3D neutron-based medical imaging, 4D lung scans, and hitting a moving tumor


Medical physicists meet in Pittsburgh

College Park, MD, July 12, 2004 -- The American Association of Physicists in Medicine (AAPM) will hold its 46th annual meeting on July 25-29 in Pittsburgh, PA at the David L. Lawrence Convention Center. Approximately 1,000 abstracts will be presented on a variety of subjects at the intersection of physics and medicine. Many of these topics deal with the development of state-of-the-art imaging and therapeutic devices, and the new techniques that go along with them.


This news release begins by summarizing some themes of the meeting, then provides a brief introduction to medical physics (including its connection to last year's Nobel Prize for magnetic resonance imaging) and finally contains detailed highlights of seven papers/sessions at the meeting.


Highlights at the meeting include: the first 3D pictures from a neutron-based imaging technique; an MRI-based method that monitors a drug's effectiveness in combating a tumor's blood supply; and a technique for targeting a tumor that moves as a patient breathes. Some general themes at this year's meeting include: the emergence of "4D scans" to improve the imaging and treatment of cancer; the development of powerful "fusion imaging" that can simultaneously show an organ's structure and function; and a far-ranging symposium on the ultimate frontiers of medical imaging and the future of radiation therapy. Additional highlights include a symposium, directed by Ehsan Samei of Duke University ([email protected]), on how medical physicists can better apply their deep knowledge of physics concepts to the science of medical diagnosis ("From Physics To Medicine," Tuesday, 10AM-12PM); and a computer-aided diagnosis symposium, directed by Maryellen Giger of the University of Chicago ([email protected]), which will showcase examples of how software automatically helps detect cancer and other diseases ("CAD," Tuesday, 4:00-5:00 PM).


Physics and medicine are close allies. Ever since the discovery of X rays and their potential for medical imaging, physicists have been vital to the advancement of medicine. Fundamental research in optics, acoustics, electromagnetism, and particle and nuclear physics has led to an array of indispensable medical tools. Magnetic resonance images (using microwaves), CAT scans (using X rays), PET scans (using gamma rays), ultrasound scans (using sound waves) and various types of radiotherapy are among the physics-based devices that help doctors diagnose and treat ailments ranging from broken bones to cancer. Modern medicine has benefited significantly from medical physics research, which thus far has led to three Nobel Prizes in Medicine/Physiology.

AAPM includes more than 5,000 members dedicated to advancing medical technology. Medical physicists working in radiation therapy commission and develop new therapeutic techniques; collaborate with radiation oncologists to design improved cancer-treatment plans; and calibrate and model therapeutic equipment to ensure that every patient receives precisely the prescribed dose of radiation at the correct location. Medical physicists contribute to the effectiveness of radiological imaging procedures by developing new imaging procedures, improving existing techniques, and assuring radiation safety of imaging procedures. Physicists working in medical imaging inspect, model and test equipment to ensure that images are acquired at the highest possible quality for effective diagnosis of possible abnormalities.


Last year's Nobel Prize in Physiology/Medicine was awarded for discoveries leading to magnetic resonance imaging (MRI). What role did medical physics play in the birth of this now widespread imaging technique? The discovery and development of MRI came about in large part from years of prior research that by today's definition falls well within the core of the discipline of medical physics. Furthermore, medical physicists successfully refined MRI instrumentation and software, and integrated it into real-world medical environments such as hospitals. When commercial machines became available in the early 1980s, medical physicists educated thousands of physicians on how to use MRI through workshops and influential journal articles. They led MRI societies and committees that helped to develop truly useful clinical applications of the technique. Medical physicists took the lead in defining and developing quality-assurance standards for both the instruments and the individuals who operate MRI equipment. Today, medical physicists work in medical settings to ensure that MRI images are as clear, informative, and high-resolution as possible. As part of teams, they develop new imaging methods, design state-of-the-art machines, and ensure the safety and comfort of the MRI procedure.


The following is a sampling of some of the many intriguing talks that medical physicists will present at the meeting.


To take pictures of the body, medical professionals conventionally use X rays, magnetic fields (MRI), ultrasound, and in some cases, radioactive isotopes (PET scans). Now, Duke University physicists and radiologists have produced the first 3D pictures from a new technique that employs elementary particles called neutrons.

Why use neutrons for medical imaging? Compared to other particles, neutrons are highly penetrating, and therefore can image deeply buried body structures that cannot be reached by other probes. In addition, neutrons can easily identify almost every naturally occurring chemical element in the body. Called Neutron Stimulated Emission Computed Tomography (NSECT), the technique involves illuminating the body with fast neutrons (those with energies between 1 and 10 MeV). The neutrons cause the nuclei of atoms and molecules in the body to emit gamma-ray photons with distinctive energies that depend on the specific chemical identities of the atoms and molecules to which the nuclei belong.

