Diagnostic imaging lets doctors look inside your body for clues about a medical condition. A variety of machines and techniques can create pictures of the structures and activities inside your body. The type of imaging your doctor uses depends on your symptoms and the part of your body being examined. They include:

• X-rays
• CT scans
• Nuclear medicine scans
• MRI scans
• Ultrasound

Many imaging tests are painless and easy. Some require you to stay still for a long time inside a machine. This can be uncomfortable. Certain tests involve exposure to a small amount of radiation.

For some imaging tests, doctors insert a tiny camera attached to a long, thin tube into your body. This tool is called a scope. The doctor moves it through a body passageway or opening to see inside a particular organ, such as your heart, lungs, or colon. These procedures often require anesthesia.

Have you ever had to get an X-ray, MRI, or other medical scan? Do you know what these tests involve? Or what they can do?

Medical scans help doctors diagnose everything from head trauma to foot pain. There are many different types of imaging technologies. Each works differently.

Some types of imaging tests use radiation. Others use sound waves, radio waves, or magnets. Learning about how medical scans work can help you feel more comfortable if you or a loved one needs one. It can also help you to know what to ask about before getting an imaging test.

X-Rays

The first revolution in seeing into the body came with X-rays. They have been used in the clinic for more than 120 years.

“X-rays are still used every day because they can do a lot,” says Dr. Kris Kandarpa, an imaging expert at NIH. They’re useful for looking at bones and finding problems in certain types of tissues, like pneumonia in the lungs.

X-ray imaging works by passing an energy beam through a part of your body. Your bones or other body parts will block some of the X-ray beams from passing through. That makes their shapes appear on the detectors used to capture the beams. The detector turns the X-rays into a digital image for a radiologist to look at.

X-ray beams use radiation. Radiation is energy that’s released as invisible particles or waves. Being exposed to very large amounts of radiation can damage cells and tissues. It may also increase your risk of developing cancer.

But modern X-ray tests use a very small dose of radiation. People are naturally exposed to radiation from many sources, such as the sky, rocks, and soil.

“A chest X-ray gives you similar amounts of radiation as you’d get in a plane flight across the Atlantic Ocean,” Kandarpa explains.

CT Scans

CT scans also use X-ray beams. But the beams rotate around your entire body to create a 3D picture. These images contain more information than a regular X-ray. The scan can be done in less than a minute. That makes it especially useful in places like the emergency department. There, doctors need to know immediately if a patient has a life-threatening condition.

Because CT scans use more X-ray beams than a normal X-ray, they often deliver a higher dose of radiation. But medical specialists have ways to calculate the smallest radiation dose needed, explains Dr. Cynthia McCollough, a CT imaging researcher at the Mayo Clinic.

“We tailor the dose to the patient’s size, and we tailor it to the reason for the exam,” McCollough says. For example, a CT scan of the chest needs less radiation than a CT scan of the stomach area.

McCollough’s lab, with four other NIH-funded teams, is working on ways to reduce the amount of radiation these scans deliver even more. Her team has used hundreds of CT scans to find the lowest radiation dose that’s needed for a radiologist to make the right diagnosis.

“We’ve found that when you take the dose way down, the images are less pretty, but they often still get doctors the right answer,” McCollough explains.

While lower doses of radiation would likely further lower risk, McCollough says that the standard doses are already quite low. That’s important for people to know, because “some patients who really need a CT scan are afraid to get it,” she says.

Fear can sometimes keep someone from getting a scan that could help improve their health, or even save their life. “Current CT doses are in a range where it’s not possible to even prove a risk exists. They’re that low,” she says.

If you’re concerned about a test that uses radiation, see questions you can “Ask Your Doctor” in the sidebox.

MRI

MRI works in a very different way. It doesn’t use X-rays. Instead, it uses strong magnets and radio waves to affect atoms in the water molecules within your body’s tissues. When the radio waves are turned off, the atoms release energy that’s detected by the MRI machine.

Atoms in different tissue types go back to normal at different speeds and release different amounts of energy. MRI software uses this information to create 3D pictures of the different tissue types.

“MRI is most helpful when you want to look at diseases that involve soft tissue, such as muscles, tendons, and blood vessels,” explains Dr. Shreyas Vasanawala, an MRI researcher at Stanford University.

