Introduction to Computed Tomography (CT), Magnetic Resonance imaging (MRI) Flashcards
What is CT ?
A method of acquiring & reconstructing an ‘image’ of a cross section of the patient by measuring the attenuation of a highly collimated x-ray beam
Overcomes the issue of superimposition
What is CT ?
Cross sectional imaging planes
Axial / Transverse – with the exception of the head
magnet gantry and noise
The first CT scanner
Godfrey Newbold Hounsfield Experiment
developed in the 1970s
took 9 hours to take an image
took 24 hours to develop and process
The first CT scanner
1024 x 1024
2048 x 2048 much more detail
ct scanner evolution
x-ray tube some have 2
bank of detectors
these move around the patient in a 360 pattern
these then fire x-ray photons through patient
attenuation measured
transmitted radiation is measured
creates data and then processed and then produce 3d representation.
What are we measuring ?
Attenuation (i)
It = Ioe-µ∆x
x - thickness of material
it - whats transmitted
1o- incident
attenuation coefficient amount of stopping power by tissue
bone has high attenuation tissue than fat or muscles would.
Pictorially
for every ct image we take we have incident radiation
Solid state detectors
measure whats transmitted
xrays produced by tube pass through patient then through detector
xray photon into light photon
using photo multiplier tube detected at the bottom of the image detector creates electrical signals
measure how much radiation has passed
light representation
High Power X-ray Tube
difference between conventional xray tube and ct tube
ct tube has a metal envelope better at dissipating heat
compartments include
ceramic insulators
direct oil cooling of spiral groove bearing
unique 200 mm anode disk
compact all metal envelope
High Power X-ray Tube
‘Flying focus’ – allowing for control of focus position on the anode
Glass tube replaced by metal ceramic technology
Circular grooves in anode support to increase contact and improve cooling
Liquid metal (gallium) vacuum bearings facilitating faster anode rotation
Rhenium / tungsten focal track
attenuation measurements taken 360degrees
many ray sums are collected
Image Reconstruction
Filtered back projection is applied in contemporary CT reconstructions
Ray sums are collected in data sets called projections (circa 1000 + rays will make a single projection)
Image reconstruction
To reconstruct the image each ‘voxel’ must be viewed from many directions (rotational intervals of approximately 10 )
‘Back projection’
Effectively reverses the attenuation process by adding the attenuation value of each ray in each projection back through the reconstruction matrix
To overcome the effects of ‘blurring’ the data are filtered prior to the back projection
Filtered Back projection
machine filters out unwanted data
scatter radiation is adding to the mean value of the vauxhall
not getting a true measurement of attenuation filters out data accurate representation.
Filtered Back projection
chopping up beam into individual sections
360 around the patient
back projection
filter out scattered radiation.
Multi-slice CT
multiple rows of detectors 200+
time taken to scan image is alot quicker than before
slight complications
acquiring big voulme of data
Isotropic Imaging no lose of data
(vauxhall) coloums rows is the same
when u reconstruct data u don’t lose much data
lose quality of the image
Dual Energy CT
2 sets of data from different tubes with different kvs
Dual Energy CT
AKA spectral CT – uses two separate photon energy spectra- creates data sets for biological tissues with different attenuation properties at different energies
Can create electron density maps and effective atomic number maps
Wide range of clinical applications including:
Virtual non-contrast imaging
Automated bone removal in CTA
Blood pool imaging (PE or MI)
Characterisation of renal stones
Etc…etc
Further reading: Spectral Computed Tomography: Fundamental Principles and Recent Developments HERE
CT Application – IMAGING
The rapid diagnosis of life-threatening injuries, strokes, blockages in arteries, or internal bleeding
The primary diagnosis of many cancers and how advanced they are. Follow up scans are used to assess how the disease is progressing
Assessment of the coronary arteries and the condition of the heart in patients with suspected heart disease
Imaging of major blood vessels of the brain, body and limbs to assess and plan treatments
Guidance for interventions such as biopsies, spinal and musculoskeletal injections
Planning of orthopaedic surgery
Specialised examinations such as CT Urograms,
Cardiac CT and CT Colonography
CT Use in Radiotherapy
make a diagnosis find the cancer
where has the cancer spread
use ct to insure patient is setup
CT Use in Radiotherapy
The use of CT in treatment planning allowed several important advances:
Greater precision in dose distribution
Dose optimisation
Patient positioning
CT has also provided for 3D dose calculations which creates precise visualisation of the geometric positions of tumours and normal tissues in patients
The radiation dose can then be calculated and optimised in order to determine the best dose distribution in the target tumour avoiding the surrounding normal tissue
CT also offers digitally reconstructed radiographs for patient position verification at the time of treatment prior to using the linac
Cone Beam CT (CBCT)
CBCT uses diverging kV x-rays and has the ability to visualise anatomical structures and acquire images over a much larger volume in a single scan than is capable with traditional fan beam CT
Due to a larger scan volume, there is increased scatter radiation and loss of contrast resolution (overall lower image quality)
Application in radiotherapy with the integration of linear accelerator mounted CBCT for radiation therapy units
This integration of CBCT imaging with radiotherapy units has allowed the patient to be imaged directly before therapy
Fan Beam CT vs Cone Beam CT
CBCT
CT Pros vs Cons
Advantages:
Detailed cross-sectional images
Improved low-contrast detectability
Use of IV contrast to further enhance difference between structures
High spatial resolution for imaging bone/fractures/trauma
Very quick scans (whole body scan in about 20 sec)
Image reconstruction/3D in various anatomical planes
Disadvantages:
High radiation dose examination
Large and expensive equipment
Images can be susceptible to artefacts
MRI – The Basics
Developed into an important imaging modality from 1978
Utilises the fact that magnetic nuclei in a static magnetic field exhibit a characteristic resonance frequency that is proportional to the field strength
Primarily MR images map the distribution of hydrogen nuclei within the body
The radiographic community have recognised MRI’s unique ability to image soft tissue & differentiate between benign and malignant tissue
MRI ?
