MRI | magnetic resonance imaging

In the modern diagnosis methods the MRI ( Magnetic Resonance Imaging ) is a key component. Specially for neurology and cancer treatment.

Although computed tomography also does the same but there is no risk of x-ray radiation and soft tissues are more enhanced in captured images.

Here will try to cover all the answers not a diagnosis but construction, installation, working, how to generate images, artifacts, and much more.

How does MRI work simple explanation

In general terms, MRI machine is more like a CT Scan machine. Which is used for imaging internal organs of the body.

Firstly it looks like CT scanner but in reality far different from it.

Despite CT Scan uses X-Ray for imaging internal organs of the body. which is ionic harmful radiation if doses are never controlled.

But MRI principle is based on NMR (nuclear magnetic resonance ) using FID (frequency induction decay) of the nucleus of hydrogen atoms.

When the body is put in a strong magnetic field its nuclei align with a magnetic field. As provide external alternating rf energy the nuclei deviate from their alignment.

As soon as external energy stop return to the original align state and radiate extra absorb energy in the form of electromagnetic radiation.

Receiving this radiation through coils and locate the location of nuclei make it possible to produce images.

The main component of machine is a magnet and classified according to the type of magnet.

Like Open magnet, close or bore magnet, permanent magnet, resistive or electromagnet, superconductive magnet.

Mostly Closed Bore magnets ( cylindrical ) are used instead of open bore magnet because of the high magnetic field achieved through cylindrical size superconducting magnet.

The field strength of the superconducting magnet varies from 1 Tesla and higher.

Open bore magnet are C – shaped or Horseshoe size permanent magnet or resistive magnet or superconductive magnet.

Although open bore magnet design reduces claustrophobia but due to less field strength ( 0.2 -1.2 Tesla) less preferable now days.

claustrophobia is a state of mind in a person who feels phobia or uncomfortable when entering tight, close or crowded spaces.

Permanent Magnet

Common magnet neodymium is magnetized in such a way it never demagnetizes. It is an ally of the alloy of iron, neodymium, and boron.

Also, have a very strong magnetic field (0.2 T to 0.4 T ). Consider the strongest permanent magnet for magnetic resonance imaging.

Magnetic Resonance Imaging permanent magnets remain in a C-shaped or horseshoe configuration.

Advantages:

1) Less operating cost: Because of no cryogenics for cooling or resistive electromagnets, therefore, less power is required.

Thus operation cost is lower and it can be turn off when not in use.

2) Price: Initial buying and setup price for the permanent magnet is comparatively less.

3) Small fringe field: This means less space and less shielding required for a magnet. Due to the low field, another equipment setup can be placed closer to the magnet.

4) Open bore: It reduces the claustrophobia and less physical footprints of the magnet. Wide bore allows to access patients in many ways.

5) Minimize tissue energy deposition: Due to lower field strength, the amount of absorbed energy (SAR) in the tissue from radio pulses is less.

SAR is a specific absorption rate that is proportional to the square of magnetic field strength.

6) Fewer artifacts: Flow/motion artifacts are less. Metal artifact induced signal distortions and signal losses are less.

Disadvantage :

1) Less homogeneous: The field strength homogeneity of a permanent magnet is not good. It comes into effect when a large FOV scan needs to be done.

2) Less contrast detection: Due to low magnetic field strength gadolinium contrast is less apparent than high field strength magnets therefore required high doses of contrast.

3) Impaired detection of calcification and hemorrhage: Due to less field strength have less ability to find-out iron accumulation and calcification.

4) Quench not possible: Once a permanent magnet is made it can not be demagnetized like electromagnets and superconductive magnets.

Resistive or Electromagnet

An electromagnet is similar to a scrapyard electromagnet which is made from turning conducting wire on an iron core and pass current through it.

There are two kinds of electromagnet exists air core and iron core and field strength can be up to 0.3 Tesla.

When current passes through winding produce heat that needs to be removed by a suitable cooling method.

Advantages:

The advantages of Electromagnet are similar to permanent magnet except for few ones.

Lightweight: Due to using electromagnet the magnet weight reduced.

Disadvantage :

The permanent magnet and resistive magnet disadvantage vary on the following points.