MRI can provide information about how the body is functioning in real time. “For example, we can measure how much blood is flowing in the vessels,” Vasanawala says. That can help doctors find small blockages or defects in the heart.

Because MRI doesn’t use X-rays, doctors would like to use it more in children. But MRI machines require you to lie motionless for a long time.

“It can be difficult for children to hold still,” says Vasanawala. If needed, general anesthesia can help get kids through the test. It makes them unconscious and unable to move. It’s typically very safe, but comes with some risks.

To help reduce the use of anesthesia, Vasanawala and his team have created a flexible, blanket-like version of MRI hardware to use with children. They coupled it with new methods for faster scanning. The soft blanket-like coil sits closely on top of the patient, providing a comforting environment. “It’s helping some kids get though exams without anesthesia,” he explains.

Other Scans

Another commonly used imaging method is called ultrasound. It sends sound waves into the body. Different types of tissue reflect sound waves differently. These differences can be picked up by an ultrasound machine and turned into a picture. Ultrasound is helpful for looking at the heart and other organs, or a developing baby.

Doctors also use tests called nuclear imaging. These tests use a tiny amount of a radioactive substance, or “tracer.” Most tracers are injected into the body, but some are inhaled or swallowed. The tracers inside the body release radiation that can be measured by a detector outside the body. The type of tracer differs depending on what the doctors want to see.

A positron emission tomography (PET) scan, for example, often uses a radioactive sugar to diagnose cancer. When cancer cells take up the radioactive sugar, they can be seen with the PET scanner.

Scientists are working to develop new types of tracers to detect different conditions, such as infections hiding deep in the body. They’re also continuing to explore other ways to make medical scans faster and deliver less radiation.

Before getting a medical scan, ask:

• What information will this test give you?
• How will the results guide my treatment?
• Does this test use radiation? If so, is the dose tailored to my size and what you need to know?
• Are there alternative tests that could be used? What are the risks and benefits of each?
• If this test is for a child, is the facility using pediatric protocols for the test?
• Is anesthesia required for this test?

X-rays are a type of radiation called electromagnetic waves. X-ray imaging creates pictures of the inside of your body. The images show the parts of your body in different shades of black and white. This is because different tissues absorb different amounts of radiation. Calcium in bones absorbs x-rays the most, so bones look white. Fat and other soft tissues absorb less, and look gray. Air absorbs the least, so lungs look black.

The most familiar use of x-rays is checking for broken bones, but x-rays are also used in other ways. For example, chest x-rays can spot pneumonia. Mammograms use x-rays to look for breast cancer.

When you have an x-ray, you may wear a lead apron to protect certain parts of your body. The amount of radiation you get from an x-ray is small. For example, a chest x-ray gives out a radiation dose similar to the amount of radiation you're naturally exposed to from the environment over 10 days.

What are medical x-rays?

X-rays are a form of electromagnetic radiation, similar to visible light. Unlike light, however, x-rays have higher energy and can pass through most objects, including the body. Medical x-rays are used to generate images of tissues and structures inside the body. If x-rays travelling through the body also pass through an x-ray detector on the other side of the patient, an image will be formed that represents the “shadows” formed by the objects inside the body.

One type of x-ray detector is photographic film, but there are many other types of detectors that are used to produce digital images. The x-ray images that result from this process are called radiographs.

How do medical x-rays work?

To create a radiograph, a patient is positioned so that the part of the body being imaged is located between an x-ray source and an x-ray detector. When the machine is turned on, x-raystravel through the body and are absorbed in different amounts by different tissues, depending on the radiological density of the tissues they pass through. Radiological density is determined by both the density and the atomic number (the number of protons in an atom’s nucleus) of the materials being imaged. For example, structures such as bone contain calcium, which has a higher atomic number than most tissues. Because of this property, bones readily absorb x-rays and, thus, produce high contrast on the x-ray detector. As a result, bony structures appear whiter than other tissues against the black background of a radiograph. Conversely, x-raystravel more easily through less radiologically dense tissues such as fat and muscle, as well as through air-filled cavities such as the lungs. These structures are displayed in shades of gray on a radiograph.

