How do mri coils work

RF coils can be used as transmitters and receivers. When used as a transmitter, they generate an oscillating or rotating magnetic filed that is perpendicular to the Bo, or static main magnetic field. When used as a receiver, RF coils detect the MR signal. In the past, most RF coils had both transmit and receive capabilities.

They have a great signal-to-noise ratio for tissues placed adjacent to the coil. The further the tissue is from the coil, the less sensitive it is. Surface coils are used for spines, shoulders, temporomandibular joints and other smaller body parts.

Volume coils are designed to provide a homogeneous RF excitation across a large volume, like whole-body imaging. Volume coils can also be smaller, and are used for the head and other extremities. Other coils are designed as only the receiver of the RF signal. The diagrams show the direction of the B 1 field.

Surface coils are very popular because they are a receive only coil and have a good signal-to-noise ratio for tissues adjacent to the coil. In general, the sensitivity of a surface coil drops off as the distance from the coil increases. Here is an example of an image of the lower human spine obtained with a surface coil. Here is a picture of a flat circular surface coil with its connecting cable. The cable will connect the coil to the imager. This is a picture of a surface coil molded to conform to the back of the knee.

The bird cage coil is the most routinely used volume coil. It is the coil of choice for imaging the head and brain. Here is a picture of the human head in a bird cage coil.

All of the head images in this hypertext book were obtained using a bird cage coil. The single turn solenoid is useful for imaging extremities, such as the breasts and the wrist. This animation window entry shows a single turn solenoid imaging coil around the human wrist.

The detail icon will provide you with more information on the construction of a single turn solenoid. RF detectors on MRI systems have evolved considerably since the s. Initially, linear analog detectors and single channel digitizers were used.

These were replaced with quadrature analog detectors with two channel digitizers. With the more recent availability of fast digitizers, single channel digitizers followed by digital quadrature detection is more common.

Linear analog detectors produce only the M x' or the M y' magnetization as the signal as a function of time S t. The signal is then digitized. Linear detection requires that the digitization rate for the signal be at least two times the highest frequency in the signal.

The factor of two is because half of the signal must be discarded to allow discrimination between positive and negative frequencies in the signal.

The reader is encouraged to review the chapter on Fourier Transforms to prove this statement. Quadrature analog detectors separates out the M x' and M y' signals coming from the RF coil. For this reason it can be thought of as a laboratory to rotating frame of reference converter. Both the M x' and M y' signals are digitized producing a complex signal as a function of time. For this reason, the digitization rate need only be equal to the largest frequency in the signal.

Again, the reader is encouraged to review the chapter on Fourier Transforms to prove this statement. The heart of an analog linear or quadrature detector is a device called a doubly balanced mixer DBM.

The doubly balanced mixer has two inputs and one output. For this reason the device is often called a product detector since the product of Cos A and Cos B is the output. The analog linear detector consists of one DBM, a filter, and an amplifier. There are two inputs and two outputs on the device. There are some potential problems which can occur with this device which will cause artifacts in the image. These will be addressed in the chapter on Image Artifacts. It is more common to find the following detection scheme on state-of-the-art detectors.

This frequency is digitized or oversampled using a high speed digitizer. Once digitized, the rotating frame signals M x' t and M y' t are created using a digital quadrature detector and a digital filter. The use of digital quadrature detection removes the possibility of a quadrature ghost artifact, to be discussed in the Artifact Chapter. The intermediate frequency and digitization is typically 1 MHz, resulting in oversampling and the generation of too much data to be conveniently stored.

Digital filtering eliminates the high frequency components from the data, and decimation reduces the size of the data set.

The following flowchart summarizes the effects of the three steps by showing the result of performing an FT after each step. A closer examination of oversampling, digital filtering, and decimation can reveal how a combination of steps can be used to reduce the wraparound problem. Oversampling Oversampling is the digitization of a time domain signal at a frequency much greater than necessary to record the desired field of view. For example, if the sampling frequency, f s , is increased by a factor of 10, the field of view will be 10 times greater, thus eliminating wrap around.

