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01 December 2002

Where no one has seen before

By Uday Tipnis and Aniket Joshi

Involved instrument engineers and scientists are excited. Expanding utilities and potentials of magnetic resonance imaging (MRI), as relevant diagnostic tools in not only clinics but also other applications, are the reason.

MRI is an excellent noninvasive tool for diagnosing most diseases in most parts of the body. However, it is an expensive investigation. The long-term prospect as a clinical tool depends on the results of clinical research and the extent to which manufacturers are willing to invest in developing these techniques.

MRI has gradually supplemented modality in diagnostic imaging, complementing computed tomography, or CAT, in central nervous system investigations. MRI scanners are good at looking at the nonbony parts, or soft tissues, of the body.

MRI has great potential, and its developmental progress will be thrilling over the coming years. The emphatic properties of MRI are soft tissue differentiation and multiplanar display.

The instrumentation community wants to be involved in the opportunities MRI provides, such as ultrafast scanning time down in the millisecond range, MRI fluoroscopy, and functional MRI imaging.

THIS IS ROCKET SCIENCE

All fundamental particles such as protons, electrons, and neutrons possess certain in herent properties such as charge, mass, and spin. These particles move and act like tiny spinning tops.

Spin: A basic characteristic of protons is that of spin. In addition to a proton, a hydrogen nucleus contains a positive charge. A spinning charge creates a magnetic field. Located in a random direction, each proton's magnetic field will cancel the other out.

However, when a patient is in the magnetic field of an MRI unit, protons become orientated either parallel (low-energy state) or antiparallel (high-energy state) to the magnetic field. More protons will align in the low-energy than in the high-energy state. Because of this, there will be a net magnetic moment when all charges are in a single direction in the low energy or parallel direction of the magnetic field.

This magnetic moment means that when placed in a magnetic field, the hydrogen atom has a strong tendency to line up with the direction of the magnetic field.

Radio waves-Larmor frequency: When energy in the form of radio waves flows to and into the patient, the low-energy parallel direction of the protons can change to high-energy anti parallel. The more energy added, the more the protons flip to the high-energy direction.

A radio frequency (RF) pulse in the amount that displaces the net magnetic field 90° is a 90° pulse, and the RF is very specific and has its own name: the Larmor frequency.

Magnetic moments: The charge and spin of the hydrogen nucleus gives rise to a magnetic field. It has a North and a South Pole. The nucleus is thus a dipole with a moment. In most materials such as soft tissues, these moments are randomly directed, and their sum is zero. But when the patient lies in a strong external magnetic field, all of them align in the direction of the field. The z axis of the MR image is always horizontal.

Precession: In addition to charge, spin, and magnetic moment, when each nucleus is under the influence of a magnetic field, it behaves like a wobbling toy top or gyroscope. The wobbling of a toy top as it comes to a stop is precession. How fast the magnetic moment precesses is the frequency of precession, or the Larmor frequency.

Cartesian coordinate system: A unique advantage of MRI is the ability to image in different planes. These planes define by using the three-dimensional Cartesian coordinate system. The z axis is along the long axis of the MRI magnet. The x axis goes from side to side, and the y axis is vertical.

Gradient fields: In order to form an image, the source and strength of a signal must be determined. A gradient field, which is a weak magnetic field, accomplishes this by super imposing on the main magnetic field. The gradient adds to the main magnetic field on one side and subtracts from the other.

Therefore, each distant point in the patient is in a slightly different field from all of the others. Because protons will precess at a rate proportional to their particular magnetic field, those at different points along the gradient will precess at different frequencies, depending on the site.

The resulting signals, when encoded, will determine the point of origin. There are three gradients: slice selecting, frequency encoding, and phase-encoding gradient, in the z, y, and x directions, respectively.

To automatically create an image, narrow predetermined radio wave frequencies transmit into the body. This is slice selection. A fixed gradient applies along the desired image plane. This is the frequency encoding gradient.

Upon selecting the frequency encoding gradient, the system can manipulate the other two gradient fields to separate the signal from different points along the slice.

Two-dimensional Fourier transform: The result of these various manipulations with the gradient is a highly complex signal in the form of amplitude vs. time. To simplify this, we use the two-dimensional Fourier transformation. This is a mathematical manipulation by which the computer changes the signal-to-signal strength vs. frequency.

HEAVY METAL BANDS PLAY FIELD

A MRI imaging system consists, first, of a magnet. There are three types: permanent, resistive, and superconducting.