When are medical x-rays used?

Listed below are examples of examinations and procedures that use x-ray technology to either diagnose or treat disease:

Diagnostic

X-ray radiography: Detects bone fractures, certain tumors and other abnormal masses, pneumonia, some types of injuries, calcifications, foreign objects, dental problems, etc.

Mammography: A radiograph of the breast that is used for cancer detection and diagnosis. Tumors tend to appear as regular or irregular-shaped masses that are somewhat brighter than the background on the radiograph (i.e., whiter on a black background or blacker on a white background). Mammograms can also detect tiny bits of calcium, called microcalcifications, which show up as very bright specks on a mammogram. While usually benign, microcalcifications may occasionally indicate the presence of a specific type of cancer.

CT (computed tomography): Combines traditional x-ray technology with computer processing to generate a series of cross-sectional images of the body that can later be combined to form a three-dimensional x-ray image. CT images are more detailed than plain radiographs and give doctors the ability to view structures within the body from many different angles.

Fluoroscopy: Uses x-rays and and a fluorescent screen to obtain real-time images of movement within the body or to view diagnostic processes, such as following the path of an injected or swallowed contrast agent. For example, fluoroscopy is used to view the movement of the beating heart, and, with the aid of radiographic contrast agents, to view blood flow to the heart muscle as well as through blood vessels and organs. This technology is also used with a radiographic contrast agent to guide an internally threaded catheter during cardiac angioplasty, which is a minimally invasive procedure for opening clogged arteries that supply blood to the heart.

Therapeutic

Radiation therapy in cancer treatment: X-rays and other types of high-energy radiation can be used to destroy cancerous tumors and cells by damaging their DNA. The radiation dose used for treating cancer is much higher than the radiation dose used for diagnostic imaging. Therapeutic radiation can come from a machine outside of the body or from a radioactive material that is placed in the body, inside or near tumor cells, or injected into the blood stream.
Click here for more information on radiation therapy for cancer.

Are there risks?

When used appropriately, the diagnostic benefits of x-ray scans significantly outweigh the risks. X-ray scans can diagnose possibly life-threatening conditions such as blocked blood vessels, bone cancer, and infections. However, x-rays produce ionizing radiation—a form of radiation that has the potential to harm living tissue. This is a risk that increases with the number of exposures added up over the life of the individual. However, the risk of developing cancer from radiation exposure is generally small.

An x-ray in a pregnant woman poses no known risks to the baby if the area of the body being imaged isn’t the abdomen or pelvis. In general, if imaging of the abdomen and pelvis is needed, doctors prefer to use exams that do not use radiation, such as MRI or ultrasound. However, if neither of those can provide the answers needed, or there is an emergency or other time constraint, an x-ray may be an acceptable alternative imaging option.

Children are more sensitive to ionizing radiation and have a longer life expectancy and, thus, a higher relative risk for developing cancer than adults. Parents may want to ask the technologist or doctor if their machine settings have been adjusted for children.

Medical imaging has led to improvements in the diagnosis and treatment of numerous medical conditions in children and adults.

There are many types - or modalities - of medical imaging procedures, each of which uses different technologies and techniques. Computed tomography (CT), fluoroscopy, and radiography ("conventional X-ray" including mammography) all use ionizing radiation to generate images of the body. Ionizing radiation is a form of radiation that has enough energy to potentially cause damage to DNA and may elevate a person’s lifetime risk of developing cancer.

CT, radiography, and fluoroscopy all work on the same basic principle: an X-ray beam is passed through the body where a portion of the X-rays are either absorbed or scattered by the internal structures, and the remaining X-ray pattern is transmitted to a detector (e.g., film or a computer screen) for recording or further processing by a computer. These exams differ in their purpose:

• Radiography - a single image is recorded for later evaluation. Mammography is a special type of radiography to image the internal structures of breasts.
• Fluoroscopy - a continuous X-ray image is displayed on a monitor, allowing for real-time monitoring of a procedure or passage of a contrast agent (“dye”) through the body. Fluoroscopy can result in relatively high radiation doses, especially for complex interventional procedures (such as placing stents or other devices inside the body) which require fluoroscopy be administered for a long period of time.
• CT - many X-ray images are recorded as the detector moves around the patient's body. A computer reconstructs all the individual images into cross-sectional images or “slices” of internal organs and tissues. A CT exam involves a higher radiation dose than conventional radiography because the CT image is reconstructed from many individual X-ray projections.

