is the CT number dependent on?
Energy (spectrum)
What is airs CT number (hounsfield unit)
-1000
What is waters CT number (hounsfield unit)
0
What is fats CT number (hounsfield unit)
-10s
What is muscles CT number (hounsfield unit)
10-low 100s
What is bones CT number (hounsfield unit)
1000-3000
What does the CT number want to relfect?
- The nature of the tissue in that voxel
- usually assumed but calibration needed (work out what 0 and 1000 are as a reference reading)
Outline how windowing (opacity manipulation) works?
CT numbers (Hounsfield Units) represent tissue density and are mapped to grayscale.
Window width sets the range of values displayed, and window level centers it on specific tissues.
Below the range is black, above is white, and within is gray.
Narrow widths highlight subtle differences, while wide widths show broader ranges.
Adjustments optimise views for lungs, bones, or soft tissues, aiding diagnosis.
What is a problem with assigning grey scale values to voxels in windowing?
- There are at least 2000 CT numbers but humans can only see 40 shades
- We need to think about the structures that we are interested in and assign these 40 shades (a window)
- Work out the averal density (level/depth of the window) of the things we are interested in and whether we need a wide or narrow window (range)
Page 3, CT 2 for examples of common CT scans
What was a study done using CT into non-invasive diagnosis of tumours?
- Looked at lung cadavers, cancer tumours
- Scan post mortem to see the position of the different tumours
- Then dissected out to see if they could find them
- Was a good way of identifying the tumours but can also be used when treating tumours using radiotherapy
- Can 3D scan the tumours and focus the x-rays on the right area preserving surrounding tissue
- De Clerk et al 2004
What is effective radiation dose measured in?
Millisieverts (also rad, rem, roetgen, sievert)
Why do we need to consider radiation when scanning different body areas?
They have varying sensitivities to radiation
What does effective dose refer to (radiation)?
Refers to the dose averaged over the entire body
What is the mSv of chest x-rays?
0.1
What is the mSv of cranial CT?
1.5
What is the mSv of abdominal CT?
5.3
What is the mSv of chest CT?
5.8
What is the mSv of chest, abdominal and pelvic CT?
13.9
What is the mSv of cardiac CT (angiogram)?
6.7-13 depending on how much data we want to collect
What is the mSv of naturally occuring doses from cosmic rays and radioactive substances?
3
What is a study into anatomical research using existing patient data of lung shape and sexual dimorphism?
- Interested in sexual differences in lung size, shape and breathing mechanics
- In order to optimise treatment and diagnoses for males and females, need to understand the dtails
- Used prevoiusly taken CT scans and found that there was a different in lung shape
- More pointed apex for men (more diaphragm usage)
- More columnar for women (more intercostal muscle usage)- might be due to pregnancy inhibiting diaphragm use
- Torres-Tamayo et al 2018
What is a study into anatomical research using existing patient data of the size of pterygomaxillary fissures?
- The trigeminal nerve is important for general sensation from the cranial cavity
- If have a headache, trigeminal nerve is conveying the information
- Cluster headaches can be treated with a microstimulator in the pterygomaxillary fissure to stimulate the corresponding ganglion
- Wanted to see if the microsimulator would actually fit in the ganglion
- Found it was of sufficient size in both men and women
- Puche-Torres et al 2017
What is a study into anatomical research using existing patient data of hepatic vein variations?
- Hepatic vein in the liver has different patterns of attributes
- These can be subdivided into subtypes for drainage passage
- Important for transplants and re-sectioning
- Anything over 3cm in diameter needs to be reconstructed for proper drainage
- Lin et al 2023
Give 3 examples where comparative anatomy research is important.
- Zoological
- Archaelogical
- Palaentological
What is an example of a zoological approach of comparative anatomy research?
- Found variations in the bony labarynth of the inner each shape in humans and chimpanzees
- Gunz et al 2013
Why are dried skulls unuseful for MRI studies?
They don’t have hydrogen atoms which are important in MRI reconstruction
What is an example of a palaeontological approach of comparative anatomy research?
- Fossilised remains are the bones or skeleton elements replaced by minerals inside a rocky matrix
- Rocky matrix is either too hard to break apart without damaging the fossil or too crumbly
- Therefore can use CT scans and form a 3D reconstruction which can be rotated to see different bones and aspects that we couldn’t physically extract
- E.g. the skeletal anatomy of the early diapsid, Feralisaurus corami from the middle Triassic- Cavicchini et al 2020
How are CT’s useful in visualising voids?
- CT provides a non-invasive and non-destructive way to investigate the shape and volume of voids
- Can fill voids with wax and then destroy the specimen (disruptive however)
- With CT’s we can visualise and fill the voids with a virtual cast
What are 3 advantages of CT and voids?
- Non-destructive
- Can determine shape as well as size (either volume or cross-sectional area)
- Readily distinguishes bine from air or fluid
What are 3 advantages of CT in image reconstruction?
- Both 2D slices and 3D reconstructions produced
- Slices can be stacked to ‘reconstruct’ 3D visualisations
- 3D visualisations can be segmented to show internal features
How was CT helpful in visualising the maxillary sinus of macaques?
- Allowed visualisation of the maxillary sinys shape and measurement of its volume
- (voidal reconstruction)
- Marquez and Laitman 2008
How was CT helpful in imaging the frontal sinuses of bovids?
- Goat and sheep sinuses go into the bony core of their horns in order to lighten them
- So there is a hollow space in the horns which is hard to visualise the shape of this space
- CT shows complex, small interconnected chambers of this sinus void
- Farke 2010
How can you use CT to visualise volumes?
- If 2D techniques are used it is easy to add slices together to produce 3D reconstructions
- i.e. go from displaying cross-sectional areas to displaying spatial information
- CT allows non-invasive vivisection
- We can examine structrues as if they were dissected straight out of the body
How does segmentation or the extraction of 3D representation work in CT?
- Identifying voxels based on their CT number that are occupied by structures of interet
- Space occupied by that structure or tissue can be regarded as the union of voxels associated with those pixels where the x-ray attenuation is above the appropriate theshold (thresholding)
- Easy when thresholds are very different
- But also means object has multiple boundaries if surface rendering for example outer surface compared to voidal trabeculae in the bone
What are two applications of mobile CT scanners?
- For clinical purposes (like patients who don’t normally have access to a facility with a CT scanner)
- For research (like bringing a CT scanner to a museum rather than take the specimen to the scanner)