Permanent magnets are magnetized ferro magnetic materials. These magnets are the least expensive to operate but are extremely heavy (100 tons) and limited to field strengths of about 0.35 T (1 T = 10,000 gauss - about 20,000 times the strength of the earth's magnetic field) or less.

Resistive magnets are considerably lighter and are capable of field strengths of about 0.5 T. Their drawback is the large expense for the constant electrical current they use and the need for extensive cooling as a result of their large current use that results in high heat.

The superconductive magnets can achieve much higher magnetic field strengths - whole body units up to 4 T and small bore research magnets up to 16 T and above. These magnets utilize the property of certain materials, which lose their electrical resistance fully below a specific temperature.

The commonly used superconducting material is niobium titanium alloy, for which the transition temperature lies at 9 K (-264°C). To prevent an external magnetic field or the current passing through the conductors from destroying superconductivity, these conductors must be cooled down to a temperature significantly below this point, at least to half the transition temperature.

RF transmitter system: In order to activate the nuclei so they emit useful signals, energy must convey into the sample via the transmitter system. The system consists of RF transmitter, power amplifier, and transmitting coils.

The RF voltage gates with the pulse envelopes from the computer to generate RF pulses that excite the nuclei. These pulses amplify to various levels (100 watts to several kW), depending on the imaging method and feed to the transmitter coil.

The RF coils can either be a single coil serving as both transmitter and receiver or two separate coils that are electrically orthogonal (at right angles to each other).

The latter configuration has the advantage of reduced pulse breakthrough into the receiver during the pulse. In both cases, all coils generate RF fields orthogonal to the direction of the main field. The coils tune to the nuclear magnetic resonance frequency and usually stay isolated from the rest of the system by enclosure in an RF shielding cage.

GROWING AN IMAGE FROM PULSE

Detection system: The function of the detection system is to detect the nuclear magnetization and generate an output signal for processing by the computer. The receiver coil usually surrounds the sample and acts as an antenna to pick up the fluctuating nuclear mag netization of the sample and convert it to a fluctuating output voltage.

Following the receiver coil is a matching network, which couples to the preamplifier in order to maximize energy transfer into the amplifier. Eventually, and after a series of conditioning steps, a complex (two-channel) signal merges into two strings of digital numbers. A converter output passes the string to the computer in serial form for processing.

Gradient system: Spatial distribution information evolves by using the fact that the resonance frequency depends on the field strength. By varying the field in a known manner through the specimen volume, it is possible to select the region of interest.

The strength of the signal at each frequency reflects the density of the hydrogen nuclei in the plane within the object where the magnetic field corresponds to that frequency. MRI methods exploit this property by way of carefully controlled, well-defined gradients to obtain information.

Both RF and gradient coils have similar shapes. RF coils are the antennas of the MRI system that broadcast the RF signal to the patient and/or receive the return signal. RF coils are only receivers and transmitters.

Surface coils are simply a loop of wire, commonly circular or rectangular, that surrounds the region of interest. Typical usage for these coils involves spines, shoulders, and other relatively small body parts.

Paired saddle coils commonly image knees. These coils provide better homogeneity of the RF field than surface coils. They are also x and y gradient coils.

The Helmholtz pair coils consist of two circular coils parallel to each other. They also work as z gradient coils and occasionally as RF coils. There are also birdcage and shim coils.

Imager system: The imager system consists of the computer for image processing, display system, and control console. The timing and control of RF and gradient pulse sequences for relaxation time measurements and imaging-in addition to image reconstruction, data processing, filtering tasks, and display and storage -are the jobs the computer carries out.

The minicomputer provides the imaging sequence in the system. Therefore, the computer must have sufficient memory and speed to handle large image arrays and data processing, in addition to interfacing facilities.

A SHORT HISTORY OF LONG UPS

Within a short span, MRI has shown promise of attaining these futuristic possibilities:

• The future will see the evolution of intervening MRI therapy, where the surgeon has access to real-time images of the patient and the procedure. For example, as a probe inserts into a patient, the surgeon can see exactly where the instrument is traveling.
• Future potential for MRI also includes nonproton imaging of substances such as phosphorus and sodium.
• The MRI process, which now requires a substantial length of time to produce an image (30-60 minutes), will be reduced to milliseconds with the introduction of such methods as turboflash and echo planar imaging. IT

Behind the byline

Uday Tipnis and Aniket Joshi are both at VES's Institute of Technology in Chembur, Mumbai, India.

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