1) High power consumption: Electromagnet uses current to create a magnetic field therefore it consumes more power than a permanent magnet.

2) Cooling required: Similar to as above electromagnets produce heat due to resistive conductor winding required cooling for temperature control.

Superconducting Magnets

Nowadays superconducting MRI machine is very common and popular among diagnostic centers and hospitals.

It uses the same cryogenic technology to make superconducting magnets. Also used in Quantum computers, LHC ( Large Hadron Collider ) like projects.

The superconductive magnet is similar to a resistive magnet only difference is that the coil is submerged inside the liquid helium to reduce conductors’ electric resistance.

When conductors temperature reaches around 4K become superconductive. The field strength range varies from 1 Tesla and higher.

Advantages:

1) Less scan time : High field strength reduces the scan time.

2) High field homogeneity: High field homogeneity provides a large FOV area, chemical shift fat suppression, echo-planar imaging, and MR spectroscopy.

3) High SNR: Signal to noise ratio is higher which improves the image quality of the images.

4) Detection of calcification and hemorrhage: high field strength ability to detect focal areas of calcification, iron accumulation, and hemorrhage inside the tissue.

5) Less contrast dose: Due to high field strength less contrast is required for enhancement.

Disadvantage :

1) Higher setup cost: Initial installation and setup cost is higher. Compare to permanent magnet price is more than double.

2) Higher running cost: Cryogenic system runs continuously witch increases the running cost. Also, need to top up helium on regular basis.

Now low boil-off machine models available need to top up in two or more years.

3) Large fringe field required: Because of higher magnetic strength and cryogenics required bigger area for setup.

Superconducting MRI Magnet Design

To achieve higher magnetic strength ( 1 T – 12 T or higher) at low power consumption ( comparatively ) superconducting magnets are very popular in the scientific and healthcare community.

Understanding superconducting magnets first needs to know about few things which are used to create.

What is magnetic susceptibility

The measurement of magnetization of any material when it is kept in an external magnetic field.

It is denoted by X= J / B , where X (chi) is susceptibility, J = polarization and B = applied external field strength.

When matter kept in an external magnetic field it’s atom polarization ( magnetization ).

Either oppose the external field and align in opposite direction called diamagnetism or align with applied field lines known as paramagnetic or ferromagnetic matter the degree of augmentation. 

What is superconductivity

Unique physical properties of matter in which at certain temperature (critical temperature Tc) current is free to flow without any resistance and generated magnetic field due to current flow expelled outside of matter.

Practically current never flow freely for an infinite time as mentioned in theory because of two reasons.

Firstly winding is not completely made from a superconducting material. Also, joint soldiers create resistance in the path.

Secondly, MRI uses NbTi ( Niobium- Titanium ) alloy witch is type two semiconductors.

Therefore Meissner effect not able to expel magnetic flux completely and resist the current flow.

Even due to the above reasons the magnetic field decay is around 1 mT per year. The simply magnetic field of superconductive MRI never dies naturally.

Cryogenic and Refrigeration

Similar to refrigeration only difference is cryogenics maintain extremely low temperatures.

According to the U.S. National Institute of Standards and Technology define the field of cryogenics comes into the picture when the temperature is below −180 °C (93 K; −292 °F).

The gases used in cryogenics like helium,  hydrogen, neon,  nitrogen, oxygen have boiling point below 92°K (-180 °C) while refrigerant gases such as Freon and other hydrocarbons have boiling point above 92K (-180 °C).

Mostly superconducting MRI uses Nb-Ti alloy wire which has a critical temperature below 9.4°K to be a superconductor.

In the beginning, to maintain critical temperature (Tc), there were two cryostats chambers. The inner one with liquid helium and the outer chamber filled with liquid nitrogen.

Both need to refill very frequently because of high boil-off rate.

Later when technology further develops without nitrogen introduce. Only liquid helium need to top up in 2-3 year.

Now the devices coming with ZBO (zero boil-off) means no need to even top up liquid helium ideally.

The reason behind it is the development of Gifford-McMahon (GM) cryocooler. Although cold head and compressor still required service on regular basis for filter change and other service purposes.

Magnetic Resonance Imaging Gantry

Mainly scanner gantry consists of winding’s of superconducting material NbTi (niobium-titanium) merged into liquid helium cryostat structure.

