1 . LA EVICCIÓN Y LOS VICIOS REDHIBITORIOS
B. LAS CONSECUENCIAS JURÍDICAS
1. INDEMNIZACIÓN DE DAÑOS Y PERJUICIOS
The RF probe is essentially a coil of wire not unlike the gradient, shim, and primary coils of the MRI system. It differs, however, in that it must accommodate a high-frequency alternat-ing current of 10 to 200 MHz, dependalternat-ing on B0
Y Coils
Figure 12-8 Y gradient coils produce a gradient magnetic field through the patient from front to back. For transverse imaging, this field is usually the phase-encoding gradient magnetic field, BΦ.
intensity, so that it can produce a radio signal at the Larmor frequency.
Furthermore, the RF probe must usually be precisely designed to behave as both a trans-mitter and receiver of RF. The design of the RF probe is one of the more critical engineering
features of an MRI system. Such design has developed into a very exact science.
The signal generated for the RF probe comes from a device called a frequency synthesizer.
This is the master frequency source for the MRI system. It provides a tunable frequency band from which the Larmor frequency can be accu-rately determined for each individual examina-tion. This small but precise signal is then amplified by a transmitter that feeds RF energy into the coil.
The simplest RF probe is a coil of wire wrapped around a patient or placed on the body but separated by a covering. The inten-sity of the emitted RF signal and the sensitivity to the signal received from the patient are max-imum in a volume approximately equal to the diameter of the coil.
Outside of this coil diameter, signal intensity and sensitivity decrease rapidly. Such a simple type of coil is easily adaptable to a permanent magnet imaging system (Figure 12-10). This arrangement adds to the simplicity of a perma-nent magnet imaging system and improves sig-Figure 12-9 When all three gradients are
ener-gized at the same time, an oblique plane is imaged.
XY plane RF coil
Figure 12-10 A simple, circular radiofrequency (RF) coil can be used with a vertical field permanent magnet imaging system because the coil axis is in the XY plane.
nal-to-noise ratio (SNR) at a given magnetic field strength.
For most superconducting electromagnetic imaging systems, the primary static magnetic field is along the axis of the patient. To be used, the simple circular coil has to penetrate through the patient either laterally or antero-posteriorly (Figure 12-11). Because patients object to such treatment, other coil designs and shapes had to be devised.
Initially, the most widely used design for the RF probe was a saddle coil (Figure 12-12, A).
In such a coil, the intensity of emitted RF and the sensitivity of the received signal are nearly uniform within the confines of the coil. The degree of such uniformity predomi-nantly results in great measure from the pre-cise spacing of the loops of the saddle.
However, this type of coil has low signal sen-sitivity.
Quadrature Coils
Quadrature coil design improves SNR by detecting the MR signal from multiple direc-tions, as shown in Figure 12-13. They view the signal as though they were a pair of stereo lenses. The result is better sensitivity to the MR signal relative to linear coils.
Quadrature coils are also constructed to be more homogeneous for RF transmission and reception. For this reason, quadrature coils have replaced the saddle design for virtually all homogeneous applications (e.g., head, body, knee). There are several types of quad-rature coils, which are all much more compli-cated than the saddle coil. One version, the
“birdcage” resonator, is widely used (Figure 12-12, B).
There are basically two types of RF probes.
Homogeneous volume coils are typically used both to transmit RF and to receive the MR sig-nal. These include the head and body coils and other special application coils, such as an upper extremity/shoulder coil shown in Figure 12-14.
Figure 12-11 Radiofrequency probes for electro-magnetic imaging systems are complicated because a simple circular coil would have to pass through the patient for its axis to be in the XY plane.
Figure 12-12 A, The saddle coil. B, The “birdcage” resonator type of quadrature coil.
A B
Head, body, or extremity coils are known as homogeneous coils; surface coils are referred to as inhomogeneous coils.
Most inhomogeneous coils are surface coils, and the word inhomogeneous is used because these coils do not transmit RF in a homogeneous fash-ion. For this reason, they usually receive only.
When these coils are used, the RF is transmitted by the head or body coil. The transmitting coil is then electrically silenced or decoupled while the surface coil receives the signal.