Stands for:
Magnetic
Involves magnetic fields
Resonance
Involves resonance – frequency has to be matched to a natural process
Imaging
Produces images
Generating the MR signal
magnetic forces randomly distributed.
place patient in mr machine
create a magnetic field - strong magnet
see 2 physical principles
alignment of magnetic field in same direction not randomly distributed
additional movement
- net magnetic vector (nmv)
Some other terms
MR
Any MRI exam
MRA
MR Angiography – blood vessels
MRV
MR Venography – veins only
MRA
MR Arthrography – joints
MRS
MR Spectroscopy – chemical composition of the brain
fMRI
Functional MRI – how the brain works
iMRI
Interventional MRI – real-time image guided surgery
MRI hardware
open magnetic - no claustrophobia with patients.
MRI Hardware
Inside a ‘Static Magnet’
series of coil wire generate magnetic fields
Inside a ‘Static Magnet’
Inside a ‘Static Magnet’
Net Magnetisation Vector
Vector components – a framework for explaining MRI
In MR, z direction is defined by direction of B0 (longitudinal)
No difference between x and y (both are perpendicular to B0) - called transverse plane
Measuring M0
Can’t measure it while it is lined up with B0
(B0)z ~ 1T
M0 ~ 0.000001 T insignificant field
If we can move M0 we can measure it
(B0)xy = 0 T
M0 ~ 0.000001 T a measurable field
Measuring M0
Can’t measure it while it is lined up with B0
(B0)z ~ 1T
M0 ~ 0.000001 T insignificant field
If we can move M0 we can measure it
(B0)xy = 0 T
M0 ~ 0.000001 T a measurable field
Larmor Frequency
Frequency of precession is called Larmor frequency and depends on external magnetic field strength
Larmor Equation
0 = B0
γ = 42 MHz per Tesla
Effect of RF Wave
The observed effect of rf irradiation is to rotate M0 away from z axis and down towards x-y plane
‘Flip Angles’
Pulse of rf radiation leaves M0 exactly in transverse plane
called a 90° pulse
If strength of rf is doubled (or rf is switched on for twice as long) M0 will turn through 180°
pulse is then called a 180° pulse
Smaller flip angles are easily produced
The magnetism from the patient can be read at a flip angle of as little as 10°
Signal Generation
After 90° pulse, magnetization vector lies in the x-y plane, and is still rotating at ω 0
Receiver coil looking only at x-y plane detects changing magnetic field as magnetization rotates
A current is generated in the coil, producing the MR signal
Free Induction Decay
MR signal after 90° pulse is switched off known as a Free Induction Decay (FID)
Oscillates at Larmor frequency
Decays to zero exponentially
Relaxation
Weighting the images can produce different types of inherent contrast
MRI Pros vs Cons
Advantages:
Does not use ionising radiation
Images acquired directly in multiple planes (sagittal, coronal, transverse and oblique)
Superior soft tissue contrast compared to CT/US/RNI so excellent to image the brain, spine, joints etc.
Allows images to be weighted to visualise fat, water etc.
Accurate anatomical, metabolic and functional information – advanced techniques such as MR spectroscopy allow for precise tissue characterisation rather than merely ‘macroscopic’ imaging
Some angiography can be done without the use of intravenous contrast media (unlike CT)
Disadvantages:
Longer scans
Less comfortable and may be a suboptimal patient experience – small bore, noise, claustrophobia
Large equipment, very expensive and less readily available
Not safe for patient with certain implants, pacemakers, foreign bodies
Higher safety considerations for staff – MRI safe/compatible equipment
MR images are more susceptible to artefacts due to the sensitivity of the magnet – patient motion, external disturbances
MRI Use in Radiotherapy
It can provide additional information in order to more precisely define tumour localization for treatment planning using radiation therapy
It has better soft-tissue contrast than CT
Provides better visual discrimination between tissues that should be treated and those that should not
However MRI cannot identify the mass attenuation coefficient or attenuation characteristics for high-energy photons, X-rays, and gamma-rays, as this is critical for precise dose calculation
Increased used of MRI technology in radiation therapy
MR Linac
The Magnetic Resonance Linear Accelerator (MR Linac) combines an MRI scanner and linear accelerator to precisely locate tumours, tailor the shape of X-ray beams in real time and accurately deliver doses of radiation to moving tumours
Hybrid Imaging
Fusion of two or more imaging modalities to form a new technique
Fusing imaging technologies synergistically combining structural and molecular imaging
PET/CT
SPECT (Single Positron Emission CT)/CT
PET/MRI
SPECT/MRI