Like a thermos flask, the cryostat structure consists of several layers. Most outer walls and helium chamber is made from stainless steel, because of strength and non-magnetic quality.

A vacuum chamber is made from glass-reinforce polymer or steel and a cold shield is made from thick aluminum sheets.

Inside the liquid helium chamber apart from the main coil, there are superconductive shim coils and active shielding coils for removing or minimizing the fringe field effect.

There is a set of gradient coils also but placed inside the bore but outside of the flask.

Superconductive winding wire is made from 80-90 % of NbTi and 10-20% of copper. the several strains of NbTi in parallel keep inside the copper tube and thus made a single superconductive material wire.

More than 10 T (tesla) strength scanner uses Nb3Sn (niobium-tin) alloy or MgB2 (magnesium diboride). Because the main advantage is their critical temperature (Tc) is higher around 39 °K.

What is Ramping Process in MRI

The process to achieve full magnetic strength of superconducting scanner’s main magnet. The whole process takes at least 2-3 days to be complete.

There are mainly two steps :

Firstly filled liquid helium chamber with helium at least up to 75%, so that conductor coils can be dip into the liquid helium to achieve critical temperature (Tc) of the conductor.

Secondly need to flow current into the superconductor coil to produce the magnetic field as mentioned below picture.

As shown in the picture the arm of the superconducting coil parallel with the heater called the persistence switch.

Also, a parallel connection to the arm to connect with an external current source is called a plunger.

The heater is rise the temperature just above the superconducting temperature so that arm resistance reaches resistance around 100 Ohm.

Then External source connected through plungers. Due to the magnetic coil conductor is now become a superconductor the applied current flow directly through a superconducting coil and very less through the persistence resistor arm.

External source current slowly increased within 24 to 48 hours until reach up to 500 A to 1000 A. At this level, the complete field strength is achieved.

Now as soon as the heater turns off the resistance of the persistence arm also becomes zero and the current start to flow in a closed superconducting loop.

At this time external current source is removed.

What is Shimming

In any magnet in-homogeneity present in the magnetic field due to magnetic material, construction, surrounding environment, or electrical reason.

In short, the construction of a magnet with a uniform magnetic field is not possible.

Shimming is the process to make a magnetic field homogeneous. All kinds of mri scanners required shimming.

Homogeneity means inside the scanning area of main magnetic field is uniformly distributed.

The measuring unit is parts per million (ppm) and measured by a specific diameter of spherical volume (DSV).

For any magnetic resonance imaging scanner, the homogeneous field tolerance typically must be under ±1 ppm.

Types of shimming

According to different machines like a permanent magnet, resistive magnet, and superconductive magnet, there are two types of shimming methods are used.

Active Shimming

Mainly used in the superconductive and resistive magnet of magnetic resonance imaging scanner.

Superconductive has 5 to 25 numbers of superconductive coils along with main superconductive winding.

Each coil can be powered independently during the shimming process. Also has persistence switch to be placed in persistent superconducting mode once the desired field correction has been done.

Unlike resistive coil shims, the current in superconducting shims and the magnetic fields cannot be altered easily once setup.

There are three shim coils in resistive magnet similar to gradient coils in x, y, and z directions. One shim coil for each dimension.

Some resistive magnet scanners also have additional resistive coils for higher-order shimming. At least 5 coils are used for second-order shimming.

The resulting magnetic field from each coil is proportional to the direction and amount of current in the coil.

During the shimming process run 3D gradient echo sequence which acquires volumetric data of the region to be shimmed.

An algorithm analyzes the collected data to calculate the amount of current needed in each shim coil to magnetic field homogeneity.

Passive Shimming

Only used in permanent magnet magnetic resonance imaging scanners.

During the installation process carry a jig and set of ferromagnetic pallets to place in the specific section of the magnet using a jig.

According to jig specific trays, slots, number, and size of elements are fed in shimming software.

The software measure and guide for the field mapping is then repeated and the shims tweaked or adjusted as needed.

After several repeating cycles desired level of homogeneity is achieved. 

The main disadvantage of passive shimming is the homogeneity varies when temperature varies.

Why shimming Required in MRI

Magnetic field homogeneity directly affects the quality of the final image.

