Magnetic Resonance Imaging By Dr. Amr A. Abd-Elghany

Why MRI? Which one is Brain MRI? Which one is Brain CT?

CT axial only then reconstruct the slices MRI real images not reconstructed

History of MRI Raymond Vahan Damadian (born March 16, 1936, NY USA) is an Armenian-American medical practitioner and inventor of the first MR (Magnetic Resonance) Scanning Machine. Damadian was the first to perform a full body scan of a human being in 1977 to diagnose cancer. Damadian discovered that tumors and normal tissue can be distinguished in vivo by nuclear magnetic resonance (NMR) because of their relaxation times. Damadian invented an apparatus and method to use NMR safely and accurately to scan the human body, a method now well known as magnetic resonance imaging (MRI). Damadian has received several prizes

What is MRI? MRI Scan is a radiology technique that uses magnetism, radio waves & computer to produce image of body structures. MRI scan uses radiofrequency waves & strong magnetic field rather than x-rays to provide remarkably clear & detailed pictures of internal organs & tissues. MRI test done with a large machine that uses a magnetic field & pulses of radio wave energy to make pictures of organs & structures inside the body. In many cases MRI gives information that cannot be seen as well as with an x-ray, ultrasound or CT scan.

Basic principle of MRI

Principle Images are created by the interaction between the external magnet in the machine and the internal magnet in the human body. There are 3 physical principles concerning MRI: 1. The moving charged body is surrounded by magnetic field directly proportional with the size of the body. 2. The proton is largely greater in size and number (80% body weight is water) than the electron. 3. The small magnets will take the direction of the larger magnets. Radio waves generators emit radio waves having the same frequency as the frequency of the protons. The hydrogen protons will be moved.

The protons will be returned back with different speeds loosing the energy gained by radio waves giving a signal. According to the time window (T1 short, T2 long) of collecting the received signal the different parts will appear. Adequate H+ mobile ---- signal ---- image. Watery spaces ---black----- T1 weighted image. Low signal ----- black. High signal ----white. T1 T2 Low Low Low High High Low High High

Protons after RF (resonance) Proton alignment with magnetic field Protons after RF (resonance)

T1 & T2 weighted image T1 T2

Brain MRI T1 weighted image T2 weighted image

For T1 low and T2 low ----non movable protons. E.g. Cortical bone, Mature fibrous tissues (ligaments, tendons). Calcifications (physiologically, pathologically). For T1 low and T2 high ----- Fluids. E.g. water, CSF, urine, vitreous eye, …….. T1 high and T2 low ----- Fat (subcutaneous, bone marrow,..). T1 high and T2 high ---- blood (hemorrhage).

Cortical bone ---------------------------------- Ligament CSF white----T2

The resultant image depends on Motion: non mobile hydrogen-----no signal. Amount: minimal hydrogen (air) ----no signal. H+

Blood Protons of the blood are faster than the other types so that T1& T2 can not be discriminated. Flash technique is used to see the blood. No contrast media is used (advantage than CT especially for patients who have renal failure, allergy for iodine compounds). MR angiography (MRA)

MRI brain hemorrhage MRI cholangiography

MRI Urography No contrast media.

Types of MRI Machines Closed MRI

2. Open MRI Disadvantages of open MRI The magnet is weak so the scanning time is relatively long but the resolution of the images the same as closed one.

3. Extremity MRI advantages of Extremity MRI You can investigate the hand or leg (only small parts wrist, elbow, ankle, joints) to reduce the load on the big device.

4. Dynamic MRI The bed rotates from upright to recumbent (lie down position), stopping at any angle in between.

Patient with low back pain after surgery Does a lie down only scanner see the patient problem? No EX: Spinal instability on standing position

The receiver and transmitter coils The surface coils have 2 conditions: 1. it has to surround the part of interest. 2. this part should not be moved during the imaging process even due to breathing. Breast surface coil Head coil

Chest surface coil Knee surface coil

Advantages of MRI No radiation hazards (EM waves). Superior soft tissue contrast. Multi-planner imaging capability (we can take images for the patients at any position lie down, upright, oblique). Absence of artifact from adjacent bones as CT (no lines appears as CT). Tissue characterization based on signal.

Contraindications of MRI Magnetic field strength=120 times gravity Cardiac pacemaker. Metal objects: Within the eye. Near the spinal cord. Cochlear implants. Insulin pumps. Neurostimulators.

MRI magnet room is extremely powerful MRI magnet room is extremely powerful. The magnetic will bring all magnetic metal toward it, sometimes at high speed. Leave any metal object outside of the building. Check with MRI Operator if access is needed in the room with the magnet.

Safety Zones Zone 1 : This is a public access area with no restrictions. Zone 2 : This is a semi restricted area where patients and hospital staff can interact. Zone 3 : This area is completely physically restricted from non MR personnel especially the general public.