CTs Flashcards
What is CT?
- Use of X-rays
- Means obtaining an image corresponding to slice or cut through the body
- The image corresponds to a cut transaxial cross-section – there is no overlap of information – good contrast
What is the X-ray source?
The X-ray tube is an electrical device consisting of cathode (-)(filament – causes emission of electrons (thermionic emission) and anode (+) (target – produces the X-rays). It converts electrical energy into X-rays and heat. Electrical current flows through from cathode at high voltage to anode which causes the electrons to lose energy to the positive nuclei there, which in turn results in the generation of X-rays (energy is taken from electrons and is converted into photons). The closer to the nucleus the electron passes, the more energy the electron loses, and it is deflected to continue moving in a different direction at a lower energy or stopped altogether.
The cathode and anode are contained within an envelope to provide vacuum and insulation. You need an electrical supply to the X-ray tube to provide energy. We need direct current (DC) to allow the electrons to flow from cathode to anode (and not the other way round) – it helps produce smooth and consistent X-rays. The quality and quantity of X-rays are controlled by adjusting the potential difference across the tube, tube current and exposure time.
How is a 3D image constructed?
Attenuation of X-rays are recorded to build a 3D representation of the scanned tissue.
How do CTs work?
- X-ray rube and detectors sit in a motorised gantry that rotates around a patient. As they rotate, the detectors capture electronic data which is then processed by a computer before being displayed and stored as an image (convert X-rays to electronic signal then processed by computer). The scanner doesn’t take an image – instead, it collects data and constructs that image. The image is made up of many different slices through the patient. The detector is small in size for better spatial resolution
- The patient moves through the machine in the axial direction
- In CT, we measure attenuation coefficient: how much X-rays have been absorbed/attenuated vs how much X-rays was released from the X-ray tube (so it reflects the degree to which the X-ray intensity is reduced by a material)
- Projections are 2D views taken by the machine at different angles all-around the body. The attenuation coefficient is sampled at each detector to generate a projection. For each next projection, the tube and detectors are rotated a small amount and the measurements are repeated
- The image is reconstructed by complex mathematics within a computer system – one of the techniques are called filtered back projections (basically puts on a filter to prevent blurring; removes some noise around the image, making it clearer) and iterative reconstruction (it’s starting to replace filtered back projections as it can build in corrections – it works by comparing the estimated image to the projection data collected)
- Pixels are 2D representations. Since CT produces 3D images, each pixel represents a volume in the patient – this volume is called a ‘voxel’. When you take a thick slice, the intensity of the pixel will be average throughout the voxel to produce the 3D image – this can cause small bits of detail to be missed. So, if you suspect something in a certain area, you may want to go back in thinner slices.
-Each pixel is assigned a CT number measured in Hounsfield units – it’s measured using the attenuation coefficient (mu/density).
CT number= [mu(tissue) – mu (water)] / mu(water) x 1000
The mu values we need to remember are water (0) and air (-1000)
What is helical/spiral scanning?
The patient is moved through a rotating X-ray source and detector in the gantry (fast motorised gantry rotation); there is continuous table feed. An axial slice of data is obtained as the patient moves through the machine.
This data can be reconstructed by a computer to forms 3D volume, which can then be re-sliced digitally to obtain thinner slices and slices in different planes. (X-rays pass through the patient and are detected by the detectors)
- CT traces a helical pathway. This results in 3D data set which can be reconstructed into sequential images (produces one continuous volume set of data for an entire region; data from multiple slices from patient are obtained).
- Data is put together to create a normal CT image
- Pitch: radiation dose administered during helical scanning depends on the speed of the patient through the scanner
What is multi-slice helical scanning?
- Advancement of single slice helical
- Rotating X-ray source and have multiple detector rows, instead of just 1 (in the motorised gantry that rotates).
- Allows multiple layers of data to be obtained simultaneously
- Allows multiple slices
- Higher quality
- Faster scan
- More coverage in each rotation
- Gives 3D image
Why are artefacts relevant?
Make sure you’re aware of artifacts as it will complicate interpretation of a CT scan. E.g. metal, high concentration of IV contrast can cause artifacts. Ask patient to hold their breath to minimise artefacts
What is CT windowing?
-The denser the tissue, the brighter it appears
-Tissue density is measured using Hounsfield units (HU). This is defined as: water = 0; air = -1000.
This is useful in determining composition of a mass or determining whether a fluid is blood
- The relative densities in CT: Metal>Bone>Soft tissue>Fluid>Fat>Air
- A ‘window’ (small range of tissue densities) can be set to look at certain tissues which can be represented by a full greyscale spectrum, allowing subtle differences in density to be seen more easily. E.g if a window with a level of 0 HU and a width of 400HU is set, the window will have a range of -200 to 200. Any tissue with a density of -200HU or less will be black; anything with a density of 200HU or more will look white. The image produced can be looked at different windows
- We can have a soft tissue window; lung tissue window (we can see pneumothorax here as lung tissue would be whitish whilst the air pockets will appear black); bone window (can see differences in the high densities of medulla and cortex)
How is contrast used in CT?
