CT Flashcards
The circular structure that the patient and table travel through during the CT scanning process is called what?
- Collimator
- Console
- Gantry
- Couch
Gantry
What is the main advantage of using the slip ring technology in CT machines?
- Faster scanning
- Reduced patient dose
- Better quality x-ray beam
- Higher resolution image
Faster scanning
Which of the following controls how much of the detector is exposed during CT imaging and severly limits the amound of scatter reaching the detectors?
- Pre-patient collimator
- Post-patient collimator
- Bowtie filter
- Detector array
Post-patient collimator
What is the CT number of water?
- 1000
- -1000
- 0
- 1
- 0
Does bone have a more positive or negative HU than water?
More positive
Does fat have a more positive or negative HU than water?
More negative
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
Define CT number
a normalized value of the calculated x-ray absorption coefficient of a pixel in a computed tomogram
Expressed in Hounsfield units, where the CT number of air is −1000 and that of water is 0.
Define MPR
Multiplanar reformation (MPR) is the process of using the data from axial CT images to create nonaxial two-dimensional images
Define MIP
maximum intensity projection (MIP) is a method for 3D data that projects in the visualization plane the voxels with maximum intensity that fall in the way of parallel rays traced from the viewpoint to the plane of projection.
Main difference between x-ray and CT
An x-ray takes a single image at one angle, whereas CT takes multiple images at multiple angles as it spins around the patient.
Main drawback of CT vs X-ray
However, this does mean it is a large dose of xrays which is absorbed by the body.
3 clinical uses of CT are?
- Presence, size and location of tumours
- Organs in the pelvis, chest and abdomen
- Colon health (CT colongraphy)
- Vascular condition/blood flow
- Pulmonary embolism (CT angiography)
- Abdominal aortic aneurysms (CT angiography)
- Bone injuries
- Cardiac tissue
- Traumatic injuries
What is an overview of the sequence of events occuring in a CT scan
- X-Ray tube and detectors (ceramic material) rotate around the patient
- Radiation is attenuated as it passes through the patient, remnant beam is measured by detectors
- Beam converted into an electrical signal which is amplified then digitised
- Data sent to computer
- Data processed (using maths algorithms/processes)
- Data displayed, manipulated and recorded.
What is one basic difference between the x-ray tubes of the CT vs Xray housing
X-ray tube= bigger graphite backing to the anode because there is so much heat generated in CT
Give an overview of the generations of scanners
- 1st generation scanners
- Step translate scanner
- X ray tube, and detector at the bottom
- Xray tube generate and would move 5mm, and do the same (and then move 10*, and do the same 36x)
- 2nd generation scanners
- Step translate scanner
- Diverging beam, to cover larger areas
- 3rd generation scanners
- 4th generation scanners – expensive, and used for very specific types of imaging
- 5th generation scanners (EBCT)
- Multi-slice technology – most common, found in most hospitals
Describe the 3rd generation CT scanners
Slip ring technology.
Spirals around the patient- and table slowly moves round.
Lowers radiation dose for patient, and is quicker
how is multi-slice CT different from 3rd gen
Multi-Slice CT- Based on 3* generation.
- But multiple bites of detectors along in the X and Z direction.
- Original had 4 slices thick, and 4 rows.
- Now have 256 slice scanners- so can scan whole body in up to 90 seconds = CT fast!
