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Introduction to MRI Magnetic Resonance Imaging

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1 Introduction to MRI Magnetic Resonance Imaging
The magnet is a huge super-cooled one The absorption and release of electromagnetic energy by hydrogen protons while in the magnetic field is the ‘resonance” The computer receives the signal from the spinning protons as mathematical data; the data is converted into a picture through the Fournier transform mathematical formula. That’s the "imaging" part of MRI. Nuclear magnetic resonance (NMR) is a physical phenomenon which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation.

2 Bore 1.5-3.0 Tesla Super conductive Magnet, RF coils
Main components of a MRI machine The tube is referred “the bore” bathed in liquid helium. Pictured is the new 3T GE Discovery unit recently installed at the PRH Main magnet The bore contains the superconductive magnet bathed in liquid helium(270⁰ C) creating magnetic fields of 0.5 to 3.0 tesla. One tesla = 10,000 gauss: the earth’s magnetic field = 0.5 gauss. The cold is insulated by a vacuum. Main Radiofrequency Coil which generates RF signals. With the Magnet an electromagnetic field is created Gradient Coils There are three smaller magnets within a MRI machine called gradient magnets. These magnets are much smaller that the primary magnet (about 1/1000 as strong), but they allow the magnetic field to be altered very precisely. It is these gradient magnets that allow image "slices" of the body to be created. By altering the gradient magnets, the magnetic field can be specifically focused on a selected part of the body. Surface Coils are receiver coils or antennae for RF energy signals generated by the TR TE and flip motion of the H protons

3 Surface Coils are receiver coils or antennae for RF energy signals generated by the TR TE and flip motion of the H protons

4 Understanding and Reading MRIs
Like X ray, MRI is based on a discovery in the physic lab: when the nuclei of hydrogen atoms--single protons, all spinning randomly--are caught suddenly in a strong magnetic field, they tend to line up like so many compass needles.  If the protons are then hit with a short, precisely tuned burst of radio waves, they will momentarily flip around.  Then, in the process of returning to their original orientation, they resound with a brief radio signal of their own that announces the presence of a specific tissue.  The intensity of this emission reflects the number of protons in a particular "slice" of matter

5 How is this MRI Image produce?

6 When patients slide into an MRI machine, they take with them the billions of atoms that make up the human body. For the purposes of an MRI scan, we're only concerned with the hydrogen atom, which is abundant since the body is mostly made up of water and fat. These atoms are randomly spinning, or precessing, on their axis, like a child's top. All of the atoms are going in various directions, but when placed in a magnetic field, the atoms line up in the direction of the field.

7 When a strong magnetic field is applied*.
These hydrogen atoms have a strong magnetic moment, which means that in a magnetic field, they line up in the direction of the field. Since the magnetic field runs straight down the center of the machine, the hydrogen protons line up so that they're pointing to either the patient's feet or the head. About half go each way, so that the vast majority of the protons cancel each other out -- that is, for each atom lined up toward the feet, one is lined up toward the head. Only a couple of protons out of every million aren't canceled out. This doesn't sound like much, but the sheer number of hydrogen atoms* in the body is enough to create extremely detailed images. It's these unmatched atoms that we're concerned with now. *Measured in teslas on the order of 0.5 to 3.0 (the earth’s magnetic field strength is 10 thousand time less than this (T 1.5 tesla or gauss vs Earth 0.5 gauss) **There are 4.7 x 10 to the 27th power hydrogen protons in the human body, more than the est. stars in the Universe. One in a million means 10 to the 21th power protons do not align and are subjects of the RF

8 the MRI machine applies a radio frequency (RF) pulse that is specific only to hydrogen. The system directs the pulse toward the area of the body we want to examine. When the pulse is applied, the unmatched protons absorb the energy and spin again in a different direction. This is the "resonance" part of MRI. The RF pulse forces them to spin at a particular frequency, in a particular direction. The specific frequency of resonance is called the Larmour frequency and is calculated based on the particular tissue being imaged and the strength of the main magnetic field.

9 It is during the time between the radiofrequency turned off to return to normal position that energy signals are generated which can be detected and transferred to a computer.

