Magnetic resonance imaging (MRI) uses strong magnetic fields and radio waves to produce detailed images of the inside of the body without using ionizing radiation. Dr. Damadian first proposed using MRI to externally scan the human body for signs of malignancy in 1969. An MRI machine contains a magnet to align hydrogen atoms, gradient coils to spatially encode signals, and radio frequency coils to transmit and receive signals used to form images. A computer system controls the imaging process and reconstructs images from the MRI signals. Contrast agents containing gadolinium may be used to improve visibility of structures.

1. MAGNETIC RESONANCE
IMAGING
(MRI)
JASERAH SYED
MSc I

2. INTRODUCTION
•MRI is a spectroscopic imaging technique used to produce human body images.
•Also known as NUCLEAR MAGNETIC RESONANCE IMAGING (NMRI) or MAGNETIC RESONANCE
TOMOGRAPHY (MRT)
•MRI does not use ionizing radiation but rather strong magnetic fields and radio frequencies (range
3-300GHz)
•Magnetic field of about 1.5-3T (tesla) applied i.e 50,000 times more strong than earths magnetic
field. (0.00003T)

3. RAYMOND DAMADIAN
Dr. Damadian was the first
to describe the concept of
whole-body NMR
scanning, as well as
discovering the tissue
relaxation differences that
made this feasible. In
1969, he first proposed
the idea of using nuclear
magnetic resonance
technology to scan the
human body externally for
early signs of malignancy.
In 1974, he received the
first patent in the field of
MRI.

4. PRINCIPLE
• Magnetic resonance imaging (MRI) makes use of the magnetic properties of certain atomic nuclei.
• The human body is made up of fats and 90% water, hence it consists of 63% hydrogen atoms.
• The hydrogen nuclei behave like compass needles that are partially aligned by a strong magnetic
field in the scanner. When placed in a large magnetic field, hydrogen atoms have a strong tendency
to align in the direction of the magnetic field. Inside the bore of the scanner, the magnetic field runs
down the center of the tube in which the patient is placed, so the hydrogen protons will line up in
either the direction of the feet or the head. The majority will cancel each other, but the net number
of protons is sufficient to produce an image.
• ENERGY ABSORPTION :
• The MRI machine applies radio frequency (RF) pulse that is specific to hydrogen. n The RF pulses are
applied through a coil that is specific to the part of the body being scanned.

5. • RESONANCE :
• The gradient magnets are rapidly turned on and off which alters the main magnetic field. n The
pulse directed to a specific area of the body causes the protons to absorb energy and spin in
different direction, which is known as resonance.
• When the RF pulse is turned off the hydrogen protons slowly return to their natural alignment
within the magnetic field and release their excess stored energy. This is known as relaxation.
• MEASURING THE MR SIGNAL :
• the moving proton vector induces a signal in the RF antenna n The signal is picked up by a coil
and sent to the computer system. the received signal is sinusoidal in nature n The computer
receives mathematical data, which is converted through the use of a Fourier transform into an
image.
•

6. COMPONENTS OF MRI
a) A magnet which produces a very powerful uniform magnetic field.
b) Gradient Magnets which are much lower in strength.
c) Equipment to transmit radio frequency (RF).
d) A very powerful computer system, which translates the signals transmitted by the coils.

7. INSTRUMENTATION OF MRI

8. THE MAGNET
• The most important component of the MRI scanner is the magnet.
• They are superconducting magnets.
• When hydrogen atoms are subjected to the magnetic field of 1.5T-3T they align themselves either
in parallel or anti parallel.
• Most of the hydrogen atoms align parallel (low energy) and few align anti parallel (high energy)
and the magnetic moment is in the direction of primary magnetic field.

9. ALIGNMENT OF PROTONS
IN MAGNETIC FIELD

10. PRECESSION Larmor frequency (f) =
gB/2π

11. THE GRADIENT COILS
• They generate secondary magnetic fields.
• Located within the wall of primary magnet.
• They are positioned in opposite to each other to create positive or negative pulse.
a) Z gradient – runs along the long axis to produce axial images.
b) Y gradient – runs along the vertical gradient to produce coronal images.
c) X gradient – runs along the horizonatal axis to produce sagittal images.