- Need to ensure dose is kept low
Describe the Gantry of the CT Scanner
- Houses x-ray source, detector array, collimator assembly and possibly also high voltage generator
- Aperture of max 90cm diameter to allow max gantry angulation and reduce patient anxiety due to claustrophobia (although maximum patient size on table will be determined by the weight the table can hold – around 22 stone)
- Can be angled +/- 30 degrees (on average)
- To help with radiation dose
Describe the X-ray tube of the CT scanner
- X-ray production just the same as general radiography
- Rapid sequential exposures required so massive amounts of heat produced
- Large diameter anode discs (typically 20cm) with layered alloy and thick graphite backing to aid heat sink
- Modern units have pulsed beams to reduce heat loadings and prolong tube life
- Typically, between 80-140 kV
- Not continuous beam of photons
- Scanners have either liquid (oil) or air-cooled tube housing designs
- CT x-ray tube life expectancy is only about a year or so
Describe the slip rings of CT
- Faster conventional axial scanning
- Interscan delay governed only by time taken for table to move to new position (~1sec)
- Spiral scanning
- Continuous table feed/movement
What is Aquisition
- the process through which a single continuous set of spiral scan data is acquired without an intervening pause
- getting the data from the patient
What is interpolation
- modification of scan data before reconstruction in order to synthesize a set of data that yields a clinically acceptable representation of a cross-section of anatomy
- need to then modify the data into an image which is clinically diagnostic
Describe the collimation beam of the CT
- Radiation beam is double collimated (needs to be thin)
- Tube exit –
- Pre-patient
- Controls patient dose- as low as possible
- Post patient –
- Detector entrance and matches the width of the detector to eliminate scatter degrading the image
- Controls slice thickness
What is the size of the collimation beam of CT
- Collimation is variable from 1.25mm to 80cm (potential) and is controlled by the software program
- 2, 5, 10 depending on what looking at normally
- Looking at fractures/bones or small organs such as the pituitary gland
Describe the detector array
- Solid-state type, made from ultra-fast ceramic material (most efficient)
- Every phone which hits the material = lots of information/signal
- Lower the dose, as more efficient
- Efficiency is important as it determines tube loading and controls patient dose
- Have high sensitivity for best low contrast resolution
- Not sensitive enough, don’t get as good resolution as we need
- Detectors not tightly packed = interspace between (reduces cross talk and therefore increases image quality)= reduced detection efficiency and increased patient dose
- Need tightly packed, but space between. Otherwise computer cant work out where radiation is coming from in the patient
- Multiple detector arrays allow the collection of two or more data image sets simultaneously
- Can reduce heat loading of the x-ray tube
Difference between fixed and adaptive arrays
Fixed- all ceramic detectors in X axis are the same thickness i.e. all 2.5mm (so smallest slices we can generate are 2.5 mm). If we needed 5mm slice, 2 detectors together etc.
Adaptive arrays- thin detectors in the middle, thicker to the outside. This means, thin scans can only use middle detectors to scan.
Describe the beam attenuation of the CT scan
- The patient remains stationary on the examination table while the X-ray tube rotates in a circular orbit around the patient in a plane perpendicular to the long-axis of the patient
- Some will be attenuated, some will not
- The intensity of the x-ray beam striking the patient is known as the incident intensity
- This is proportional to the tube output (mAs)
- The beam is attenuated (same as general x-rays) by the patient dependent on the different tissue densities
What is attenuated radiation measured by
detectors situated opposite the x-ray tube
Ideal characteristics of CT detector arrays
- High detection efficiency, absorbing all the emergent x-ray beam’s energy to which they’re exposed
- High conversion efficiency, enabling all the detected energy to contribute to image formation
- A wide dynamic range, so that all x-ray intensities, from weakest to strongest, are converted into proportional output signals
WHat are CT detectors
- The detectors are ceramic scintillators, which respond to x-ray exposures by emitting pulses of visible light in proportion to the absorbed energy
- Every time a photon hits the detector, it hits pulses of light, which is converted to an electrical signal. More light = stronger x-ray photon
- The light is converted into electrical signals by photodiodes, coupled to the scintillators
The intensity of the x-ray beam after passing through the patient is called the,……. and is inversely proportional to the attenuation of the x-ray beam
transmitted intensity,
The decrease in intensity following penetration into matter, expressed numerically, is the ………. (µ) i.e. the percentage of radiation remaining after the x-rays pass through the object
attenuation coefficient
in image construction, what is the pre-processing phase
-
Pre-Processing
- Raw data is produced by processing scan data
- 1st stage is to undertake a series of dose measurements
- For each dose measurement the computer has to know the position of tube and detector and hence the voxels the measured beam has passed through
- This information is known as raw data
In image construction, what is the convultion phase
- Convolution
Algorithm (mathematical formula) applied to raw data to provide a sharp or smooth image to enhance the appearance of the tissue of interest (determined by protocol selection prior to scanning the patient)
- Primary mathematical model in the creation of computerised medical images is the Fourier Transformation
- This is simply conversion of data to more useful recognisable form using mathematical algorithms
- In the case of CT these are created using the different image densities and enhancing them to create a recognisable grey scale
in image construction, what is the back-projection phase
-
Back- Projection
- A complicated process in which the convolved scan data is projected back into the image matrix
- Each pixel is assigned a numerical value (the CT number), which is the average of all the attenuation values within the corresponding voxel
- This number is compared to the attenuation value of water, which is assigned an attenuation value (HU) of 0
- The numbers are displayed on a scale called …………. ??
Hounsfield Units (HU)
- Dense bone = ?? HU
- Water = ?? HU
- Air = ?? HU
- Dense bone = +1000 HU,
- water = 0 HU,
- air = -1000 HU
Describe windowing on a CT scan image
-to distinguish between shades of grey
A narrow window width = high contrast scan
0- +100 would show as grey. Above would show as pure white
-100 - -1000 = pure black
What is Multiplanar Reformatting (MPR)
We can only scan in axial planes. The original cross-sectional images are used to produce image data in another body plane (coronal/sagittal).