The MR signal detected from a patient con-sists of two components: signal and random noise. The signal comes from only the slice of tissue being excited.
The noise detected by the RF coil comes from all the tissue within the sensitive volume of the coil. For example, with the body coil, the
MRI technologist may excite and subsequently detect an MR signal from a slice through the liver, but the noise detected by the body coil comes from the liver, chest, and abdomen within the body coil. This reduces the SNR because of the higher level of noise.
All coils except the body coil improve SNR by reducing the detected noise. Tissue not being examined is eliminated from the coil’s sensitive volume. For example, the head coil does not detect noise from the liver because the liver is far from the coil (i.e., outside its sensi-tive volume). The other coils also improve SNR through increased signal detection by having the signal better “fill” the coil volume.
Body Coils
The most robust RF probe is the body coil. This statement is justified because the body coil can Figure 12-13 Quadrature detection improves signal-to-noise ratio (SNR) with multiple pairs of coils.
image any part of the human anatomy. The body coil is wound on a former and is just inside the gradient coils. The body coil is always a trans-mit/receive device (Figure 12-15). However, it is often possible to design coils for imaging specific anatomy with better SNR and thereby produce better images than those produced with a single body coil alone.
Head/Extremity Coils
A common alternative to the body coil for imag-ing cranial anatomy is the head coil. An
extrem-ity coil is available to obtain high-resolution images from the lower extremity. A typical quadrature birdcage head coil (Figure 12-16) can be attached to a neck coil and a thoracic/
lumbar surface spine coil to produce images of the total spine.
Surface Coils
For most imaging systems, the RF probe serves as both the transmitter of the RF and the receiver of the MR signal. One notable excep-tion is the surface coil.
A B
C D
Figure 12-14 A basic set of radiofrequency coils would include coils to image the head, body, extremities, and spine. These coils are for A, spine and soft tissue neck studies; B, chest, abdomen, and pelvis imaging; C, imaging across the glenohumeral joint; and D, soft tissue, skullbase, neck, and brain imaging. (Courtesy GE Medical Systems.)
A surface coil is a specially designed coil that is usually flat but can also be other shapes and is used to obtain high SNR images of anatomy close to the surface. The disadvan-tage of its use is reduced field of view (FOV).
The surface coil may be encased in a rubber-ized or plastic matrix to make it somewhat pli-able or in a hardened composite material for increased stability. The coil is placed on the surface of the patient at the anatomical region under investigation.
Surface coils come in many different sizes and shapes and are usually fabricated for spe-cific anatomy. A minimal complement of
sur-face coils consists of several flat, circular devices of varying diameters and a “license plate” coil for spine imaging.
Other surface coils include individual probes designed specifically for extremities, joints, or orbit (Figure 12-17). Special coil designs now include breast (Figure 12-18), prostate (Figure 12-19), and almost any other organ of interest. A high-resolution prostate image is shown in Figure 12-20.
When in use, the surface coil is positioned inside the head or body RF probe on the patient but is insulated from the skin. These devices are normally used only as MR signal Figure 12-15 A body probe of a quadrature detection phased-array design to image the cervical, thoracic, and lumbar spine simultaneously. (Courtesy Hitachi Medical Systems America.)
Figure 12-16 Quadrature design, birdcage, head coil shown with oncology mask. (Courtesy Midwest RF.)
Figure 12-17 Typical surface coils from small circular (A) to rectangular coils for spine (B) to large rectangular (C) available for magnetic resonance imaging. (Courtesy Siemens Medical Systems.)
A B C
receivers and rely on the head or body coil to transmit the RF. This type of use requires that the two coils be decoupled from one another so that they do not interfere with sensing the weak MR signal or that one does not “intercept” the RF transmission of the other.
Lack of decoupling can cause the receiving coil to heat up, possibly destroy the surface coil electronics, and potentially burn the patient. Special hardware is often used to actively decouple the two coils by detun-ing one of the coils while the other is operating.
Surface coils provide better contrast reso-lution and better spatial resoreso-lution.