Radiography of the vascular system of the brain after injection of a contrast medium.

A rapid, low-dose, digital imaging system using a small intraoral sensor instead of radiographic film, an intensifying screen, and a charge-coupled device. It presents the possibility of reduced patient exposure and minimal distortion, although resolution and latitude are inferior to standard dental radiography. A receiver is placed in the mouth, routing signals to a computer which images the signals on a screen or in print. It includes digitizing from x-ray film or any other detector. (From MEDLINE abstracts; personal communication from Dr. Charles Berthold, NIDR)

Radiography of the URINARY BLADDER.

As in many aspects of medicine, there are both benefits and risks associated with the use of CT. The main risks are those associated with

1. test results that demonstrate a benign or incidental finding, leading to unneeded, possibly invasive, follow-up tests that may present additional risks and
2. the increased possibility of cancer induction from x-ray radiation exposure.

The probability for absorbed x-rays to induce cancer or heritable mutations leading to genetically associated diseases in offspring is thought to be very small for radiation doses of the magnitude that are associated with CT procedures. Such estimates of cancer and genetically heritable risk from x-ray exposure have a broad range of statistical uncertainty, and there is some scientific controversy regarding the effects from very low doses and dose rates as discussed below. To date, there is no evidence of genetically heritable risk in humans from exposure to x-rays. Under some rare circumstances of prolonged, high-dose exposure, x-rays can cause other adverse health effects, such as skin erythema (reddening), skin tissue injury, and birth defects following in-utero exposure. But at the exposure levels associated with most medical imaging procedures, including most CT procedures, these other adverse effects do not occur.

Because of the rapidly growing use of pediatric CT and the potential for increased radiation exposure to children undergoing these scans, special considerations should be applied when using pediatric CT. Among children who have undergone CT scans, approximately one-third have had at least three scans.

Radiation Dose from CT Examinations

The quantity most relevant for assessing the risk of cancer detriment from a CT procedure is the "effective dose". The unit of measurement for effective dose is millisieverts (abbreviated mSv). Effective dose allows for comparison of the risk estimates associated with partial or whole-body radiation exposures. It also incorporates the different radiation sensitivities of the various organs in the body.

Radiation dose from CT procedures varies from patient to patient. The particular radiation dose will depend on the size of the body part examined, the type of procedure, and the type of CT equipment and its operation. Typical values cited for radiation dose should be considered as estimates that cannot be precisely associated with any individual patient, examination, or type of CT system. The actual dose from a procedure could be two or three times larger or smaller than the estimates. Facilities performing "screening" procedures may adjust the radiation dose used to levels less (by factors such as 1/2 to 1/5 for so called "low dose CT scans") than those typically used for diagnostic CT procedures. However, no comprehensive data is available to permit estimation of the extent of this practice and reducing the dose can have an adverse impact on the image quality produced. Such reduced image quality may be acceptable in certain imaging applications.

Estimates of the effective dose from a diagnostic CT procedure can vary by a factor of 10 or more depending on the type of CT procedure, patient size and the CT system and its operating technique. A list of representative diagnostic procedures and associated doses are given in Table 1.

Table 1 - Radiation Dose Comparisons

1. Average effective dose in millisieverts (mSv) from McCollough CH, Bushberg JT, Fletcher JG, Eckel LJ. Answers to common questions about the use and safety of CT scans. Mayo Clin Proc. 2015;90(10):1380-92.

The effective doses from diagnostic CT procedures are typically estimated to be in the range of 1 to 10 mSv. This range is not much less than the lowest doses of 5 to 20 mSv received by some of the Japanese survivors of the atomic bombs. These survivors, who are estimated to have experienced doses only slightly larger than those encountered in CT, have demonstrated a small but increased radiation-related excess relative risk for cancer mortality.

