The development of MRI revolutionized the medical world. Since its discovery, doctors and researchers have refined techniques to use MRI scans to assist in medical procedures and also help in research.
This article looks specifically at MRI scans. We also have articles in our knowledge center about CT scans, PET scans, and ultrasound scans.
Here are some key points about MRI scanners. More detail and supporting information is in the main article.
- MRI scans are a non-invasive and painless procedure
- Raymond Damadian created the first MRI full body scanner, which he nicknamed the "Indomitable"
- The cost of an MRI scanner starts at $150,000
- Japan has the most MRI scanners, with 46.5 per one million citizens.
What is an MRI scan?
An MRI scan uses a large magnet, radio waves, and a computer to create a detailed cross-sectional image of the patient's internal organs and structures.
The scanner itself typically resembles a large tube with a table in the middle, allowing the patient to slide into the tunnel.
An MRI scan differs from CT scans and X-rays because it does not use ionizing radiation that can be potentially harmful to a patient.MRI uses
An MRI scanner can be found in most hospitals and is an important tool to analyze body tissues.
The development of the MRI scan represents a huge milestone for the medical world, as doctors, scientists, and researchers are now able to examine the inside of the human body accurately using a non-invasive tool.
The following are just some of the examples where an MRI scanner is used:
- Abnormalities of the brain and spinal cord
- Tumors, cysts, and other abnormalities in various parts of the body
- Injuries or abnormalities of the joints, such as back pain
- Certain types of heart problems
- Diseases of the liver and other abdominal organs
- Causes of pelvic pain in women (e.g. fibroids, endometriosis)
- Suspected uterine abnormalities in women undergoing evaluation for infertility
There is little to no preparation required for patients before an MRI scan. On arrival at the hospital, doctors may ask the patient to change into a gown. As magnets are used, it is critical that no metal objects are in the scanner, so the patient will be asked to remove any metal jewelry or accessories that may interfere with the machine.
Sometimes, patients will be injected with intravenous (IV) contrast liquid to improve the appearance of a certain body tissue.
The radiologist will then talk the individual through the MRI scanning process and answer any questions they may have about the procedure.
Once the patient has entered the scanning room, they will be helped onto the scanner to lie down. Staff will ensure that they are as comfortable as possible by providing blankets or cushions.
Earplugs or headphones will be provided to block out the loud noises of the scanner. The latter is very popular with children as they can listen to music to calm any anxiety.During
During an MRI scan
Once in the MRI scanner, the MRI technician will speak via the intercom to ensure the patient is comfortable. They will not start the scan unless the patient is ready.
During the scan, it is imperative to stay still. Any movement will disrupt the images created, much like a camera trying to take a picture of a moving object. Loud noises will come from the scanner, which is perfectly normal. If the patient feels uncomfortable during the procedure, they can speak to the MRI technician via the intercom and request the scan be stopped.After
After an MRI scan
After the scan, a radiologist will examine the images to check whether any further images are required. If the radiologist is satisfied, the patient can go home. The radiologist will prepare a short report for the doctor, who will make an appointment to discuss the results.How does MRI work
An MRI scanner contains two powerful magnets; these are the most important parts of the equipment.
The human body is largely made of water molecules, which are comprised of hydrogen and oxygen atoms. At the center of each atom lies an even smaller particle called a proton, which serves as a magnet and is sensitive to any magnetic field.
Normally, the water molecules in our bodies are randomly arranged, but upon entering an MRI scanner, the first magnet causes the body's water molecules to align in one direction, either north or south.
The second magnetic field is then turned on and off in a series of quick pulses, causing each hydrogen atom to alter its alignment and then quickly switch back to its original relaxed state when switched off. The magnetic field is created by passing electricity through gradient coils, which also cause the coils to vibrate, resulting in a knocking sound inside the scanner.
Although the patient cannot feel these changes, the scanner can detect them and, in conjunction with a computer, can create a detailed cross-sectional image for the radiologist to interpret.
Functional magnetic resonance imaging (fMRI)
Functional magnetic resonance imaging or functional MRI (fMRI) uses MRI technology to measure brain activity by monitoring blood flow in the brain. This gives an insight into the activity of neurons in the brain as blood flow increases in areas where neurons are active.
This technique has revolutionized brain mapping by allowing researchers to assess the brain and spinal cord without the need for invasive procedures or injections of drugs.
fMRI helps researchers learn about the function of a normal, diseased, or injured brain.
