Medical Imaging Technology: Techniques and Applications
Amy Schnelle
Computer Science
University of Wisconsin-Platteville
Abstract
Since the advent of the x-ray more than one hundred years ago, medical imaging has progressed from accidental discovery to a mainstay of modern medicine. This progress has led to easier, earlier, and less invasive diagnosis as well as a better understanding of the human body. The technology that makes medical imaging possible is continuing to develop at a rapid place, making it faster, cheaper and more user-friendly. This technology includes more advanced hardware, image capturing techniques, and visualization of captured images. Some of the most commonly used medical imaging techniques are CT (computed tomography),MRI (magnetic resonance imaging), radiography, ultrasound, and nuclear imaging. Approaches to image rendering include real-time volume rendering, finite-element modeling, deformable models, and tensor-mass models. In recent years a greater number of applications for medical imaging have arisen, including computer-aided diagnosis, surgery simulation, and drug development. As medical imaging continues to advance,so will the quality of health care for people around the world.
Why Medical Imaging is an Important Field of Study
Medical imaging is a large and diverse field composed of technologies related to image capture, visual rendering, and applications such as computer-aided diagnosis, surgery simulation and drug development. It is a field that requires the input of medical professionals, physicists, electrical engineers and computer scientists. With a collaborative effort, medical imaging has the potential to continually revolutionize the medical field.
Easier diagnosis is one of the many benefits of medical imaging. Before many of the current imaging systems, doctors had to rely on the patient’s description of symptoms and external cues. Most disease and disorder symptomscan vary from patient to patient. Moreover, each patient’s description of the same symptoms is different. External symptoms are also not a very easy way to diagnosis a condition. They can also vary from person to person and in many cases no external symptoms are visible.
Modern medical imaging systems can detect disease before any symptoms are noticed by physician or patient. A good example of this is cancer, which is much more easily treated if caught early. If cancer goes undiagnosed until symptoms are noticed, it has often metastasized and can be beyond treatment. As the technology continues to improve, evenearlier diagnosis will be the reality.
Less invasive diagnosis and procedures are great benefits to patients. Exploratory surgery, which would be necessary without imaging to see inside the body, is much riskier than imaging due to destruction of tissues, various levels of anesthesia, possibility of infection, and surgeon error. Less invasive procedures are also commonplace today. Arthroscopic surgeries, for example, greatly increase recovery time and decrease tissue damage. Medical imaging also plays an important role in surgery planning by giving the surgeon a glimpse into what the surgery will entail.
The applications of medical imaging also provide great benefits to society. Imaging can help prove safety and efficacy of new drugs. Surgeons can gain more training and practice with surgery simulation. Physicians can use computer-aided diagnosis techniques to assist them in diagnosis problems. With computer-aided diagnosis, doctors with little or no experience in a rare problem will be able to increase their diagnostic abilities. For all the above reasons, and many more, medical imaging has helped to and will continue to improve health care for everyone [11], [16].
The History of Medical Imaging
Medical imaging began in 1895, when Professor Roentgen accidentally discovered x-rays while experimenting with Crookes tubes. He went on to win the first Nobel Prize in Physics in 1901 for his discovery. X-ray technology continued to slowly progress up until the Second World War. Afterwards, breakthroughs in X-ray technology, such as image intensifiers, contrast media, and fluorescent screens improved the quality and safety of X-rays [13].
Nuclear imaging and magnetic resonance both began in the 1950’s. Anger developed the gamma camera in 1958, which led to exploration in the field of nuclear imaging. In 1946, Felix Bloch and Edward Purcell discovered the presence of magnetic resonance in solids and liquids. Nuclear magnetic resonance took shape from that time through the 1970’s, but was mainly used for chemical and physical molecular analysis.
Magnetic resonance was not used for medical purposes until a physician named Raymonde Damadian discovered that malignant body tissue had a different spectrum than normal tissue. This discovery, as well as recent advances in electronics and computing, started the exploration of MRI for medical purposes. This new technology was first realized in 1974, when Damadian took an MRI of a tumor in a rat [9].
The ultrasound was first developed in the 1960’s thanks to sonar development during World War II. Ultrasound was unique at the time, because it provided an imaging device without radiation. This made it ideal for monitoring fetal development. The ultrasound also had the additional feature of being able to produce real-time images.
Another medical imaging breakthrough made possible by digital computers was the CT, or computed tomography, scan. The CT scan was introduced in 1972, by Godfrey Hounsfield who later received the Nobel Prize for his work. One problem with the original CT scan was its speed. To take a single slice and convert it to digital image, took days. Since that time, there have been many advances in CT technology, and modern scans take only seconds [13].