Risk Estimates

The effective doses from diagnostic CT procedures are typically estimated to be in the range of 1 to 10 mSv. This range is not much less than the lowest doses of 5 to 20 mSv estimated to have been received by some of the Japanese survivors of the atomic bombs. These survivors, who are estimated to have experienced doses slightly larger than those encountered in CT, have demonstrated a small but increased radiation-related excess relative risk for cancer mortality.

The risk of developing cancer as a result of exposure to radiation depends on the part of the body exposed, the individual’s age at exposure, and the individual’s gender. For the purpose of radiation protection, a conservative approach that is generally used is to assume that the risk for adverse health effects from cancer is proportional to the amount of radiation dose absorbed and that there is no amount of radiation that is completely without risk. This conservative approach is called the “linear non-threshold” model. The amount of dose depends on the type of x-ray examination. A CT examination with an effective dose of 10 millisieverts (abbreviated mSv; 1 mSv = 1 mGy in the case of x-rays.) may be associated with an increase in the possibility of fatal cancer of approximately 1 chance in 2000. This increase in the possibility of a fatal cancer from radiation can be compared to the natural incidence of fatal cancer in the U.S. population, about 1 chance in 5 (equal to 400 chances in 2000). In other words, for any one person the risk of radiation-induced cancer is much smaller than the natural risk of cancer. If you combine the natural risk of a fatal cancer and the estimated risk from a 10 mSv CT scan, the total risk may increase from 400 chances in 2000 to 401 chances in 2000. Nevertheless, this small increase in radiation-associated cancer risk for an individual can become a public health concern if large numbers of people undergo increased numbers of CT screening procedures of uncertain benefit.

There is considerable uncertainty regarding the risk estimates for low levels of radiation exposure as commonly experienced in diagnostic radiology procedures. This is because the risk is quite low compared to the natural risk of cancer. At low doses, the radiation-related excess risk, which is thought to be proportional to dose, tends to be dwarfed by statistical and other variation in the background risk level. To obtain adequate evidence for a statistically valid estimate of cancer risk from exposure to low doses of radiation would require studying millions of people for many years. There are some who question whether there is adequate evidence for a risk of cancer induction at low doses. Some scientists believe that low doses of radiation do not increase the risk of developing cancer at all, but this is a minority view.

What is optical imaging?

Optical imaging uses light and special properties of photons to obtain detailed images of organs, tissues, cells and even molecules. The techniques offer minimally or non-invasive methods for looking inside the body.

What are the advantages of optical imaging?

- Optical imaging significantly reduces patient exposure to harmful radiation by using non-ionizing radiation, which includes visible, ultraviolet, and infrared light. Because it is much safer than techniques that require ionizing radiation, like x-rays, optical imaging can be used for repeated procedures to monitor the progression of disease or the results of treatment.

- Optical imaging is particularly useful for measuring multiple properties of soft tissue. Because of the wide variety of ways different soft tissues absorb and scatter light, optical imaging can measure metabolic changes that are early markers of abnormal functioning of organs and tissues.

- Optical imaging can be combined with other imaging techniques, such as MRI or x-rays, to provide enhanced information for doctors monitoring complex diseases or researchers working on intricate experiments.

What types of optical imaging are there and what are they used for?

Endoscopy uses an endoscope, which is a flexible tube with a light source to illuminate an organ or tissue. An endoscope can be inserted through a patient’s mouth and into the digestive cavity to find the cause of symptoms such as pain, difficulty swallowing, or gastrointestinal bleeding.

Optical Coherence Tomography (OCT) is a technique for obtaining sub-surface images such as diseased tissue just below the skin. Ophthalmologists use OCT to obtain detailed images from within the retina. Cardiologists also use it to help diagnose coronary artery disease.

Photoacoustic Imaging delivers laser pulses to a patient’s tissues; the pulses generate heat, expanding the tissues and enabling their structure to be imaged. The technique can help monitor blood vessel growth in tumors, detect skin melanomas, and track blood oxygenation in tissues.

Diffuse Optical Tomography (DOT) and Imaging (DOI) are non-invasive techniques that use light in the near-infrared region to measure tissue properties such as total hemoglobin concentration and blood oxygen saturation. Because DOT and DOI work well in soft tissue, the techniques are widely used for breast cancer imaging, brain functional imaging, stroke detection, photodynamic therapy, and radiation therapy monitoring.