Unfortunately digitizing at 10 times the speed also increases the amount of raw data by a factor of 10, thus increasing storage requirements and processing time. Digital Filtering Filtering is the removal of a select band of frequencies from a signal. For an example of filtering, consider the following frequency domain signal. Frequencies above f o could be removed from this frequency domain signal by multiplying the signal by this rectangular function. In MRI, this step would be equivalent to taking a large FOV image and setting to zero intensity those pixels greater than some distance from the isocenter.

Digital filtering is the removal of these frequencies using the time domain signal. Recall from the chapter on Fourier Transforms that if two functions are multiplied in one domain i. To filter out frequencies above f o from the time domain signal it must be convolved with the Fourier transform of the rectangular function, a sinc function.

See the chapter on Fourier Transforms. This process eliminates frequencies greater than f o from the time domain signal. Fourier transforming the resultant time domain signal yields a frequency domain signal without the higher frequencies.

Decimation Decimation is the elimination of data points from a data set. Decimating the digitally filtered data above, followed by a Fourier transform, will reduce the data set by a factor of five. High speed digitizers, capable of digitizing at 2 MHz, and dedicated high speed integrated circuits, capable of performing the convolution on the time domain data as it is being recorded, are used to realize this procedure.

I am often asked, how safe is MRI? As with every piece of technology, there are risks and benefits. Those pieces of technology in wide use generally have a high benefit to risk ratio, while those with a low benefit to risk ratio are generally used more sparingly.

Although MRI does not use ionizing radiation to produce images there are still some important safety considerations which one should be familiar with. These concern the use of strong magnetic fields, radio frequency energy, time varying magnetic fields, cryogenic liquids, and magnetic field gradients.

These are sumarized in the animation window table. The limits were revised again in and are summarized in the following table. MRI personnel often forget about the dangers associated with ferromagnetic objects near the MRI magnet. Learn More.

They directly impact the spatial and temporal resolution, sensitivity, and uniformity in MRI. Advances in RF hardware have resulted in a variety of designs optimized for specific clinical applications. This review looks at the fundamental principles of an MRI RF coil from the perspective of clinicians and MR technicians and summarizes the current advances and developments in technology. Technical Efficacy : Stage 6 J. Imaging ;— Starting with the initial studies by Lauterbur 1 and Mansfield and Grannell, 2 magnetic resonance imaging MRI has seen a tremendous growth as a diagnostic and research imaging modality.

MRI is unique in its ability to move beyond anatomical imaging, as it allows visualizing metabolic functions and chemical processes via spectroscopic imaging, and offers advanced methods to measure physiologic properties such as tissue oxygenation, flow, diffusion, and perfusion.

While advances in the MRI hardware such as increased field strength and improved gradient performance have been substantial, advances in the radiofrequency RF technology have also proved to be valuable to improve the resolution and shorten the duration of MRI examinations. This review on MRI RF Coils and their basic principles is written from the perspective of clinicians and MR technicians, and is divided into three sections.

Every MRI system Fig. The basic components of any MRI system: The main magnet produces the B 0 field, necessary to align the spins and achieve equilibrium. All the components are controlled and interfaced with the user via a console.

In general, RF coils act like a broadcasting station: they transmit and receive signals. As a patient is positioned in an MRI scanner, the tissue obtains a small magnetization that aligns with the magnetic field, M 0. A RF transmit coil Tx generates an RF pulse that produces a small magnetic field perpendicular to the main magnetic field, which rotates net magnetization away from its alignment with the main magnetic field.

The RF receive coil Rx detects the precessing magnetization resulting in an induced electric current via electromagnetic induction. This precession frequency corresponds to the frequency range of radio waves MHz.

A The receive coil takes up the response from the excitation, amplifies, and digitizes ADC it. These coil circuits need to be correctly tuned and matched. Tuning a coil means adjusting the capacitance and sometimes the inductance so that the frequency of the electrical resonance of the coil circuit matches the frequency of the nuclear MR of the spins in the tissue. At the electrical resonance, a small external perturbation—due to the precessing magnetization—produces a large response from the coil, much like rubbing the lip of a wine glass can produce a loud tone.