12. GRADIENT COILS

13. RADIO FREQUENCY COILS (RF)
• The RF coils are located within the magnet assembly and relatively close to the patient’s
body. These coils function as the antennae for both transmitting signals to and receiving
signals from the tissue. There are different coil designs for different anatomical regions. The
three basic types are body, head, and surface coils.
Transmitter :
•The RF transmitter generates the RF energy, which is applied to the coils and then
transmitted to the patient’s body. The energy is generated as a series of discrete RF pulses.
•The transmitters must be capable of producing relatively high power outputs on the order of
several thousands.
•Therefore, a 1.5 T system might require about nine times more RF power applied to the
patient than a 0.5 T system. One important component of the transmitter is a power
monitoring circuit. That is a safety feature to prevent excessive power being applied to the
patient’s body.

14.  Receiver :
A short time after a sequence of RF pulses is transmitted to the patient’s body, the
resonating tissue will respond by returning an RF signal. These signals are picked up
by the coils and processed by the receiver. The signals are converted into a digital
form and transferred to the computer where they are temporarily stored.

15. RF COILS
 Surface coils are used to
receive signals from a
relatively small
anatomical region to
produce better image
quality than is possible
with the body and head
coils.
 Surface coils can be in
the form of single coils
or an array of several
coils, each with its own
receiver circuit operated
in a phased array
configuration.
 This configuration
produces the high
image quality obtained
from small coils but
with the added
advantage of covering a
larger anatomical region
and faster imaging.

16. COMPUTER SYSTEM
• A digital computer is an integral part of an MRI system. The production and display of an
MR image is a sequence of several specific steps that are controlled and performed by the
computer.
•Acquisition Control
•The first step is the acquisition of the RF signals from the patient’s body. This acquisition
process consists of many repetitions of an imaging cycle. During each cycle a sequence of RF
pulses is transmitted to the body, the gradients are activated, and RF signals are collected.
Unfortunately, one imaging cycle does not produce enough signal data to create an image.
Therefore, the imaging cycle must be repeated many times to form an image. The time
required to acquire images is determined by the duration of the imaging cycle or cycle
repetition time—an adjustable factor known as TR—and the number of cycles. The number of
cycles used is related to image quality. More cycles generally produce better images.

17. •Image Reconstruction :-
•The RF signal data collected during the acquisition phase is not in the form of an image.
However, the computer can use the collected data to create or “reconstruct” an image.
•This is a mathematical process known as a Fourier transformation that is relatively fast and
usually does not have a significant effect on total imaging time.
•Image Storage and Retrieval :-
•The reconstructed images are stored in the computer where they are available for additional
processing and viewing. The number of images that can be stored—and available for
immediate display—depends on the capacity of the storage media.
•Viewing Control and Post Processing :-
•The computer is the system component that controls the display of the images. It makes it
possible for the user to select specific images and control viewing factors such as windowing
(contrast) and zooming (magnification).
•In many applications it is desirable to process the reconstructed images to change their
characteristics, to reformat an image or set of images, or to change the display of images to
produce specific views of anatomical regions.
•These post-processing (after reconstruction) functions are performed by a computer. In some
MRI systems some of the post processing is performed on a work-station computer that is in
addition to the computer contained in the MRI system

18. IMAGE PRODUCTION

20. GADOLINIUM CONTRAST MEDIUM
(MRI CONTRAST AGENTS)
• Gadolinium contrast medium is used in about 1 in 3 of MRI scans to improve the
clarity of the images or pictures of your body’s internal structures. This improves the
diagnostic accuracy of the MRI scan. For example, it improves the visibility of
inflammation, tumours, blood vessels and, for some organs, blood supply.
• OTHER DYES :-
•IRON OXIDE : Superparamagnetic - Two types of iron oxide contrast agents exist:
superparamagnetic iron oxide (SPIO) and ultrasmall superparamagnetic iron oxide
(USPIO). These contrast agents consist of suspended colloids of iron oxide
nanoparticles and when injected during imaging reduce the T2 signals of absorbing
tissues. SPIO and USPIO contrast agents have been used successfully in some
instances for liver tumor enhancement.
•IRON PLATINUM (SUPERPARAMAGNETIC)
•MANGANESE

21. GANDOLIUM IMAGES
If the blood ocular barrier
is disrupted the
gandolinium leaks into the
brain showing bright
images.
Eyes yield information
about strokes: MRI
scans revealed that a
chemical called
gadolinium, used to
improve images, leaked
into the eyes of stroke
patients.

22. REFRENCES
1. http://www.sprawls.org/mripmt/MRI02/index.html
2. https://en.wikipedia.org/wiki/Magnetic_resonance_imaging
3. https://www.youtube.com/watch?v=Ok9ILIYzmaY
4. https://www.youtube.com/watch?v=g6FpkrERaPY
5. https://www.youtube.com/watch?v=djAxjtN_7VE&t=1248s
6. http://mriquestions.com/gradient-coils.html