Especially when a large FOV scan is done then shading, spatial distortion, blurring, intensity loss, curved slice profiles, and zebra banding artifacts appear.

For magnetic resonance spectroscopy (MRS) less than 1 ppm magnetic field homogeneity is required.

Fat and water resonances are separated at less than 3.5 ppm. magnetic field irregularities or distortions more than 3.5 ppm shows poor fat suppression.

Gradient Coil

Those coils encoded proton frequency in a 3D plane (x, y, and z directions ) in such a way so that RF signal can be read from a particular slice or region for imaging.

To achieve this there are three sets of different coils in each plane or direction known as X, Y, and Z gradient.

There are three gradient pulse-width modulated (PWM) amplifiers for each direction gradient coils.

Z Gradient Coil

The principally z-gradient coil is a maxwell coil configuration where two magnetic coils carry current in opposite directions of each other.

Distance between coils should be 1.73 x r, where r is the radius of the coil.

Z- Gradient considers horizontal in bore type MRI and vertical in open MRI.

The strength of the z gradient is zero at the ISO center of the main magnet and linearly increase in z+ and z- direction.

X and Y Gradient Coils

Clearly, x and y gradient coils of any magnetic resonance imaging machine is an example of Golay coils.

For any given direction ( X or Y ) Golay coils consist of 4 inner arcs and 4 outer arcs connected with straight wires (parallel to the z-axis ).

Only inner four arcs create gradient field for the given axis. In the modern machines versions Golay coils used as fingerprint design coil winding.

Ideally coils length is 2.18r with 120° arc where r is the radius of the main coil with the distance between coils 0.78r.

For open MRI flat or disk size gradient coils are used because above mentioned cylindrical gradient coils can not be used.

Disk size gradient coils can be used either for x- gradient or y- gradient only need to rotate 90º.

Eddy current in MRI

Frequently changing magnetic field in gradient coils and RF coils produce eddy current.

According to the definition, the change in the flow of current in any conductor produces varying flux causes of induced current (eddy current) in the conductor.

That eddy current in the conductor which oppose to the flow of current in the conductor.

Eddy current in magnetic resonance imaging causes two major issues.

Firstly the heating effect is similar to our induction stove. Any metallic object near the MRI including tissues of a patient body.

Also increase boil-off rate of liquid helium.

To minimize the effect of eddy current in gradient coils place the same set of gradient coils between the image gradients and bore.

The only difference is the flow of current keeps opposite in direction to image gradients.

For further cooling maintain the water flow coming through the chiller between the coils.

Secondly, reduce the speed and efficiency of gradient pulse switching results in gradients become distorted.

Thus artifacts shearing, shading, scaling, blurring and misregistration appear in the final image.

Although a smart solution for this problem is to add signals which compensate for the distorted signal due to eddy current from gradient output.

Radiofrequency (RF) and RF Coils

Magnetic resonance imaging creates images using radio frequencies. When transmitter rf coils energies using radiofrequency then energy absorbed by body tissues nuclei.

Later nuclei release the energy in the form of radio frequency. That is absorbed by receiving rf coils and again nuclei align with the main magnetic field.

The frequency of the signal must be kept similar or near to similar of Larmor frequency.

Which is depending on the applied main magnetic field. In any MRI the following modules are used for radiofrequency.

Frequency Generator

Using a numerically controlled oscillator (NCO) using PLL (phase lock loop) the frequency generator generates the Larmor frequency carrier waves.

Witch is later sent into two sections one for further processing of rf transmitter signal and second is rf receiver as reference for demodulation of the signal.

Pulse Modulation

In this module data waves pulses generated (1 to 5 mili seconds and kHz range) using a frequency synthesizer.

Later data wave pulses mixed with carrier wave using modulation and make RF pulses sequence.

RF Pulse Amplifier

Amplifier increase rf pulse power to drive rf coils up to 10 to 30 kW so that the magnetic field (B1) strength reaches up to 10 to 50 μT.

Inside the amplifier provided transmit attenuator to adjust the flip angle of the signal amplified.

All this managed by changing or adjust in amplifier gain.

Quadrature Hybrid Coupler

Simply receive the amplified signal output and split it into two I and Q signals.

Where I is the original signal in-phase and Q signal is quadrature in a phase of first of signal.

