MRI

Magnetic resonance imaging (MRI) is a type of scan that uses strong magnetic fields and radio waves to produce detailed images of the inside of the body

1. magnetic resonance imaging (MRI) JASERAH SYED MSc I

2. introduction MRI is a spectroscopic imaging technique used to produce human body images. Also known as NUCLEAR MAGNETIC RESONANCE IMAGING (NMRI) or MAGNETIC RESONANCE TOMOGRAPHY (MRT) MRI does not use ionizing radiation but rather strong magnetic fields and radio frequencies (range 3-300GHz) Magnetic field of about 1.5-3T (tesla) applied i.e 50,000 times more strong than earths magnetic field. (0.00003T)

3. Raymond damadian Dr. Damadian was the first to describe the concept of whole-body NMR scanning, as well as discovering the tissue relaxation differences that made this feasible. In 1969, he first proposed the idea of using nuclear magnetic resonance technology to scan the human body externally for early signs of malignancy. In 1974, he received the first patent in the field of MRI.

4. Principle Magnetic resonance imaging (MRI) makes use of the magnetic properties of certain atomic nuclei . The human body is made up of fats and 90% water, hence it consists of 63% hydrogen atoms. The hydrogen nuclei behave like compass needles that are partially aligned by a strong magnetic field in the scanner . When placed in a large magnetic field, hydrogen atoms have a strong tendency to align in the direction of the magnetic field. Inside the bore of the scanner, the magnetic field runs down the center of the tube in which the patient is placed, so the hydrogen protons will line up in either the direction of the feet or the head. The majority will cancel each other, but the net number of protons is sufficient to produce an image . ENERGY ABSORPTION : The MRI machine applies radio frequency (RF) pulse that is specific to hydrogen. n The RF pulses are applied through a coil that is specific to the part of the body being scanned.

5. RESONANCE : The gradient magnets are rapidly turned on and off which alters the main magnetic field. n The pulse directed to a specific area of the body causes the protons to absorb energy and spin in different direction, which is known as resonance. When the RF pulse is turned off the hydrogen protons slowly return to their natural alignment within the magnetic field and release their excess stored energy. This is known as relaxation. MEASURING THE MR SIGNAL : the moving proton vector induces a signal in the RF antenna n The signal is picked up by a coil and sent to the computer system. the received signal is sinusoidal in nature n The computer receives mathematical data, which is converted through the use of a Fourier transform into an image.

6. Components of mri A magnet which produces a very powerful uniform magnetic field. Gradient Magnets which are much lower in strength. Equipment to transmit radio frequency (RF ). A very powerful computer system, which translates the signals transmitted by the coils.

7. Instrumentation of mri

8. The magnet The most important component of the MRI scanner is the magnet. They are superconducting magnets. When hydrogen atoms are subjected to the magnetic field of 1.5T-3T they align themselves either in parallel or anti parallel. Most of the hydrogen atoms align parallel (low energy) and few align anti parallel (high energy) and the magnetic moment is in the direction of primary magnetic field.

9. Alignment of protons in magnetic field

10. precession Larmor frequency (f) = gB /2 π

11. The gradient coils They generate secondary magnetic fields. Located within the wall of primary magnet. They are positioned in opposite to each other to create positive or negative pulse. Z gradient – runs along the long axis to produce axial images. Y gradient – runs along the vertical gradient to produce coronal images. X gradient – runs along the horizonatal axis to produce sagittal images.

12. Gradient coils

13. Radio frequency coils ( rf ) The RF coils are located within the magnet assembly and relatively close to the patient’s body. These coils function as the antennae for both transmitting signals to and receiving signals from the tissue. There are different coil designs for different anatomical regions. The three basic types are body, head, and surface coils . Transmitter : The RF transmitter generates the RF energy, which is applied to the coils and then transmitted to the patient’s body. The energy is generated as a series of discrete RF pulses. The transmitters must be capable of producing relatively high power outputs on the order of several thousands. Therefore , a 1.5 T system might require about nine times more RF power applied to the patient than a 0.5 T system. One important component of the transmitter is a power monitoring circuit. That is a safety feature to prevent excessive power being applied to the patient’s body .