- Phases of a scan refer to when the images are taken relative to the administration of the contrast. The contrast is usually iodine based IV (with exceptions for head CTs and patients with poor renal failure).
- There are some risks with contrast e.g contrast induced nephropathy (characterised by increased creatinine by 25% within 3 days; can cause complications of kidney disease); anaphylaxis and allergy.
- Contrast will be injected in the vein of the arm to outline the blood clot. The contrast will absorb the X-rays (greater attenuation) due to high density so it appears white, which will allow us other structures such as vessels to show up.
- The phases are pre-contrast, arterial phase, venous phase, and delayed phase
- Arterial phase comes before venous phase (despite venous administration) because the contrast goes to the right heart, to pulmonary circulation and back to the left heart and the rest of the body before it reaches the entire venous system
- The chest is usually imaged in the arterial phase
- The abdomen is usually imaged in the venous phase
- Liver lesions are usually imaged with multi-phase scan
- No contrast: kidney/urethral stones, arterial calcifications
- Arterial: abdominal bleeding, aortic aneurysm, arterial stenosis/occulations, hypervascular liver metastasis, pancreas tumour
- Venous: screening, hyper vascular liver metastasis, abscess formation, venous thrombosis
- Delayed: uteral obstruction or leaks, characterisation of liver tumour
What are the radiation risks?
- Radiologist must ensure the scan is justified
- Radiographer must minimise the amount of radiation the patient is exposed to
- Millisievert is used to measure radiation dose
- The radiation dose in CT is 5-10 times higher than with planar X-rays
- Radiation exposure carries 2 types of risks: deterministic and stochastic
- Deterministic = dose related effects on fast dividing cells (e.g GI tract and bone marrow) where it’s useful
- Stochastic = random effects that are independent of dose and cause DNA changes (e.g from diagnostic tests)
What are the limitations of X-rays in comparison to CT?
The problem with X-rays is that it’s 3D, there is superimposition of structures; differentiation of structures with low contrast resolution (density of structures are similar so its harder to see these structures separately). CT can see exactly where each structure is e.g anterior, posterior due to the cross sections.
Why is CT useful?
- CT can be very useful for making acute diagnosis of stroke – allows us to differentiate between a clot or haemorrhage so that we can thrombolyse a clot or not go anywhere near thrombolysis in a haemorrhagic stroke. Due to the dense nature of an acute haemorrhage, it will show up on the scan as a bright white area
- CT can be used to monitor disease progression and response to therapy e.g. cancers
- CT can be used to define the area that needs to be treated in radiotherapy to spare as much healthy tissue as possible
How can CT be used with PET?
- PET scanners are used for functional anatomy – they detect the presence of radiolabelled tracers e.g glucose. They are not so good for anatomy
- So we use CT with this to help with anatomy. If we take both scans (PET and CT), we can superimpose the PET image over the CT images – this allows us to localise areas of high uptake
How can CT be used for pulmonary embolism?
- Pulmonary embolism is the obstruction of a pulmonary artery by a substance that has travelled from somewhere else in the body, such as thrombus from a distant vein, air or fat. This leads to decreased lung perfusion which in turn results in decreased blood oxygenation and the accumulation in the right ventricle of the heart
- CT pulmonary angiogram: This allows us to obtain an image of the pulmonary arteries. Iodine based contrast will be injected in the vein of the arm of the patient at a high rate to outline the blood clot. We use the image in the arterial phase. The contrast will absorb the X-rays (greater attenuation) due to high density so it appears white, which will allow us other structures such as vessels to show up. A normal scan will show the contrast filling the pulmonary vessels appearing white. An embolus will appear dark in place of the contrast, blocking the space where blood should be flowing into the lungs. If the interventricular septum appears bowed, it indicates back pressure usually due to embolus in the embolus in the pulmonary trunk obstructing outflow. In patients with pulmonary embolism, the contrast will also flow to the liver.
The patient should hold their breath for about 10-20 seconds to prevent motion within lung. For a patient to get a CT, it should be justified because it is a much higher exposure to radiation in comparison to chest X-rays (5 to 10 times more). There is a possibility in induction of cancer by DNA mutation by the X-rays.
How can CT be used for aortic dissection?
- An aortic dissection is a tear in the inner wall of the aorta (tunica intima) which creates 2 channels for blood flow – one is normal lumen if aorta and the other is into the wall where blood becomes stationary. Blood entering the wall can constrict the aortic lumen, reducing the blood flow to the rest of the body. It can also cause further weakness and dilation of the wall, potentially leading to aneurysm.
- Contrast enhanced CT is used to find aortic dissections. The findings show double lumen (because 1 has stationary blood which has greater attenuation), dilation of the aorta, Mercedes-benz sign (3 intimal flaps). Also, in the descending aorta, there is a grey region which isn’t contrast enhanced – this is intramural haematoma due to past aortic dissection.