Matching the impedance of the coil ensures the maximum power transfer from the power amplifier to the coil, minimizing the reflected power from the coil, and ensuring that the greatest possible fraction of the power is delivered to the spins.

The preamplifier is an electronic circuit that increases the amplitude of the received signal to a level that can be digitized. Coupling in electronics is the often undesirable transfer of energy from one medium to another. As the transmit power is orders of magnitude greater than the received signal, the preamplifier of the Rx coil needs to be protected from damage due to the high power of the RF transmission. When a forward DC bias is applied to the PIN diode, the resonant parallel LC circuit inserts a high impedance in series with the coil loop, blocking current flow at the Larmor frequency during the transmit phase.

Depending on the size of the resonant coil and the operating frequency, one or more blocking networks are used per coil element. In the early days of MRI, two somewhat parallel paths towards more efficient MRI experiments evolved: one was to improve the gradient technology and pulse sequence design to increase spatial resolution and decrease imaging time, and the other one was the development of RF coil technology.

Regardless of the field strength, the key requirement of any receiver coil is achieving the maximum SNR in order to obtain the best possible image quality. The SNR fundamentally represents the statistical confidence one can have in the robustness of the appearance of features in the image or spectrum, in the case of MR spectroscopy. If the signal is far stronger than the random variations in the intensity due to noise, then one can have confidence that apparent features in the image a bright spot in a particular location reflect the physical characteristics of the sample.

When detected by the receiver, the noise may start as Gaussian white noise, but will generally take on a Rician shape and be further altered by the reconstruction process, particularly with Array Coils. Every physical experiment includes either random or systematic noise, which can seriously affect the accuracy of the measurement.

The degree to which noise affects an experiment is generally characterized by the SNR Fig. Noise in MRI can be considered a random signal, which is superimposed on top of the real signal. Due to its random character, the mean value is zero, which gives no indication of the noise level, and so the quantitative measure of the noise level is conventionally the standard deviation of the noise.

Noise in the image appears as a grainy random pattern similar to snow on a TV screen. It represents statistical fluctuations in signal intensity that do not contribute to image information, and have two basic sources: Brownian motion of molecules in the human body and electronic noise of the receiver, which both add up. The signal of every MRI experiment is originated from dipoles that rotate in the transverse plane. The rotating magnetization induces a voltage at the terminals of the loop, which can cause a current in a single loop or coil.

Linearly polarized RF coils detect the rotating magnetization MR signal along a single direction. Quadrature circularly polarized coil arrangements detect the MR signal in orthogonal directions. In , Hoult first explained and discussed the NMR receiver.

Soon after the first publications, researchers recognized that a small region of interest ROI could also be imaged by a small RF coil, closely fitted to the ROI. The major breakthrough towards modern RF coil design was explored by Ackerman et al in by placing a small coil on the surface of the sample, close to the ROI, a significant SNR gain was achieved.

These coils, first introduced in by G. Surygan and later by J. The resulting trend towards small surface coils, initiated by miniaturization of RF coil loop size, necessitated consideration of noise dominance.

Noise dominance determines where the loss, and therefore the lower SNR, comes from: the sample or the coil. In the early days, cooling RF coils was a way to maintain the RF coil in sample noise dominance, but due to higher field strength and thereby higher operating frequencies, the sample noise generally maintains dominance in recent MRI systems without the need for cooling, providing coil dimensions are not too small. Next, switched loop elements arranged in arrays, which received the MR signal individually by detuning the other elements in the array, were developed to adjust the sensitivity pattern of such an array, in order to optimize the single receiver for use in arrays.

As long as the SNR maintains high enough, root sum of squares signal combination is generally adequate. However, when SNR per element is low ie, in diffusion tensor imaging, 22 the weights need to be obtained with a separate high SNR scan: At a point where only one loop detects significant signal, the other loop still contributes noise, and should therefore not be included in the reconstruction.