A surface coil has improved contrast resolution because of higher SNR. A surface coil has improved spatial resolution because of smaller FOV. A surface coil image of the cervical spine, for instance, has a pixel size of 0.4 × 0.4 mm for a 10-cm FOV and a 256 matrix (Figure 12-21).
Because a smaller volume of tissue is being imaged, the pixel size for a given matrix with a
surface coil is always less than that for a whole-body RF probe.
Spatial Resolution/Pixel Size FOV = 10 cm = 100 mm Matrix size = 256 ¥ 256
Pixel size = = 0.39 mm 2 pixels per line = 0.78 mm/lp Hence: spatial resolution
= (0.78 mm/lp)-1
= 1.28 lp/mm
= 12.8 lp/cm
Disadvantages of surface coil imaging include limited FOV and positioning. Because the sur-face coil is smaller than the head or body probe, it has a smaller sensitive volume, which results in a restricted FOV. This is Figure 12-18 Bilateral breast coil for magnetic
resonance mammography. (Courtesy Siemens Medical Systems.)
100 mm 256
Figure 12-19 Endorectal coils for prostate imag-ing. (Courtesy Siemens Medical Systems.)
acceptable if only a small region of anatomy is to be imaged.
However, positioning the surface coil requires more time. To maximize the signal from the small volume, the MRI technologist
must pay closer attention to patient setup. The surface coil must be as orthogonal or perpen-dicular to the XY plane as possible for maxi-mum sensitivity. Just a slight tilt in the coil can result in significant loss of signal. With Figure 12-20 High-resolution, T2 weighted, three-dimensional fast spin echo (FSE) image of the prostate acquired with an endorectal coil and pelvic phased-array showing carcinoma (low intensity) on the left. (Courtesy William Bradley, San Diego, CA.)
experience, MRI technologists learn to posi-tion surface coils easily and quickly.
Surface coils have limited FOV.
The principal objection in the use of surface coils is the limited FOV. This deficiency has
been overcome with the development of phased-array or multicoil systems. If several surface coils are connected and positioned so that they have minimal coupling, a single large image can be the result. Each coil must have its own receiver channel for processing that single image of the cumulative sensitive vol-umes of all the coils (Figure 12-22).
In this situation, the SNR and spatial reso-lution of each individual surface coil is main-tained in the composite image. For example, if four surface coils are used to image the tho-racic and lumbar spine, each having a 10-cm FOV acquired at a matrix size of 256 × 256, the composite image could have up to a 40-cm FOV with a 256 × 1024 matrix (pixels are added only along the coils). Current routine applications of this technology include imag-ing of the spine (Figure 12-23), pelvis, and breast.
CHALLENGE QUESTIONS
1. What is the purpose of shim coils in a superconducting MRI system?
2. Surface coil imaging results in better SNR and better spatial resolution. What are its principal limitations?
Figure 12-21 A high-resolution image of the cer-vical spine. (Courtesy Larry Rothenberg, New York, NY.)
COIL COIL COIL COIL
Combined reconstruction Digital Receiver 2
Digital Receiver 3 Digital Receiver 4 Digital Receiver 1
COMPOSITE IMAGE Figure 12-22 An array of four surface coils positioned to image the entire spine.
3. A superconducting MRI system operating at 3 T is said to have a field homogeneity of ± 1 ppm. How should that be expressed in millitesla?
4. A surface coil with an 8-cm FOV is used to image the orbit with a 512 × 512 recon-struction matrix. What is the limiting spatial resolution for this examination?
5. A gradient magnetic field has a maximum amplitude of 25 mT/m, with a rise time of 100 ms. What is the slew rate?
6. What can be done to improve the spatial resolution of an MRI system?
7. For sagittal plane imaging, which gradient coil must be energized (GX, GY, GZ)?
8. Surface coils are said to result in better contrast resolution. Why?
9. For a transverse image, is the read gradient magnetic field (BR) frequency encoded or phase encoded?
10. What is a homogenous RF coil?
Figure 12-23 Composite image of the entire tho-racic and lumbar spine acquired with multiple sur-face coils. (Courtesy GE Medical Systems.)