MR Physics Flashcards
When was MRI scanning developed?
Between 1950 and 1973
First MR images of human tissue was published by Peter Mansfield in 1977
What is a magnetic moment?
The magnetic moment (µ) is a vector quantity used to measure the tendency of an object to interact with an external magnetic field - in MRI the object is the hydrogen nucleus
What is Larmor frequency?
Refers to the rate of precession of the magnetic moment of the proton around the external magnetic field
The equation states that the frequency of precession of the nuclear magnetic moment is directly proportional to the product of the magnetic field strength (B0) and the gyromagnetic ratio (g)
What is the contrast in an MRI scan generated by?
Different body tissues exhibiting different magnetic properties
Scans visualise this difference in magnetic behaviour between brain matter, fat etc
The contrast is the ratio between high magnetic signal and low magnetic signal
What type of signal do bone and fluid have?
Bone = high signal (bright)
Fluid = low signal (dark)
What contrast is used in MRI and why are they used?
Gadolinium
Can be used to highlight certain tissues or pathologies
What are the two types of contrast?
Exogenous (gadolinium injection)
Endogenous (natural properties of tissue, i.e., without injections)
How does the value of the pixel reflect the MR signal?
Higher the pixel value, the brighter the signal
When we do image processing, we are manipulating sets of numbers, what is the arrangement of the numbers called?
Matrix or sometimes called a two dimensional array
How many axes do MRI scans have?
3 = x, y, z
This means we are dealing with stacks of matrices
What is the spatial resolution of an MR scan?
Each pixel quantifies an amount of physical space - in structural brain MRI scan this is typically 1mmx1mm
The size of the pixels is referred to as the spatial resolution
What does higher resolution mean in terms of pixels?
Higher resolution = smaller pixels = clearer image
What is a voxel?
3D volume of anatomy rather than a flat square (pixel)
What is the raw data format of MRI?
DICOM file
To perform analysis on the raw data what needs to happen?
Need to convert raw scans into other types which are more standardised and compatible with image processing tools
The single most common of these is the nifti format (.nii) which can be further compressed down into nifti-gz (.nii.gz) to save storage space
Describe the basic MRI hardware
Made up of 4 components
1. The magnet
2. The gradient coils
3. Radiofrequency transmitter and receiver
4. Computer
What is the function of the gradient coils?
The main function is to spatially modulate the main magnetic field in a predictable way, thereby causing the Larmor frequency of spins to vary as a function of position
The use of stronger gradients permits smaller anatomical features to be detected and enables faster scanning
Gradient coils are used to produce deliberate variations in the main magnetic field (B0). There are three sets of gradient coils, one for each direction. The variation in the magnetic field permits localisation of image slices as well as phase encoding and frequency encoding
What is the RF transmitter and receiver?
The radiofrequency (RF) transmitter is the generator of the radiofrequency current which is delivered to the transmitting coil
This creates a signal which is used to excite protons in the imaging field
Radiofrequency coils can be both transmitters and receivers of the radiofrequency signal or receivers alone
A radiofrequency coil that performs both of these actions is called a transmit receiver coil or a transceiver coil
How are the gradients generated?
Three sets of gradient coils one for each direction/axes, through which large electrical currents are applied repeatedly in a carefully controlled pulse sequence
What is the basis of MRI scanning?
Placed in a magnetic field which affects the water atoms in the brain
Excites hydrogen nuclei in water atoms with radio wave
Receive radio wave responses from atoms and dump data into a matrix (K-space)
Perform mathematical operation to convert the raw data into an image
What is nuclear spin?
Nuclei with unpaired neutrons and/or protons have nuclear spin
The nuclear spin state is the orientation of the spin-generated magnetic field relative to an external magnetic field
Spin combines with nuclear charge to produce a magnetic moment
Describe the basic principle of MR imaging
For imaging purposes the hydrogen nucleus (a single proton) is used because of its abundance in water and fat
The hydrogen proton can be likened to the planet earth, spinning on its axis, with a north-south pole. In this respect it behaves like a small bar magnet. Under normal circumstances, these hydrogen proton “bar magnets” spin in the body with their axes randomly aligned
When the body is placed in a strong magnetic field, such as an MRI scanner, the protons’ axes all line up. This uniform alignment creates a magnetic vector oriented along the axis of the MRI scanner
When additional energy (in the form of a radio wave) is added to the magnetic field, the magnetic vector is deflected. The radio wave frequency (RF) that causes the hydrogen nuclei to resonate is dependent on the element sought (hydrogen in this case) and the strength of the magnetic field
The strength of the magnetic field can be altered electronically from head to toe using a series of gradient electric coils, and, by altering the local magnetic field by these small increments, different slices of the body will resonate as different frequencies are applied
When the radiofrequency source is switched off the magnetic vector returns to its resting state, and this causes a signal (also a radio wave) to be emitted. It is this signal which is used to create the MR images.
Receiver coils are used around the body part in question to act as aerials to improve the detection of the emitted signal. The intensity of the received signal is then plotted on a grey scale and cross sectional images are built up
What is a magnetic moment?
The measure of the object’s tendency to align with a magnetic field
What are spin up and spin down states?
Protons align in two orientations separated in energy by an amount proportional to the magnetic field and in relative numbers determined by Boltzmann statistics
In the spin-up state, the detected component of nuclear angular momentum is in the same direction as the external magnetic field. In the spin-down state, it is in the opposite direction