Raman Spectroscopy relies on Raman scattering of visible, near-infrared, or near-ultraviolet light. The laser light interacts with molecular vibrations in the material, with shifts in energy revealing the material’s chemical properties. Applications include identifying chemical compounds and the structure of materials and crystals. During surgery, Raman gas analyzers are used to monitor the mixture of gases used for anesthesia.

Super-resolution Microscopy encompasses a number of techniques to obtain very high-resolution images of individual cells. One example is photoactivated localization microscopy (PALM), which uses individual fluorescently marked molecules to create a super-resolution image comprised of a compilation of individual molecules in a cell or tissue.

Nuclear medicine is a medical specialty that uses radioactive tracers (radiopharmaceuticals) to assess bodily functions and to diagnose and treat disease. Specially designed cameras allow doctors to track the path of these radioactive tracers. Single Photon Emission Computed Tomography or SPECT and Positron Emission Tomography or PET scans are the two most common imaging modalities in nuclear medicine.

DEXA stands for Dual Energy X-Ray Absorptiometry (or Densitometry). DEXA is a procedure that measures the amount of bone, muscle, and/or body fat.

The DEXA determination of bone density or body composition (muscle and fat composition) is performed with an instrument that uses low-energy X-rays. For bone density measurements, scans are typically performed of the lumbar (lower) spine, the hip, and the wrist. For body composition measurements, a scan of the entire body is performed. The DEXA instrument is very open, consisting of a table and scanning arm (in the shape of a ‘C') that houses the X-ray tube on one end of the arm and the detector on the other end.

There is no specific patient preparation for these studies. However, please consider the following regarding patient eligibility for DEXA studies:

• DEXA studies cannot be performed if the patient has had a nuclear medicine study within 1 week or an X-ray procedure using contrast within 72 hours. Therefore, DEXA studies should be scheduled before other X-ray or nuclear medicine procedures. If these other procedures are performed before the DEXA procedure and the time elapsed does not meet our protocol, the DEXA procedures will not be performed, and the study will be rescheduled.
  
• A woman of child-bearing age must have pregnancy ruled out before undergoing the test.
  Often, this means having a pregnancy test performed within 1 week of the DEXA procedure.
  The length of time in the clinic will depend on the number of procedures that are performed. Usually, the appointment is for either 1/2 hour or 1 hour.

The patient should wear light clothing to the clinic so he/she can change into a surgical scrub uniform for the study. After changing into a scrub uniform, the patient will have weight and height measured. For such measurements and for scans, a patient will remove shoes. The individual scanning procedures are performed with the patient either lying on top of or sitting next to the scanning table.

For each scan, the technologist must perform two steps. The first step is the scan (scan acquisition); the second step is analysis of the scan (scan analysis) to determine the information contained within the scan. The DEXA scans are therefore more complicated than standard X-rays. The DEXA scan is performed to actually measure the amount of bone, muscle, or fat.

After the technologist has completed and satisfactorily analyzed the scans, the patient can change back into street clothes and leave the clinic.

All scans are reviewed by one of the Nuclear Medicine physicians. A report of the DEXA scan results will be entered into the hospital's computer and record system. This is usually done the same day as the scan.

The controlled use of radioisotopes has advanced medical diagnosis and treatment of disease. Interventional radiologists are physicians who treat disease by using minimally invasive techniques involving radiation. Many conditions that could once only be treated with a lengthy and traumatic operation can now be treated non-surgically, reducing the cost, pain, length of hospital stay, and recovery time for patients. For example, in the past, the only options for a patient with one or more tumors in the liver were surgery and chemotherapy (the administration of drugs to treat cancer). Some liver tumors, however, are difficult to access surgically, and others could require the surgeon to remove too much of the liver. Moreover, chemotherapy is highly toxic to the liver, and certain tumors do not respond well to it anyway. In some such cases, an interventional radiologist can treat the tumors by disrupting their blood supply, which they need if they are to continue to grow. In this procedure, called radioembolization, the radiologist accesses the liver with a fine needle, threaded through one of the patient's blood vessels. The radiologist then inserts tiny radioactive "seeds" into the blood vessels that supply the tumors. In the days and weeks following the procedure, the radiation emitted from the seeds destroys the vessels and directly kills the tumor cells in the vicinity of the treatment.