Functional MRI is also used in clinical practice as, unlike standard MRI scans which are useful for detecting structural anomalies in tissues, a functional MRI scan can help detect anomalous activity in those tissues. As such, it is used to assess the risks posed by brain surgery by helping a surgeon to identify the regions of the brain involved in critical functions such as speaking, moving, sensing, or planning.
Functional MRI can also be used to determine the effects of tumors, stroke, head and brain injury, or neurodegenerative diseases such as Alzheimer's.MRI FAQ
It is important to stay still during the scan.
How long will an MRI scan take?
MRI scans vary from 20-60 minutes depending on what part of the body is being analyzed and how many images are required.
If, after the first MRI scan, the images are not clear enough for the radiologist, the patient may be asked to undergo a second scan straight away.
I have braces/filings, should I still undergo the scan?
Although braces and fillings are unaffected by the scan, they may distort the image. The doctor and radiographer will discuss this beforehand. The MRI scan may take longer if additional images are required.
Can I move while I am in the MRI tunnel?
No, it is important to stay as still as possible while in the MRI scanner. Any movement will distort the scanner and, therefore, the images produced will be blurry. In particularly long MRI scans, the MRI technician may give a short break halfway through the procedure.
I am claustrophobic, what can I do?
The doctor and radiologist will be able to talk the patient through the whole procedure and address any anxieties. Open MRI scanners are available in some locations to help patients with claustrophobia.
Do I need an injection of contrast before my MRI scan?
Some patients may need to have a contrast agent injected before the scan to improve diagnostic accuracy by highlighting certain issues, such as inflammation.
Can I have an MRI scan if I am pregnant?
Unfortunately, there is not a straight answer to this question. The doctor should be informed before the scan. There has been relatively little research on the effect of MRI scans on pregnancy. However, guidelines published in 2016 have shed more light on the issue.
It is recommended that MRI scans should be restricted during the first trimester unless the information is considered essential. MRI scans taken during the second and third trimester are safe at 3.0 tesla or less.
The guidelines also state that inadvertent exposure to MRI during the first trimester has not been associated with long-term consequences and should not raise clinical concern.
Topic 1: Medical Diagnostic Imaging
History and evolution
The history of medical imaging has included a number of different imaging systems and innovators, but it is fairly clear that the story really starts with Wilhelm Rontgen (1845-1923).
In 1895, Wilhelm Rontgen was experimenting with discharging electrical current in 'Crookes' tubes, when he noticed that a barium platinocyanide coated screen across the room was glowing, despite the tube being encased in cardboard.
Whilst experimenting with holding items between the tube and the screen, he soon found that the mysterious rays would pass through his flesh, but cast an outline of his bones on the screen. These findings were published in the Wurzburg Physical-Medical Journal, and as the world realised the medical value of the X-ray, Rontgen was awarded the first Nobel prize for physics in 1901.
The X-ray went on to become a vital tool to the medical profession (although its dangers were not appreciated until much later), but the next steps in internal medical imaging were not made until the end of World War II.
The development of magnetic resonance is attributed to the work of Felix Bloch and Edward Purcell in 1946, who independently discovered the presence of magnetic resonance in solids and liquids. They were able to create devices that would show images of these fields, and they were awarded the Nobel prize in 1952 for their work.
During the period 1950-1970, nuclear magnetic resonance was developed for chemical and physical molecular analysis, rather than medical applications. In 1967, the next developments in NMR were made by Paul Lauterbur, who found that a photographic-style image could be generated using magnetic resonance (previously this had only been possible using light and x-rays).
At about the same time, a physician named Raymonde Damadian discovered that malignant body tissue had a different spectrum than normal tissue. In 1974 he was able to produce a crude image of a tumor in a rat, and in 1976 he produced an image of the human body which took four hours to complete. These early MRI scanners were able to display results in tomographic layers, and would certainly have been impossible to construct without the development of digital computers and electronics.
Meanwhile, a different medical imaging system had been developed by British engineer Godfrey Hounsfield of EMI Laboratories. Hounsfield invented Computed Tomography(CT) imaging in 1972. This scanning technique involves combining x-ray images from a detector that rotates 360 degrees around the subject. A dedicated computer extrapolates this information into a two-dimensional image of the scanned 'slice'.