10 Numbers By a series of calculations using the Fourier transform equations, the individual radio frequencey amplitutes are given a number (digitalized).

11 Transforming Radio Frequencies into Images?
Let’s step back, and look at a device that is familiar to all of us…the Radio

12 The RF used in MRI are the same range as in common radio communication
The RF used in MRI are the same range as in common radio communication. These are non ionizing radiation unlike used in conventional x-ray and CT scans. How the common radio works can take some of the mystery out of how MRI images are generated.

13 Encoded RF “1060 on your dial” RF decoded
When you turn on a radio you hear sounds because the transmitter at the radio station has converted the sound waves into electromagnetic waves, which are then encoded onto an electromagnetic wave in the radio frequency range. A microphone converts the sound wave into an electrical signal. The electrical wave is used to encode or modulate a high-frequency "carrier" radio wave. The carrier wave itself does not include any of the sound information until it has been modulated The signal is transmitted by a radio broadcast tower. Your radio contains an antenna to detect the transmitted signal, a tuner to pick out the desired frequency, a demodulator to extract the original sound wave from the transmitted signal, and an amplifier which sends the signal to the speakers. The speakers convert the electrical signal into physical vibrations (sound).

14 X 1021 / 5mm slice of tissue MRI proton RF energy is encoded into black and white pixels instead of “notes”. In the K space, which is like a sounder mixer, the many individual RFs are layered, assigned a number(digitalized), and converted by the Fournier transform into visual shades of grey to produce a composite image.

15 Setting up the Machine Some practical information. For each study the technical has to set the radio frequencies, what gradient coils to use and position the patient in the bore tube square to the main magnet and RF coil, inject contrast agents, and provide for patient safety. The patient needs to remain still for the minute examination. The loud banging noise, is the result of the vibration of gradient coils in the machine caused by rapid pulses of electricity. Patient comfort can be improved with ear plugs or ear microphone music. Claustrophobia can be dampened by preemptive anti-anxiety medication. Pictured is the original “Open” MRI scanner during Igor’s first OJT session.

16 Surface coil Brain study. Notice the “surface coil” which acts as an antenna to pick up the RF signal and fine tune the resultant data sent to the computer “mixer”

17 The value of those numbers (brightness of the dots) dictate how bright the wave that that point represents should be plotted on the MR image. Now were getting down to the point: an MR image is simply the superposition of thousands of these waves, all plotted on the same image, one o top of the another. When you add them all together, you get the image (like the shoulder in this example). It sounds improbable, but its true. Perhaps MRI should stand for Magical Remarkable Images.

18 Reading the Images We will concentrate on 3 series of MRIs: T1, T2, T2 fat suppressed and Proton Density as seen in all three cardinal planes

19 The emitted energy of the protons once the RF is stopped is affected by in which tissue (the “lattice”) it resides: fat, muscle, ligament, bone, brain, etc.

20 RF Pulse sequence parameters which the technician adjusts
The three pulse sequence parameters are Repetition time (TR) measured in msec Echo time (TE) measured in msec Flip angle measured in degrees The flip angle a is used to define the angle of excitation for a field echo pulse sequence. It is the angle to which the net magnetization is rotated or tipped relative to the main magnetic field direction via the application of a RF excitation pulse at the Larmor frequency. It is also referred to as the tip angle, nutation angle or angle of nutation. The radio frequency power (which is proportional to the square of the amplitude) of the pulse is proportional to a through which the spins are tilted under its influence. Flip angles between 0° and 90° are typically used in gradient echo sequences, 90° and a series of 180° pulses in spin echo sequences and an initial 180° pulse followed by a 90° and a 180° pulse in inversion recovery sequences.