Radioisotopes emit subatomic particles that can be detected and tracked by imaging technologies. One of the most advanced uses of radioisotopes in medicine is the positron emission tomography (PET) scanner, which detects the activity in the body of a very small injection of radioactive glucose, the simple sugar that cells use for energy. The PET camera reveals to the medical team which of the patient's tissues are taking up the most glucose. Thus, the most metabolically active tissues show up as bright "hot spots" on the images (Figure). PET can reveal some cancerous masses because cancer cells consume glucose at a high rate to fuel their rapid reproduction.

PET Scan

PET highlights areas in the body where there is relatively high glucose use, which is characteristic of cancerous tissue. This PET scan shows sites of the spread of a large primary tumor to other sites.

Magnetic resonance imaging (MRI) uses a large magnet and radio waves to look at organs and structures inside your body. Health care professionals use MRI scans to diagnose a variety of conditions, from torn ligaments to tumors. MRIs are very useful for examining the brain and spinal cord.

During the scan, you lie on a table that slides inside a tunnel-shaped machine. Doing the scan can take a long time, and you must stay still. The scan is painless. The MRI machine makes a lot of noise. The technician may offer you earplugs.

Before you get a scan, tell your doctor if you

• Are pregnant
• Have pieces of metal in your body. You might have metal in your body if you have a shrapnel or bullet injury or if you are a welder.
• Have metal or electronic devices in your body, such as a cardiac pacemaker or a metal artificial joint

Medical ultrasound falls into two distinct categories: diagnostic and therapeutic.

Diagnostic ultrasound is a non-invasive diagnostic technique used to image inside the body. Ultrasound probes, called transducers, produce sound waves that have frequencies above the threshold of human hearing (above 20KHz), but most transducers in current use operate at much higher frequencies (in the megahertz (MHz) range). Most diagnostic ultrasound probes are placed on the skin. However, to optimize image quality, probes may be placed inside the body via the gastrointestinal tract, vagina, or blood vessels. In addition, ultrasound is sometimes used during surgery by placing a sterile probe into the area being operated on.

Diagnostic ultrasound can be further sub-divided into anatomical and functional ultrasound. Anatomical ultrasound produces images of internal organs or other structures. Functional ultrasound combines information such as the movement and velocity of tissue or blood, softness or hardness of tissue, and other physical characteristics, with anatomical images to create “information maps.” These maps help doctors visualize changes/differences in function within a structure or organ.

Therapeutic ultrasound also uses sound waves above the range of human hearing but does not produce images. Its purpose is to interact with tissues in the body such that they are either modified or destroyed. Among the modifications possible are: moving or pushing tissue, heating tissue, dissolving blood clots, or delivering drugs to specific locations in the body. These destructive, or ablative, functions are made possible by use of very high-intensity beams that can destroy diseased or abnormal tissues such as tumors. The advantage of using ultrasound therapies is that, in most cases, they are non-invasive. No incisions or cuts need to be made to the skin, leaving no wounds or scars.

A method used to check the lymph system for disease. A radioactive substance that flows through the lymph ducts and can be taken up by lymph nodes is injected into the body. A scanner or probe is used to follow the movement of this substance on a computer screen. Lymphoscintigraphy is used to find the sentinel lymph node (the first node to receive lymph from a tumor), which may be removed and checked for tumor cells. Lymphoscintigraphy is also used to diagnose certain diseases or conditions, such as lymphoma or lymphedema.

Scientists at the Food and Drug Administration are studying the next generation of screening and diagnostic devices, some of which borrow from the world of entertainment. Soon, three-dimensional (3D) images in actual 3D might help your doctor find hidden tumors and better diagnose cancers, thanks to the regulatory work being done by a team at FDA’s Division of Imaging, Diagnostics, and Software Reliability.

The team is led by Division Director Kyle Myers, a physicist with a Ph.D. in optical sciences. It includes Aldo Badano, Ph.D., a world-renowned expert in display evaluation technology, and Brian Garra, M.D., a diagnostic radiologist doing research in regulatory science at FDA.

