What are the components of an MRI?
The MRI bore is a long, enclosed tube that the patient enters for the scan
The large cylindrical portion of the scanner contains the powerful magnet
The MRI magnet runs the length of the bore, providing a strong magnetic field around the patient inside the bore
The magnetic field is produced by passing current through the multiple coils resulting in a state of superconductivity
The magnetic field aligns the protons in the body’s tissues (which normally spin at random) to the magnetic field
Gradient coils create a secondary magnetic field and ensure uniform distribution of the main magnetic field and by varying the current deliberately distorts the main magnetic field
Information is gathered in the x, y, and z directions thus providing spatial information based on the changes within the magnetic field strength across the direction
Different areas of the body can be imaged by increasing or decreasing in the current of the gradient coils, which adjusts the main magnetic field
Radio Frequency coils or antennas of the MRI system transmit energy and detect the return signal of energy from the body
The RF pulse causes the magnetization of the protons to “flip” away from the main magnetic field (needs to match the resonance of the tissue)
The computer system controls the machine timing, power of the gradients, RF pulses, and specific pulse sequences are executed through the computer
What liquid cools the MRI?
Liquid helium (most common) or nitrogen
What is the tube of the MRI called?
Bore
How long is a typical MRI scan?
30-45 mins
Does the MRI create a strong magnetic field around the patient?
Yes
Signals are received by a computer and converted into an image of the part of the body being examined
Is the image quality determined by the signal to noise ratio?
Yes
SNR Determines graininess/clarity of the image
Signal – area of interest
Noise – air surrounding the area
Is there loud clicking that takes place during an MRI?
Yes
Is all metal a concern with MRIs?
Yes because of the strong magnetic field
The loud tapping noise you hear during MRI scanning is created by the turning on and off the magnet coils
Electric current flows through the coils
An oscillating magnetic field is generated
Excited hydrogen atoms release a radio frequency that is measured by a receiving coil
Can you feel the magnetic pull of the MRI?
No
What is the aperture?
Donut hole in the CT scan
Pt does inside the bore in the MRI
Does the magnet run the length of the patients body?
Yes
Do the gradient coils provide a secondary magnetic field?
Yes, what contributes to the generation of a 3D image
What is the earth’s magnetic field compared to the human body and an MRI?
Earth’s magnetic field = 0.5 Gauss
Body parts = .000001 Gauss/cc H2O
MRI magnetic field = 1.5 Tesla = 15K Gauss, 3.0 Tesla = 30K Gauss
Are superconducting magnets used to produce high-quality images?
Yes
Consists of many coils, windings of wire
Electricity passes through the magnet creating a magnetic field
Requires a lot of energy to create high-quality images, creates heat
Requires an insulated vacuum and liquid coolant to reduce hear and resistance
Superconductivity state is maintained without electrical resistance
Is most of the human body made up of water molecules?
Yes
Hydrogen and oxygen atoms are abundant in all body tissue
Protons, electrons and neutrons are in each atom
Center of each atom is the proton (tiny magnet)
Protons are constantly spinning at their own natural speed or resonant frequency
Protons move in a natural state of equilibrium - don’t all move in the same direction or in phase
Differing and unique resonance for different tissues
Do strong magnetic fields align protons in the body?
Yes, with the magnetic field’s direction
Motion of the protons at their resonance frequency in alignment with the direction of the magnetic field but spinning out of phase (not all spinning the same - what RF pulse does)
What does the radio frequency pulse do?
RF pulses tuned to the exact frequency of the protons
Energy is absorbed; exciting the protons
Radio frequency pulses are absorbed by the protons
Knocks the protons out of alignment with the magnetic field’s direction
All protons are spinning in phase together (all spinning in the same phase)
Excited protons begin to relax or recover
Realign with the magnetic field or “relax” - back to their resonant frequency
Emit stored energy as a radio signal - protons emit the stored energy as a radio signal at their resonance frequency; this emitted radio signal is detected by the scanner to create an image
How are images created?
A small amount of energy is released as the protons realign with the magnetic field
The rate of excited protons returning to equilibrium is measured - as they return to their resonant frequency (the energy they are sluffing off is measured)
The number of protons and the exact location is converted into a gray scale and provides contrast between tissues
Is the time it takes for the protons to realign with the magnetic field and the amount of energy released predictable?
Yes
Various tissues realign at different speeds and produce distinct signals
The same way millions of pixels on a computer screen can create complex pictures, millions of protons combine to create a detailed image
The key is how quickly the protons release the energy
What is the gray scale in an MRI?
Described in terms of intensity
High signal intensity = white
Intermediate signal intensity = gray
Low signal intensity = black
What are the terms used for comparison of tissues?
Hyperintense - brighter than
Isointense - similar, gray
Hypointense - darker than
What is repetition time and echo time?
Determine how the image looks
TR is the amount of time between pulse sequences applied to the same slice - controls how much recovery time is allowed for tissues before the next RF pulse (determines which tissues show up)
TE is the time between the delivery of the RF pulse and the receipt of the echo signal (controls when the measurements are collected)
Can the MRI technologist create weighted images by adjusting the timing controls?
Yes
T1 weighted - images are formed early on after the RF is stopped, tissues that have a shorter relaxation time will be more visible in the T1 image; better for anatomy and structure viewing
T2 weighted - images are formed after waiting a while in the relaxation proton phase, tissues that have a longer relaxation time will be more visible in the T2 image; better for fluid and pathology visualization
*creating contrast in the image - different from contrast agent
What is the coloring for T1 weighting?
Fluid = low signal intensity (black)
Gray matter - intermediate signal intensity (gray)
White matter - hyperintense compared to gray matter (lighter)