A common application of both I and Q signal is with birdcage or transverse electromagnetic (TEM) coils for circular polarization (CP).

In linear polarization only in-phase signal (I) is used.

Transmitter / Receiver (T/R) Switch

Sometimes single rf coil work as transmitter and receiver or there may have several transmitter and receiver coils.

Although we have several transmitter and receiver coils each signal should reach up to assign coil and time.

To make sure these adjustments switches are used.

Radiofrequency (RF) Coils

RF coils work as a transmitter and receiver or sometimes both. As a transmitter, it creates a rotating magnetic field according to provided RF pulses.

Which is perpendicular to the main magnetic field. The RF pulses provided of near Larmor frequency for few mili seconds.

That energy absorbed by nuclei in the form of alignment with RF magnetic field.

When nuclei align back with the main magnetic field emits energy earlier absorbed.

As receiver coil capture energy which induce current in the coil.

Later current amplified then converted into digital form then apply a filter to recover frequency and phase-related information.

Only receiver coils are used in multiple parallel coil sets as called large array for parallel imaging.

Commonly used coil arrays are switchable arrays, Phased arrays, and Parallel Arrays.

Although the same RF coil can work for transmitter and receiver both but modern smart choice MRI uses separate coils for it.

But common transmitter and receiver coil still use for head and knee imaging.

What are channels in MRI

A completely independent set of amplifiers, filters, analog to digital converter (ADC) circuitry, demodulation, or mixer with individual image processing capability. 

The output of each channel partial view of the entire region being scanned, Subsequently combined output of all channels to produce the final image.

Eight-channel, sixteen channel or thirty-two channel are very common nowadays.

In other-hand coil segmentation is set of many parallel coils.

MRI Site Planning

For any MRI installation technically there are three basic rooms are required.

1) Magnet or gantry Room

2) Console or control room

3) Equipment room

Here ACR Manual on MR Safety has more clarity to design any site with ACR safety norms.

Although MRI scanner manufacturing companies provide complete details about site design specification for their every model.

In this article will stick to technical details.

Magnet or gantry Room

From a technical point of view, the gantry room is the most important room. Required special arrangements to place magnet and make it proper functioning.

Firstly the floor should be strong enough to carry magnet weight and maintain the level of floor equally in all directions.

For example 1.5T superconducting magnet weight is 10,000 lbs (4,500 kg) and permanent magnet of 1.5T weight is 35,000 lbs (16,000 kg).

Vibrations and Acoustic

Magnet room should be far from road, structure, elevator, or such area where vibration generates to ensure magnate stability.

During the working MRI produce noise from gradient coils during scanning ( Echo-planer, fast spin-echo, diffusion).

Although helium pump work continuously and produce noise.

For all these reasons gantry room should be soundproof for outsiders and provide earplugs for the people and patients inside the gantry room.

Faraday Cage or RF Shielding

Mainly serves two purposes first limit the electromagnetic field or radiation from gantry so that it can not interfere with other devices.

The second is just opposite to the first one to protect from RF signal interference and magnet homogeneity.

To achieve this whole room converted into a Faraday Cage which means every wall of the gantry room is covered with metal sheets including the roof and floor.

Filament bulbs or LED light sources are used. The window glass between the gantry room and console for monitoring also contains thin copper mesh in between.

RF bandstop filter is used for cables ( power and communication ) and fiber optic cable, duct, tubing, pipes use a waveguide to suppress external RF radiation.

Console or control room

Like the cockpit of the airplane, the console room is the command center of any magnetic resonance imaging scanner.

Here on the operator console and the physical window is present to observe the patient and perform scanning of body or body part.

All the controls and communication required for scanning and for handling emergency (quench button) are provided. The operator console converts MRI raw data into images.

It provides an interactive environment and controls so that operator can apply different scan techniques to get the best result out of it.

Final diagnosis, findings, and film printouts have been done from here.

Equipment room

In short, it is an extended part of the gantry room to keep all the pieces of equipment that are needed to maintain the magnetic field and gradient coil temperature within the limit.

According to control signals from the console, the gradient cabinet sends current to the gradient and RF coils.