They are studying how clinicians receive visual information and analyze it to diagnose a disease. At the center of their research are breast cancer screening devices, which are making the leap from traditional two-dimensional (2D) screening such as mammography to 3D breast tomosynthesis, 3D ultrasound and breast computerized tomography (CT). This technology is very exploratory and years away from becoming standard in your doctor’s office.

New Era in Breast Cancer Detection

There are many new technologies being developed for breast cancer screening, especially 3D alternatives that may eventually replace today’s 2D mammography. FDA has already approved two of these state-of-the-art devices: The Selenia Dimensions 3D System, which provides 3D breast tomosynthesis images of the breast for breast cancer diagnosis; and the GE Healthcare SenoClaire, which uses a combination of 2D mammogram images and 3D breast tomosynthesis images.

The technologies under development include 3D breast tomosynthesis, which artificially creates 3D images of the breast from a limited set of 2D images. Tomosynthesis reveals sections of the breast that can be hidden by overlapping tissue in a standard mammogram.

“The problem of overlapping shadows has confounded breast cancer screening because mammograms don’t show cancers that are hidden by overlapping tissue,” Myers says. And compounding the problem is overlapping tissue that can look like cancer but isn’t. “The new technologies we’re studying overcome these barriers,” she adds.

Another benefit of 3D breast tomosynthesis: It’s more accurate than mammography in pinpointing the size and location of cancer tumors in dense breast tissue, Myers says. With 3D breast tomosynthesis, doctors can detect abnormalities earlier and better see small tumors because the images are clearer and have greater contrast.

“Clinical studies have shown that 3D breast tomosynthesis can increase the cancer detection rate, reduce the number of women sent for biopsy who don’t have cancer, or achieve some balance of these two goals of this new screening technology,” she adds.

There’s also a lot of research and development in 3D ultrasound, which automatically scans the breast and generates 3D data that can be sliced and examined from any direction. Garra, who is a leader in this field, says 3D ultrasound improves breast cancer detection in women with dense breast tissue.

“Both 3D breast tomosynthesis and 3D ultrasound detect breast cancer. But for radiologists and other doctors, there are many more images to examine, and that can reduce the speed at which studies can be interpreted,” he says.

Another promising technology—the dedicated breast CT system—creates a full 3D representation of the breast. The scan is taken while the patient lies face down on a bed with her breast suspended through a cup and the X-ray machine rotates around it. For patients, the procedure is more comfortable than regular mammography because the breast isn’t compressed. Also, there’s less radiation exposure than during a CT exam of the entire chest because only the breast is exposed to X-rays.

Health care practitioners using this technology have to learn how to read and interpret hundreds of high-resolution images produced by the scanner. But what makes the task easier is that the images have less distortion than mammography, and the system is optimized to differentiate between the breast’s soft tissue and cancer tissue.

“These images will be very different from 2D mammograms. They’re truly 3D images of the breast from any orientation. You can scroll through the slices—up and down, left and right—and get a unique view of the breast like never before,” Myers says. “It gives doctors tremendous freedom in how they look at the interior of the breast and evaluate its structures. It’s almost like seeing the anatomy itself.”

New Era in How We See

How can radiologists look at these images and convert them into three dimensions? That’s where Badano’s work comes in. His research lab is exploring various display device technologies to improve how radiologists review 3D images. The studied technologies include devices supported by mobile technologies and special-purpose 3D displays developed specifically for 3D imaging systems.

“These are no longer conventional images, so you need to examine them in the 3D space,” he says. “Using a 2D display might no longer be ideal.” Device manufacturers are building on technologies developed primarily for other markets, including the gaming industry, to show 3D images in actual 3D. But the work is painstaking and far from ready for a medical use.

“As people have experienced in movie theaters and when playing videogames, 3D displays have problems, including the image resolution and added noise. When wearing 3D glasses, our brain needs to separate the images from the left eye and the right eye and reconstruct a 3D object,” Badano says. “In the lab, we’re doing experiments to see how different technologies handle these tradeoffs.”

One of the challenges is that 3D displays for medical imaging require better resolution. For a medical use, the specifications are high—“and so are the stakes,” he adds.