The first CT scanners were installed between 1974 and 1976, becoming widely available by 1980. The main importance of this was that it showed that hospitals were willing to pay for expensive medical imaging machines. The first CT scanner took several hours to aquire the raw data for a 'slice', and days to reconstruct a single image from this data! Increases in technology have decreased the time taken for a scan, with a modern CT scanner able to scan a 'slice' in about 100ms, and reconstruct an image in less than a second.
In parallel to the establishment of CT scanners in general use, Damadian continued his work, setting up a company to sell his MRI medical imaging machines. The next major innovation in the field of MRI was made by Dumoulin, who perfected MRI angiography in 1988. This scanning technique allows imaging of flowing blood without the use of contrasting agents. In 1989 echo-planar imaging was introduced, which allowed image acquisition at video rates; vital for the real-time systems which are being developed for the future.
Very recent developments in the field of MRI (1999) have been made in the development of image-guided neurosurgery, with an open-magnet MRI machine designed by General Electric Medical Systems and innovative software written by the AI laboratory at Massachusetts Institute of Technology. The function of this system is to take MRI scans and MRI angiograms, and combine these to create a three dimensional model of the patient's head and brain tissues, which is overlayed in virtual reality over the surgeon's point of view as surgery takes place.
X-rays are electromagnetic radiation ranging in wavelength from about 10 nm to 0.001 nm. They effect a photographic emulsion in the same way that light does. Absorption of X radiation by a substance depends upon its density and atomic weight. When the human body is x-rayed, the bones, which are composed of elements of higher atomic weight, absorb more radiation and cast darker shadows on the photographic plate.
Computed Tomography is based on the x-ray principle; as x-rays pass through the body they are absorbed or weakened at differing levels. Inside the CT machine is a semi-circular detector that measures the x-rays strength. This detector and the x-ray tube are mounted on a rotating frame that spins around the body of the patient taking roughly 1000 snapshots from different angles; every 360 degree rotation, a 'slice' is scanned. The x-ray tube may be focussed to a certain slice width (resolution) using lead shutters.
The software which drives the CT scanner has the complex task of taking the 1000 x-ray images and backwards reconstructing a two dimensional image of the 'slice' which was scanned. There is a dedicated computer inside the scanner devoted to this task of dealing with the raw CT data.
Magnetic Resonance Imaging
Magnetic resonance imaging is based on the absorption and emission of energy in the radio frequency range of the electromagnetic spectrum. The human body is primarily fat and water, and therefore contains approximately 63% hydrogen atoms. Magnetic resonance imaging primarily images the NMR signal from the hydrogen nuclei, which each comprise a single proton. The proton possesses a property called spin, which will cause the nucleus to produce an NMR signal.
In an MRI scanner, an enormous superconducting electromagnet (cooled by liquid helium) generates a powerful magnetic field. This field generates a steady state of hydrogen nuclei within the body. Within the magnet are gradient coils for producing a gradient in the X, Y and Z directions. Inside the gradient coils is the RF coil, which produces the magnetic pulse necessary to stimulate the hydrogen nuclei, and rotate the spins by 90 or 180 degrees. The RF coil also detects the signals from the spins within the body. (appendix 1).
The scan room must be shielded from external RF signals, to stop them from being picked up by the imager. A piece of equipment called a quadrature detector converts the signals returned from the RF coil into X and Y components. This data is fed to the computer, which builds up a cross-section image similar to that obtained from a CT scanner.
Once interpreted by the built-in algorithms, both CT and MRI machines produce two dimensional bitmap images of cross sections of the patient. One type of additional processing which software commonly does is the automatic separation and colour coding of tissue types. This is really a segmentation task that falls within the topic of computer vision.
The type of segmentation described in Image Guided Surgery is fairly simple; "each voxel must be labelled by tissue type and combined with like voxels into indentifiable structures." There isn't any explanation of how this would happen, but having read some books on segmentation of images (notably Boyle/Thomas), I think that it is most probable that Histogram Segmentation is being used here.
The histogram distribution of the values of all the pixels in the image is calculated. Then, the troughs in the graph are discovered (where fewer pixels of that colour exist), and this point is deemed to be the threshold. Any pixels with values over the threshold are grouped with the brighter pixels. Those pixels with values below the threshold are grouped with the darker pixels. A diagram of such a graph is in appendix 1.
For specific areas of the anatomy, a program may sometimes be able to label easily distinguished areas (generally by the pixel values and general shape). Other areas may be more ambiguous, with difficulty distinguishing the different pixel values. Human help can be enlisted here, with a technician picking out a group of pixels that clearly belong to a certain tissue. The software can then examine the other pixels in the image and assign them to the group that their value most closely matches.