14. Receiver : A short time after a sequence of RF pulses is transmitted to the patient’s body, the resonating tissue will respond by returning an RF signal. These signals are picked up by the coils and processed by the receiver. The signals are converted into a digital form and transferred to the computer where they are temporarily stored .

15. Rf coils Surface coils are used to receive signals from a relatively small anatomical region to produce better image quality than is possible with the body and head coils. Surface coils can be in the form of single coils or an array of several coils, each with its own receiver circuit operated in a phased array configuration. This configuration produces the high image quality obtained from small coils but with the added advantage of covering a larger anatomical region and faster imaging.

16. Computer system A digital computer is an integral part of an MRI system. The production and display of an MR image is a sequence of several specific steps that are controlled and performed by the computer. Acquisition Control The first step is the acquisition of the RF signals from the patient’s body. This acquisition process consists of many repetitions of an imaging cycle. During each cycle a sequence of RF pulses is transmitted to the body, the gradients are activated, and RF signals are collected. Unfortunately, one imaging cycle does not produce enough signal data to create an image. Therefore, the imaging cycle must be repeated many times to form an image. The time required to acquire images is determined by the duration of the imaging cycle or cycle repetition time—an adjustable factor known as TR—and the number of cycles. The number of cycles used is related to image quality. More cycles generally produce better images.

17. Image Reconstruction :- The RF signal data collected during the acquisition phase is not in the form of an image. However, the computer can use the collected data to create or “reconstruct” an image. This is a mathematical process known as a Fourier transformation that is relatively fast and usually does not have a significant effect on total imaging time. Image Storage and Retrieval :- The reconstructed images are stored in the computer where they are available for additional processing and viewing. The number of images that can be stored—and available for immediate display—depends on the capacity of the storage media. Viewing Control and Post Processing :- The computer is the system component that controls the display of the images. It makes it possible for the user to select specific images and control viewing factors such as windowing (contrast) and zooming (magnification). In many applications it is desirable to process the reconstructed images to change their characteristics, to reformat an image or set of images, or to change the display of images to produce specific views of anatomical regions. These post-processing (after reconstruction) functions are performed by a computer. In some MRI systems some of the post processing is performed on a work-station computer that is in addition to the computer contained in the MRI system

18. Image production

19. Gadolinium Contrast Medium (MRI Contrast agents ) Gadolinium contrast medium is used in about 1 in 3 of MRI scans to improve the clarity of the images or pictures of your body’s internal structures. This improves the diagnostic accuracy of the MRI scan. For example, it improves the visibility of inflammation, tumours , blood vessels and, for some organs, blood supply. OTHER DYES :- IRON OXIDE : Superparamagnetic - Two types of iron oxide contrast agents exist: superparamagnetic iron oxide (SPIO) and ultrasmall superparamagnetic iron oxide (USPIO). These contrast agents consist of suspended colloids of iron oxide nanoparticles and when injected during imaging reduce the T2 signals of absorbing tissues. SPIO and USPIO contrast agents have been used successfully in some instances for liver tumor enhancement . IRON PLATINUM (SUPERPARAMAGNETIC) MANGANESE

20. Gandolium images If the blood ocular barrier is disrupted the gandolinium leaks into the brain showing bright images. Eyes yield information about strokes: MRI scans revealed that a chemical called gadolinium, used to improve images, leaked into the eyes of stroke patients.

21. Refrences http:// www.sprawls.org/mripmt/MRI02/index.html https:// en.wikipedia.org/wiki/Magnetic_resonance_imaging https:// www.youtube.com/watch?v=Ok9ILIYzmaY https:// www.youtube.com/watch?v=g6FpkrERaPY https:// www.youtube.com/watch?v=djAxjtN_7VE&t=1248s http:// mriquestions.com/gradient-coils.html