23 T1 and T2 relaxation times
Occur simultaneously and independently of each other and form the basis of tissue contrast in MR-reconstructed images

24 T1 Low TR ( msec) Low TE (20-40 msec) T2 High TR (2,000-3,000 msec) High TE (40-70 msec) Proton density High TE (2,000-3,000msec)

25 Basic Sequences T1 to view anatomy
T2 to detect a pathologic process (edema, swelling) Proton Density (PD) great for ligamentous anatomy

26 Basic Pulse Sequences for MRI Imaging
Image type Fat Water Advantage T1 Bright Dark Anatomical detail T2 Intermediate ++ edema Fat Suppressed T2 Very Dark Very Bright ++++ edema Students are given this hand out which is also posted on the CHA website Modified from: Khanna AJ, Cosgarea AJ, Mont MA, Andres BM, Domb BG, Evans PJ, Bluemke DA, Frassica FJ. Magnetic resonance imaging of the knee. Current techniques and spectrum of disease. J Bone Joint Surg Am. 2001;83 Suppl 2(Pt 2):129. In STIR sequences, the echo time is varied to make the water appear bright (i.e., a fluid-sensitive sequence).

27 Confusing terminology:
The term Proton Density is actually an inaccurate term, as it implies that the only contrast mechanism of the image is based on differences in proton density. In fact, contrast is predominantly derived from intermediate weighting between T1 and T2.  Most, so called PD sequences have TRs that are too short to completely eliminate T1 contrast and TEs that are too long to completely eliminate T2 contrast. There are a multitude of other sequences STIR, Flair with more on the way as newer MRIs have increased magnetic flux density(tesla) from stronger magnets in the units.

28 Identify the joint, left or right, the plane and sequence
Axial T1 Sagittal T1 Coronal T2 fat suppressed. Could be a PD fat suppressed

29 Fat Suppression A very valuable tool to define whether a structure is composed of water (bright) such as blood or fat (bone marrow) Since the hydrogen nuclei of fat and water resonate at slightly different frequencies, it is possible to excite just the fat with a special RF pulse, and then destroy (“spoil”) the resulting signal with a gradient pulse. This loss of fat signal is referred to as “saturation” or fat suppression

30 TI Fat T1 WEIGHTED - an image created typically by using short TE and TR times (TR msec/TE 20=40 msec) whose contrast and brightness are predominately determined by T1 signals. Water

31 T2 Fat Water Fat Water Intermediate Bright
T2 WEIGHTED - an image created typically by using longer TE and TR times(T2: TR /TE msec) whose contrast and brightness are predominately determined by T2 signals. TAU (t) - the interpulse times (time between the 90° and 180° pulse, and between the 180° pulse and the echo) used in a spin echo pulse sequence. TE (Echo Time) - represents the time in milliseconds between the application of the 90° pulse and the peak of the echo signal in Spin Echo and Inversion Recovery pulse sequences Water

32 Fat T2 Fat Suppressed Water
FAT SATURATION (FAT-SAT) - A specialized technique that selectively saturates fat protons prior to acquiring data as in standard sequences, so that they produce negligible signal. The pre-saturation pulse is applied prior to each slice selection. This technique requires a very homogeneous magnetic field and very precise frequency calibration. See also Fat Suppression In medicine, body water is all of the water content of the human body. A significant fraction of the human body is water. Lean muscle tissue contains about 75% water. Blood contains 95% water, body fat contains 14% water and bone has 22% water.The human body is about 60% water in adult males and 55% in adult females. In diseased states where body water is affected, the compartment or compartments that have changed can give clues to the nature of the problem Water

33 Water FAT Proton Density Fat Suppressed (PD-S)
PROTON DENSITY WEIGHTED IMAGE - an image produced by controlling the selection of scan parameters to minimize the effects of T1 and T2, resulting in an image dependent primarily on the density of protons in the imaging volume. Long TR/short TE. Useful for musculoskeletal (MSK) studies Confusing terminology: The term Proton Density is actually an inaccurate term, as it implies that the only contrast mechanism of the image is based on differences in proton density. In fact, contrast is predominantly derived from intermediate weighting between T1 and T2.  Most, so called PD sequences have TRs that are too short to completely eliminate T1 contrast and TEs that are too long to completely eliminate T2 contrast.  Intermediate-weighted is a more accurate term, as is well depicted by the bladder in this image. The PD sequence is also often the 1st echo of a dual-echo T2 sequence. There are a multitude of other sequences STIR, Flair with more on the way as newer MRIs have increase magnetic flux density(tesla) due to stronger magnets in the units. Water

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