Image Capturing Technologies
Radiography
Radiography is the process of creating an image by passing x-rays through a patient to a receptor. Because x-rays penetrate solid objects, but are slightly attenuated, or weakened, by them, the receptor receives different amounts of x-rays at different locations. The picture formed by the pattern of x-rays is known as a radiograph and allows physicians to see the structure of bones and solid foreign objects within the body. The principles of radiography are also used in other imaging processes such as CT.
In traditional radiography, the image is imprinted onto a film which can be viewed by the physician against a light or later converted to a digital image. Since the image is in analog form, it must be converted to digital form. This takes place in a three step process. The first step involves scanning the image, which is typically accomplished by a laser beam. During the scanning process, the image is divided into a certain number of lines. The number of lines the image is divided into, will determine the number of pixels the image has horizontally. The next step is sampling, where the continuous analog signal created during scanning is divided at certain intervals. The intervals mark the bounadry between one pixel of the final image and the next. Therefore, the number of scan lines and sample intervals determine the number of pixels in the entire image. More scan lines and sample intervals translate into longer scan time and a larger file but a better image. The last step is conversion where an analog-to-digital converter translates the value of the analog signal at a specific sample interval. The result of this conversion is a binary number that represensts a specific pixel in the digital image [22].
With digital radiography, the detector contains a thin active matrix which creates the image digitally. This eliminates the need for analog-to-digital conversion which is time consuming. In addition, the image does not lose some of the quality that can happen when converting to digital.
Fluoroscopy is a specialization of radiography. Unlike radiography, that produces a single static image, fluoroscopy allows for real-time, continuous x-ray image production. Originally, the fluoroscopy recptor was just a fluorescent screen, but the advent of the image intensifier tube allowed for much brighter images without increased exposure. The image is viewed by passing it from the image intensifier tube to a video camera and onto a monitor. Because of its real-time capabilities, fluoroscopy is especially useful for guiding a procedure, searching through a patient, or observing a dynamic function such as joint movement [19].
Magnetic Resonance Imaging
Magnetic Resonance Imaging (MRI)uses magnetic energy and radio waves that have varying sensitivity to the presence of water within tissues. Although this may not seem very useful, it is in fact very useful because the properties and amount of water in tissues can vary greatly with injury or illness. MRI is also distinctive, because it can selectively image several different tissue characteristics [13].
The magnet itself is one of the most important components of the MRI. It produces the magnetic field that the patient is placed into during an MRI procedure. The magnet can be superconducting, resistive, or permanent. The superconducting magnetis the most common in modern MRI devices, and is capable of producing the strongest magnetic field. This field is produced by a wire that has no resistance to electrical flow. The magnet is made of many loops of this wire which carry a large current. Along with producing a strong magnetic field, the magnet also requires constant cooling with liquid helium. The strength of these magnetic fields are measured in Tesla units and range from 0.5 to 1.5 Tesla. For comparison, a 1.5 Tesla MRI system has a magnetic field 30,000 times stronger than the pull of gravity on the earth's surface[19].
The magnetic field produced by the magnetresults in tissue magnetization and tissue resonance. Inside the MRI, the patient’s tissues become temporarily magnetized. Different tissues become magnetized to different levels and at different rates. This applies to both normal and pathological tissues. During the MRI procedure, the patient’s tissues react to the strong magnetic field and resonate in the radio frequency range. This results in radio communication between the patient’s tissues, the radio frequency transmitter, and the radio frequency receiver throughout the procedure.
Image capture during an MRI is accomplished by a computer. The capturing takes place in two distinct steps: acquisition and reconstruction. During Acquisition, a series of radio frequency pulses are transmitted to the patient inside the MRI. The radio frequency signals resonating from the body are collected and then another series is completed until sufficient data is collected to create an image. The number of series completed is indicated by the TR number. The higher the TR number, the more series completed and the better the image quality. To make the MRI procedure easier to complete, many preset protocols are stored for many clinical procedures [19] [22].
After acquisition, the image must be reconstructed. This is done by a process known as Fourier transform. The Fourier transform (FT) is a mathematical technique for converting time domain data to frequency domain data, and vice versa.
Computed Tomography
Computed Tomography(CT) imaging involves taking a series of x-ray images and combining them to form a 2D or 3D cross-sectional image. Unlikeconventional radiography, CT allows a physician to see the entire structure of soft tissue structures such as organs and fat, boney structures, and blood vessels. This makes CT especially useful for detecting tumors, blood clots, and problems with muscles. Because of its 3D capabilities, CT is also highly useful forthereconstruction of facial trauma, or diagnosing problems with complex joints [8].