What isMaximum Intensity Projection (MIP)
- To separate superimposed structures/vessels
- Selects voxels along a row/column (e.g. blood vessel) in a volume of interest with a specified range of CT numbers
- The basis for CT angiography (CTA)
- Contrast dye ~650 Hounsfound Units
Purpose of Shaded Surface Display
- Provides a 3D image of the surface of a structure
- Very useful in orthopaedics as the image can be rotated and viewed from various different angles or Maxillofacial planning for surgery
True or False: CT can be 4D as well as 3D
True! 4th dimmension is movement
When is a CT scan triggered, post-contrast injection
Trigger our scan when most contrast in area of interest.
(Lung fields and mediastinum –> look at aorta
Once there is enough contrast in the region of interest, it will start scanning. Delay between injection, and triggering the scan.
What is this image showing
That a scan is only triggered when a certain HU is achieved- in this case 110 HU
What is the most common CT scan performed?
CT Head
When might a CT head be performed
- Trauma (traumatic brain injury – TBI)
- Intra-cerebral haemorrhage (ICH)
- Cerebro-vascular accident (CVA) - stroke
- Infection/inflammatory disease
- Tumours (malignant and benign)/metastases
What are the three catgeories within the GCS
Made up of three areas eye opening, verbal and motor response.
What do the different GCS scores indicate
- Maximum score of 15 indicates a normal consciousness level
- A GCS of 13 to 14 may indicate a mild TBI, with up to 15% of these patients having symptoms 1 year after the injury
- A GCS of 9 to 12 indicates a moderate injury and has a mortality rate of approximately 2% to 3%
- A GCS of 8 or less indicates severe head injury and is associated with a mortality rate of up to 36%
- The lowest score of 3 indicates a probability of death or persistent vegetative state of up to 97%
Is this extradural or subdural
- Extradural- convex (coming away from the skull)
- Outside the layers of the dura of the brain
- Midline shift- fisher line in the middle and lateral ventricle ‘back to back banana’
- Bore hole and drain stat
Is this extradural or subdural
- Subdural- follows shape of bony skull
- Bleed between dura layers of the brain
- Harder to spot- i.e. thickened skull area/mimic bone
- Trauma/long-term drug users/long-term chemo*
- Left to heal unless they cause other problems
What pathology is this
Intracranial Haemorrhage
- Traumatic brain injury i.e. RTC/Falls
- Fresh blood = bright white
- Severity/location = different treatments
What pathology is this
Left Parietal Cerebral Vascular Accident
- Stroke- darker area (not fresh blood)
- Thrombolytic treatment ASAP
- ?stroke = straight to CT, as quicker onto thrombolytic treatment better patient outcomes: ‘time is tissue’
- Most caused by infarctions (often plaque)- no blood, so starts to die
Is this likely to be a recent pathology or not
Recent- Fresh blood - high density on CT - appears white
Is this likely to be a recent pathology or not
Later-
- After 2-3 weeks, CT will show a haematoma to be of a density similar to brain
- Beyond this time, it becomes progressively less dense, until it appears the same as cerebrospinal fluid (CSF)
What is a glioma
Type of mass
- Contrast shows up as bright white, so never give patient contrast first
- Do scan, if suspicious, then contrast
What is a menigioma
Mass which sits within the meninges
What can a HRCT show up in the lungs
Detailed imaging of lung parenchyma
- Interstitial lung disease
- Bronchiectasis, airway disease
- differentiation of pulmonary nodules
- Differentiation of focal lung disease
Black bits = trapped air in alveoli/bronchi
What pathology is this
Pneumothorax
What is highlighted by this non contrast CT
- No contrast- renal colic (abdo pain, radiating along back, caused by bilateral renal stones)
What is highlighted here with the use of contrast
- Bright white contrast/calcification (iodine) shows aortic aneurysm
What is the difference between a CTA and Conventional Angiogram
CCTA also uses imaging contrast, but it’s injected through a vein in the arm and pictures of the arteries are taken from outside the body.Only intervention: injection of contrast / radiation dose same as conventional, but more information
CA a catheter is inserted into an artery (usually in the arm or leg) and threaded up into the heart. A contrast dye is then sent through the catheter to help the arteries show up on an X-ray.
What can a CTA- Circle of Willis show
- CVA/stroke = blockage of blood vessels
- Inject contrast
- Arterio-vascular malformation = crossing of blood between two
Why might a CTA be used for trauma incidents
- Severe fracture of femur
- Check for vascular component
- Gives information before theatre for fracture fixture too
Why might a whole body CT be done
Use whole-body CT in people aged 16 and over with blunt major trauma and suspected multiple injuries.
Use clinical findings and the scanogram to direct CT of the limbs in people aged 16 and over with limb trauma.