Inside the cabinet Frequency Generator, Pulse Modulation, RF Pulse Amplifier, Quadrature Hybrid Coupler, Transmitter / Receiver (T/R) Switch, X-Y-Z-Gradient circuitry (like gradient amplifiers) are present.

Cryocooler compressor for helium, heat exchanger cabinet, and universal power supplies/ gradient/RF cabinet are placed in this room.

Finally, non-freezing chilled water from the chiller is fed into all the above cabinets including gradient coils inside the gantry room.

Also keep power supply and backup in the equipment room.

Fundamental quantum physics of MRI

To understand the basics first need to understand the spin of protons, neutrons, electrons, and nuclei.

What is spin quantum number ?

Every atomic and subatomic particle spin (rotate) along its axis and the measurement of the magnitude of spin is the spin quantum number.

Spin is quantized therefore it can only measure as a limited set of discrete values and represent by the symbol “l”.

Observed value of spin angular momentum with its direction known as a spin state like up spin and down spin.

In classical physics, any spinning object has angular momentum affected by gravity. But for atomic and subatomic particles spin only affected by an electromagnetic field.

Therefore quantum particles are not spin as assumed in classical physics and from here “bizarro” world of quantum physics starts.

What is Precession ?

When a strong magnetic field is applied to nuclei, its spin aligns perpendicular to the applied magnetic field and hence gyroscopic flywheel rotation experience.

The relation between frequency (fo) and applied magnetic field (Bo) is called Larmor Equation.

fo  =  γ Bo

fo  = Precession or Larmor frequency

Bo  = Applied magnetic field

γ = Gyromagnetic ratio

What is Larmor Frequency ?

Larmor frequency comes from his famous equation which gives in his theory to explain Zeeman’s split spectral lines.

The theory proved mathematically that Zeeman’s split spectrum bands are due to rotation of electron ( charged particle ) in an elliptical orbit.

Particle precess around applied magnetic field. Also, show that precession frequency is proportional to the applied magnetic field known as Larmor Equation.

fo  =  γ Bo , where fo  = Larmor frequency

To understand the concept in real instead of classical assumption then follow quantum explanations of the concept.

What is Resonance in MRI ?

Resonance is a natural phenomenon of an atom or subatomic particles define by an induced oscillating response over the band of frequencies when applying external energy.

For induced resonance in a quantum particle spin system, the frequency of external energy should be near the Larmor (resonance) frequency. 

When the external magnetic field is applied to a quantum particle then the precession of each quantum particle is different.

Therefore consider the sum of all quantum particle spin is considered and represented by M (net magnetization).

When providing external energy through RF pulse then quantum particle absorbs energy and flip called Excitation.

After stop RF pulse nuclei come back into their original state (align to the magnetic field) called Relaxation.

Importantly nuclei induce transient oscillation observed during relaxation called “free induction decay”.

Bloch Equations and T1 / T2

Felix Bloch had a key role in the development of magnetic resonance imaging.

According to Bloch’s assumption, any sample put in the magnetic field consists of millions or billions of nuclei.

Magnetization sum represents by single vector M ( polarization) known as net magnetization.

In the three-dimensional space vector, M (net magnetization ) is also divided into three-dimensional components.

Mx, My (transverse components), and Mz (longitudinal component) were written as.

Mz(t) = C

Precession of M as cone around B, longitudinal component Mz(t) at time t is a constant C.

Mx, My transverse components rotate around z-direction with angular frequency ω.

Mx(t) = A sin ωt 

My(t) = A cos ωt

Where ω = γ B and A = scaling factor.

Also mentioned two relaxation time constants T1 and T2. Using T1 and T2 in equations.

M_x(t) = M_o e ^−t/T2 sin ωt

M_y(t) = M_o e^−t/T2 cos ωt

M_z(t) = M_o (1 − e^−t/T1)

Where M_o is value of M in equilibrium with applied magnetic field.

Magnetic Resonance Imaging (MRI) Signals

Before understand about how to create image from MRI signals.

Need to understand pulse sequences of MRI signals and kind of signals.

All the signals and their combination of pulses are not sent to a single coil.

But sent and receive signal pulse sequences to different coils at different times.

How to transfer Signals to Nuclei and Vice Versa

Atomic nuclei consist of protons and neutrons. Where proton is positively charged quantum particles.