A CT scanner consists of an x-ray tube and a CT detector that sit opposite of each other on a large ring. This ring rotates around the patient in a spiral pattern and the detector takes around 1000 snapshots of the x-ray beam per 360 degrees. Each snapshot in a rotationprovides one specific “shadow” just as in an x-ray image. All the snapshots for one rotation are processed by a dedicated processor that lines all the images up around a central axis and then stretches the image so they overlap to create a single 2D slice in the entire CT scan [15, 22].
The thickness of each slice can be adjusted between 1mm and 10mm depending on the amount of detail desired and the available storage. Since this process is costly, a dedicated processor is used to create the slices and then paste them back together after the scan completes. A modern CT scanner also has computers that control the movement of the scanner and allow the physician to control and monitor the scan [8].
Ultrasound
Ultrasound uses high frequency sound waves and their corresponding echoes to create images of the internal structures of patients. During an ultrasound, the transducer sends out millions of sound waves per second in the 1 to 5 megahertz range. When the sound waves encounter a boundary between tissues, it is echoed back. These echoes are received by the transducer and are processed to form the image. The process of ultrasound works similar to sonar and echolocation.
The light and dark areas in an ultrasound image are determined by the amplitude of the returning sound wave. In order to compensate for tissue absorption, a technique called time-gain compensation is applied to the results. Time-gain compensation is preformed by the electronic pulse amplifier. It works by amplifying the electrical pulse of echoes received by the transducer in relationship to the amount of time it takes to transmit and receive the signal. The gain of the amplifier, which can be set to a particular level or adjusted by the operator, determines how much the electrical pulses need to be adjusted to properly compensate for distance. The result of time-gain compensation is an image with brighter areas regardless if distance from transducer [19].
Because there are no x-rays involved in ultrasound, it has become very popular for checking fetal development. However, ultrasound is also used for evaluating many internal organs. Since ultrasound produces images in real-time, itis ideal for determining organ function and guiding surgical procedures [17].
Nuclear Imaging
Nuclear Imaging involves detecting radiopharmaceuticals given to a patient for diagnostic purposes and then creating images of the collected data. Nuclear imaging is especially useful for finding cancer, because radioactive substances that attach to tumors can be given and traced throughout the body to find very small metastases that may be difficult to otherwise detect. The two main types of nuclear imaging are positron emission tomography (PET) and single photon emission computed tomography (SPECT).
A PET scanning system works by detecting the location of radioactive particles that have accumulated in a certain area. When a positron from an unstable radionuclide, such asCarbon-11 or Fluorine-18, collides with an electron, both particles are annihilated and two photons are emitted. When annihilation occurs, each photon is emitted in exactly the opposite direction as the other. These photons are then detected by the PET scanners circular array of radiation detectors. If a detector pair(two detectors exactly opposite of each other) both detect a photon within a given time frame, then the location of the line between the two detectors is recorded. Multiple lines are then overlaid to produce the multi-dimensional location of the annihilation event, and ultimately the location of the radionuclide concentration [19, 22].
The radioactive substances used in SPECT are Xenon-133, Technetium-99, and Iodine-123. The process of SPECT imaging is similar to CT, but uses a gamma camera to take images of the gamma rays emitted by the particles. The path of the radioactive substances can be traced throughout the body, shedding light on the flow of blood and metabolisms. SPECT has the downside of producing less detailed images with lower sensitivity, but is more easily accessible for most patients [14, 19].
In attempts to further PET and CT technology, new hybrid devices are gaining in popularity. They are able to combine the benefits of PET and CT scanning devices to create images that clearly show both structure and function. The combination of these two imaging technologies creates a powerful new tool and also makes both technologies more affordable, and therefore more accessible [6].
Techniques for VisualRendering ofImages
Real-time Volume Rendering
Volumetric rendering allows for the visualization of the internal structure of objects, making it a highly important contribution to medicine. Although a great tool, volumetric rendering is very computationally expensive. For example, “rendering a dataset of 2563 16-bit voxels at 30 Hz requires 32 MBytes of storage, a memory transfer rate of 1 GByte per second, and approximately 5 billion instructions per second. This problem is aggravated by the continuing trend towards larger datasets. High-resolution sampling devices, faster supercomputers, and more accurate modeling techniques will make 10243 and larger datasets the norm” [11].
Volume rendering is the process of creating 2D images from a 3D collection of voxels. Voxels, or volume pixels, are the smallest distinguishable cubes that combine to form a 3D image. This 3D image is captured through one of the many image capturing techniques and is then processed using one of the many volume rendering techniques. These processes can be either indirect, where a geometrical model is first built from the data, or direct, where an image is created directly from the acquired data[12].