In one of the systems I looked at (M.I.T. / G.E.M.S. image-guided surgery using MRI), the 'slices' from the imager were combined to create a 3-D model. This technique is not limited to image-guided systems, either; often it can help anatomical study to have 3-D computer models to examine. The pixels of the 'slices' are given Z-coordinates (based on the position of the 'slice' in the scan), and become full-dimensional voxels (volume elements).
This three dimensional voxel-based object may be rotated to any desired angle using software, or the voxels may be connected using flat faces or gouraud shading to render the object at any desired quality. In the image-guided surgery software, the scanned body part (head) is rotated to match the patient's real head as seen from the surgeon's point of view.
Advantages and Disadvantages
Of the three main internal medical imaging techniques discussed, there are definite differences between the effectiveness, cost and safety of the systems.
The x-ray picture has served the medical profession for more than a hundred years, but it has two major disadvantages: Firstly, the aspect of safety. It was not generally accepted that exposure to x-rays could be harmful until the early pioneers started to suffer the marks and scars of radiation, and die. Although proper precautions are taken nowadays by x-ray operators, and the dosages delivered are much smaller, it is still not very desirable to irradiate patients, and as a consequence, the operator is restricted in the number of plates they may expose. Secondly, x-rays are less effective than both CT scanners and MRI, since they only depict a two dimensional view of the patient from a certain angle.
The Computed Tomography scan is an improvement over the x-ray picture mainly because, like MRI, a series of 'slices' may be computed showing cross-sections of the patient. CT scanners have the advantage of being based on simpler technology than MRI systems (cheaper), but they still suffer from the problem of exposing the patient to doses of ionising radiation.
Magnetic Resonance Imaging systems are better at demonstrating anatomy than CT scanners, and also more sensitive to diseased tissue. Using MRI angiography allows pictures of flowing blood without the necessity of marker agents necessary to make veins show up on x-ray/CT scans. The patient is not exposed to any radiation during the scan. My personal opinion is that MRI systems are the best of the three techniques, however they are also very expensive through being technologically advanced.
Any CT scanner or magnetic resonance imaging system used to require the patient to lie full length inside the machine, whilst the scanner generated the images. This sometimes causes problems with people who are claustrophobic or very fat; either they will not or they cannot lie in the aperture. To some extent, this is now being remedied by the introduction of open-magnet MRI machines.
Computer generated imagery (whether two dimensional or three dimensional, real-time or preliminary) has proved itself invaluable in the field of modern medicine. The main advantage is that invasive surgery is unnecessary to find out the position, shape and healthiness of internal structures. A diagnosis can be made on the strength of the digital images, which are likely to give a clearer indication of details than surgery would in any case. The benefits of invasive surgery being avoided are obvious; surgery is always a risk to the patient's health, and not operating increases their chances of recovery. If surgery does prove necessary, then computer images/models will prove invaluable to the surgeon, allowing him to plan the operation in detail beforehand, and making the surgery quicker and more effective.
Finally, there are the general benefits of any kind of digital imagery, in that the data is easily stored, compressed, or transmitted to a colleague maybe hundreds of miles away.
In terms of scope for future improvements in the computer-assisted analysis of medical imaging, obviously a computer program that was able to actually diagnose medical problems from images would be a tremendous asset to the medical profession, but realistically this kind of decision is likely to continue to be made by doctors for the foreseeable future.
In the not-so distant future is the impending design of hardware and software capable of extending the image-guided surgery techniques recently developed, to allow real-time rebuilding of the three dimensional model which assists the neurosurgeon during surgery.
The scope of the possible applications for image-guided surgery in the future is enormous; in radiotherapy, for instance, the radiotherapist can ensure that the beams of radiation converge accurately at the desired anatomical site, with minimal damage to surrounding tissue. The size and position of tumors may also be easily reviewed before and after radiation or drug treatment. Investigation has already commenced into whether three dimensional representations built from MRI scans might be more effective at early detection of breast cancer than mammograms.
There are still considerable challenges for software authors in the field of real-time updated image-guided surgery. Considerable problems still exist with creating representations of moving organs; models can quickly become inaccurate when the patient breathes. Algorithms are currently under development to try to predict tissue deformations in such cases.
|Image Guided Surgery||Eric,Grimson,Kikinis,Jolesz,Black||Scientific American||Article|
Block diagram of MRI scanner