According to Faraday’s Law of Induction, moving charge changing magnetic field induces a voltage in a nearby conductor.

Thus nuclei induced current in coil when recover from resonance.

Similarly when the oscillating current at Larmor frequency flow from coil produces a magnetic field and flip the nuclei.

What is TR and TE component of MRI signal

Simply TE is known as echo time and TR is the time of repetition signal again as explained in the image.

Gradient Echo Signal

To convert free induction decay (FID) T2 signal into gradient echo signal (GRE) applies a dephasing gradient field.

After that again rephasing gradient field applied twice the duration then dephasing gradient time.

The advantage of the gradient-echo signal is to improve in signal-noise ratio and useful for frequency encoding.

What is spin Echo (SE)

Very similar to GRE (gradient echo signal ) means after FID T2 decay two successive RF pulses 90° and 180° produce spin-echo (SE).

Stimulated Echo also alike Spin echo but uses three or more RF pulses instead of two.

Commonly used in magnetic resonance imaging. In the STEAM Stimulated Echo Acquisition Mode (STEAM) three RF pulses are applied when x, y, and z gradient signal select slice (scan area).

IR (inversion recovery) Echo

Again similar to above but 180° RF pulse included before the complete signal.

Due to this nuclei flip 180° to a main magnetic field. As result, we get T1- weighted image with good contrast quality.

Fast or Turbo Spin Echo (FSE / TSE)

The only difference between spin-echo and fast spin-echo the after 90° pulse initially 180° pulse apply before the signal.

Varying gradient phase-encoding used along with 180° pulse train.

The number of echoes acquired in a single TR interval is known as the echo train length (ETL). ETL varies from 4 to 32.

Although it may increase up to 200 for turbo/fast imaging.  

k-space definition

To understand k-space it is similar to raw data in CT Scan machine.

In other words, the K-space matrix is an x and y-axis coordinate grid in which each point stores spatial frequency and phase information received from the signal.

The signal was converted into digital form by ADC ( analog to digital converter) and applied Fourier transform to fill k-space points.

K space is Fourier form of mri image.

Each cell represent voxel of the image display on console monitor.

Types of filling K-space

Inside K space empty cells are filled with raw signals. There are several methods to arrange those raw signals into cells in a proper manner.

K-space trajectory filling

There are some common methods to fill empty cells.

Cartesian

Radial

Spiral

Zig Zag

How are MRI images produced

Finally, come this far to understand image construction from the signal for what is an MRI used for.

Now we have an understanding of the concept and components related to magnetic resonance imaging.

Also, know how to transfer/receive RF signal pulse sequence to and from nuclei or bunch of nuclei.

But for creating an image must be known as the location ( coordinates ) of the signal in 2D and 3D space.

As you know gradient does this work using x, y, and z gradient signal pulses.

Here trying to explain a simple and basic understanding of image creation and the procedure behind it.

Slice Selection Encoding

Firstly for image creation of body part in magnetic resonance imaging machine need to select the region of the body part for taking 2D cross-sectional view.

To select that area of the body part for slicing use Z – gradient frequency encoding.

When Z-gradient energies apart from the main magnetic field additional field generated along the z-axis.

Z-gradient magnetic field strongest at ISO center and reduce in Z+ and Z- direction. Accordingly, Larmor frequency also changes with body organ distance from ISO center.

Choosing the right frequency can select the body part region where a slice needs to be taken.

Frequency Decoding

Secondly need to acquire x and y coordinates for 2D image cross-section view of selected body organ slice.

To know x-axis coordinate of nuclei in 2D matrix apply frequency through x-gradient coils in x+ and x- directions.

Thus we get the row of different frequencies. But still, never get the value of individual voxel value.

Gradient Phase Encoding

Although we get different column values but still not able to locate individual voxel (pixel) value.

Here y-gradient coils energies in result get the extra magnetic field in the y-axis direction.

The spin of nuclei is not uniform anymore and never spins in the same phase called dephasing. Which is different for every row.

Combination of all these three we get the coordinate and value for each voxel (pixel) to create a 512×512 image.

Parallel Imaging (PI)

In a parallel imaging system, multiple local coils are used result reducing in phase encoding and scan time.