At points where both loops detect, the signals have to be put in phase to constructively sum the signals. The necessary amplitude and phase correction information is referred to as weighting coefficients Fig. The optimal combination of signals from an array coil requires knowledge of the spatial variation of each element's sensitivity.

Coil 1 will contribute primarily noise leading to a suboptimal combination of the pixel at point 1. If the contribution is weighted by the sensitivity at this location, the contribution of Coil 1 will be weighted close to zero, reducing the combined noise and hence increasing the SNR.

Soon after that, researchers explored the possibility to incorporate the spatial heterogeneity of the receiver elements from the array in spatially reconstructing the MR image. The spatial variance of RF fields between receivers was used to accelerate image acquisition and is called parallel imaging. In the last 10 years, several works have introduced different RF antennas for MRI, like patch antennas, dipoles, multipole antennas, and many more, enabling efficient and homogeneous B 1 field despite the shorter wave length.

The following chapter gives an overview of various types of RF coils and their fields of application, without a claim of completeness, as the field is still rapidly developing. RF Coils can also be distinguished, for example, by their homogeneity of the B 1 field or, if viewed from a usage perspective, into Surface Coils, Array Coils, or Volume Coils.

Volume coils are typically cylindrical and rely upon a sinusoidal distribution of currents arranged circumferentially around the tube and running the length of the coil to create a transverse magnetic field. Such coils as the birdcage 27 and the transverse electromagnetic TEM resonator 28 were designed to generate a very homogeneous RF excitation field B 1 across the entire covered volume.

An RF surface coil consists of a partial loop of wire having dimensions that match the area of interest and its inductance in resonance with the capacitance at the Larmor frequency. Another but similar approach to a more localized SNR is the concept of dipole antennas.

Already well described by communications or astronomy, dipole antennas are still relatively unknown in the broad MR community. Various types of dipoles used in MRI already exist: monopole antenna , 31 the folded dipole antenna , 32 circular dipole antenna , 33 fractionated dipole , 25 , 34 and combinations of loop coils and dipole antennas. A combination of several surface coils or dipole antennas is called an Array. As array coils have coil elements that operate at the same time, care must be taken to minimize crosstalk between elements, else efficiency in SNR and acceleration performance is compromised.

RF array coils are used to reduce imaging time by their ability to spatially localize signals. The SNR gain is greatest near the periphery, and as the channel count grows, the incremental SNR gains are increasingly confined to the peripheral areas closest to the coil elements. SNR along a 1D profile as a function of channel count for a few representative arrays courtesy of Ref. In the following Part B, the transmit and receive RF coil concepts are explained and analyzed in detail.

The transmit RF Coil is used to excite the spins in the sample. The secondary focus to minimize the time needed to tip the net magnetization from its previous state eg, equilibrium state. The power delivered to the coil depends on the impedance match to the transmission line from the amplifier, and the current excited in the coil that created the B 1 field, depends on the impedance.

When the frequency of the electrical resonance of the coil is the same as the center frequency of the incident RF pulse, the maximum current will flow in the coil and produce the largest B 1 field.

B 1 scales linearly with current, and the power and current relation in a matched condition is as follows:. These resistive losses R eff comprise the resistance of the coil conductor and its components R Coil , the electronic losses R Electronics , and tissue losses R Sample. Ideal homogeneous resonators create a homogeneous magnetic field, either by a uniform distribution on a sphere or by a cosine dependent distribution of currents, flowing parallel to a cylinder axis.

The closest approximations to ideal homogeneous resonators are the axial resonators eg, Helmholtz Coil, 41 Four Coil Configuration, 42 Solenoid Coil Transverse resonators are also known as volume coils. Citation, DOI and article data. Ballinger, J. Radiofrequency coils.

Reference article, Radiopaedia. Ray Ballinger. URL of Article. See also g radient coils. Read it at Google Books - Find it at Amazon 2. Delchar T. Physics in Medical Diagnosis. Read it at Google Books - Find it at Amazon 3. Understanding Magnetic Resonance Imaging.