Parallel imaging has become very popular in nowadays MRI because of advancements in technology and reduced the cost of RF-digital processing systems.

Now magnetic resonance imaging vendors making machine to support 200+ coils and 128+ receiver channels.

MRI Procedure

There are the following standard steps to perform any MRI (magnetic resonance imaging ) procedure.

There may be a few variations on case to case basis if required. For example when a need to inject contrast to a patient then repetition of scan is required or need to add a new slice.

1) Patient consent and preparation

Very first need to know the patient’s medical history, health condition, allergies, the reaction from gadolinium contrast previously, any implant like a pacemaker.

After clearing all the above provide a hospital gown to wear to the patient and remove all metallic ornaments.

2) Patient Placing into mri

In the MRI scanning room assistance explain the complete process of MRI scanning and keep calm to the patient.

In between position, the patient and place coils on body parts need to scan.

Also make sure for EKG or oxygen supply if required.

3) Patient Registration

As patient position finished and scanning room close and secure then required patient registration required on the operator console.

Here need to feed patient name, age, selection of body part need to scan, scanning protocol, and sequence.

4) Calibration and Scan planning

Before performing any scan on a patient calibration process is done in the following manner to ensure image accuracy and clarity.

transmitter gain adjustment, shimming, coil tuning, center frequency calibration, receiver gain adjustment, and dummy stimulation.

If using parallel imaging then coil sensitivity calibration also performs.

After all, the calibration sequences perform a scout scan performs and the plan position and angle of the slices need to be taken.

FOV, slice thickness, scan technique for individual slice or group of slices can be configured in this stage.

5) Acquire image sequence

All scan slice raw data acquire and store in operator console hard disk to produce images and display on a monitor.

Here radiographer makes the decision, if any finding is there, and needs to repeat the scan sequence or need to perform another sequence.

If contrast is required then after injecting contrast again perform image acquisition.

6) Post Processing

In this part acquired images further process for spectroscopy, diffusion tensor, cardiac, angiography, etc.

Remove unwanted parts of the image and filming. Digitally store study for future reference and reporting.

MRI Image Artifacts

The difference between actual and captured images is known as artifacts. It may occur due to many reasons but few important reasons are as follow.

1) Chemical shift artifacts

Seen like white and dark bands along frequency encoding direction. Appear due to changes in the chemical structure of molecules.

Like fat and water in our body have common H proton but oxygen atom in the water pulled hydrogen atom.

2) Susceptibility Artifact

When any material or tissue placed in the magnetic field inside get magnetize according to their property.

Hence the magnetic field varies and results get the artifact in the final image.

Susceptibility Artifact increase with a high magnetic field. Metal artifacts also under this category.

3) Dielectric Artifact

Occurs only greater than 3T magnetic resonance imaging due to the RF field shows shadow effect.

4) Motion Artifacts

Mapping errors in the nuclei phase cause motion artifacts.
It is because of the time mismatching between RF excitation and RF signal receiving.

During this period nuclei may have moved, due to respiration, pulsation, or motion.

When the image constructed over monitor the position of the RF signal placed in the wrong place in the image.

5) Para Magnetic Artifacts

Para magnetic material like iron comes in scanning area then resonate frequency goes beyond the range.

Therefore the region shows blank or nothing in that part of image.

6) Zebra Artifact

When the coil comes in touches with something like the patient touches the coil then pick noise which results in zebra artifact in the image.

7) Spike Artifact

If there is a bad data value in the k-space result in spike artifact.

8) Phase Wrap Artifact

When Scanning body size is large and FOV selection is less than body size.

9) Frequency Artifact

When gantry room shielding is not so perfect and outside frequency noise is coming and interfering.

Shows lines in the final image on the operator console.

Future Trends

Still, today lots of research work is going on. Many vendors already have been developed superconducting magnets above the cryogenic temperatures.

Magnesium diboride (MgB2) is new superconducting material above the 39°K temperature.

One step more ahead trying to develop superconducting magnetic resonance imaging at room temperature.

The most recent research found Tl5Sn2Ba2SiCu8O16+ has a transition temperature near 42°C (107ºF, 315°K).

One company making an MRI system using receiver coils of YiBaCuO and BiSrTiCaO. Those materials have transition temperatures in the range of 90°K.