At the AAPM meeting, Carey Floyd ([email protected]) will present the first 3-D images ever reconstructed from the emission of characteristic gamma rays stimulated by fast neutrons. The images, of an iron-copper sample, demonstrate the technique's ability to completely distinguish between the iron and copper that made up the object.

With further development, NSECT could potentially diagnose breast cancer early by looking for differences in the concentration of trace elements that are known to exist between benign and malignant tissue. NSECT could identify cancer by the way it changes concentrations of chemical elements in tissue long before the cancer has begun to cause the anatomical changes (such as the formation of dense tumors or microcalcifications) that are detected by conventional methods. The researchers estimate that an NSECT clinical system, if successfully developed, could cost a fraction of a typical clinical CT system.

While an individual neutron is more damaging to the body than a single x ray of equal energy, the researchers' preliminary calculations indicate that an accurate test for breast cancer could be performed at a dose similar to that of a current mammography examination. As an intermediate step towards this goal, the group next plans to develop a prototype system that can image the distribution of iron in the liver in order to diagnose hemochromatosis (iron overload in the liver) without the need for a biopsy. (Paper WE-D-315-6, Wednesday, July 28, 2:45 PM.)

II. COMBATING TUMORS BY UNDERSTANDING THEIR VASCULATURE is a specialty of Jeffrey Evelhoch, who works at the Pfizer labs in Ann Arbor, Michigan. Compared with the blood supply system of healthy tissue, a tumor's vasculature is more chaotic in its geometry and its vessels are wider and leakier. Knowing this, a researcher can perhaps tailor an anti-cancer drug aimed at holding down angiogenesis, the formation of new blood vessels in the tumor, that is less toxic (because it targets the more sensitive tumor) than older drugs. The method Evelhoch ([email protected]) used to evaluate drugs designed to exploit the weaknesses in the tumor vasculature is a process called dynamic contrast-enhanced (DCE) MRI, in which MRI scanning is performed before, during and after the injection of a contrast agent. From this a quantitative measure of the pharmacodynamic effectiveness of the treatment can be achieved. (Paper WE-D-305-2, Wednesday, July 28, 2 PM.)

III. FIRST, DO NO HARM is the injunction followed by medical doctors. In the realm of treating the body with radiotherapy the equivalent slogan might be "Do the least harm to healthy tissue while doing maximum damage to tumors." Since healthy tissue cannot always be spared injury during treatment, it is helpful to know which healthy tissue is the most important to the survival of the patient, so that the delivered radiation can be steered away. Conversely, the important part of tumors can be singled out for attention. To accomplish all of this, functional PET and MRI imaging---medical imaging that provides information not just about the spatial location of tissue but also its function---is vital. Further, it is important to understand how each region of a normal organ responds to radiation, such that predictions can be made about the anticipated degree of normal tissue injury. At the meeting, Lawrence Marks of Duke University ([email protected]) will report on his work using functional imaging to minimize and monitor radiation-induced normal tissue injury. The Duke results, based on several hundred patients, is one of the largest experiences exploiting this approach. (Paper WE-D-305-1, Wednesday, July 28, 1:30 PM.)

Breast cancer is the second leading cause of cancer death in American women. To better detect and diagnose breast cancers, Tao Wu of Massachusetts General Hospital/Harvard Medical School ([email protected]) and his colleagues are merging two breast-imaging techniques: contrast enhancement and digital breast tomosynthesis. The combined method can also potentially improve the ability to detect breast lesions, as well as distinguish between benign and malignant lesions.

An emerging 3D imaging technique, digital breast tomosynthesis (DBT) has recently been shown in studies of over 400 women at the Massachusetts General Hospital to provide much clearer images than conventional 2D mammography. DBT unmasks cancers that are ordinarily obscured by normal tissue on traditional 2D mammograms. Contrast imaging involves the injection of an agent, such as iodine (in x-ray imaging) or gadolinium (in MRI), that concentrates in abnormal breast tissue and "lights up" those regions in subsequent images.

Combining 3D DBT and contrast-enhanced imaging in recent experiments, Wu and colleagues obtained DBT images of a breast tissue specimen before and after it was injected with an iodine-based contrast agent. The pre-injection image was subtracted from the contrast-enhanced image, clearly revealing the precise distribution of the contrast agent. Contrast-enhanced regions of the specimen were more clearly displayed and structures more sharply defined on DBT images.

To reach the goal of clinical in vivo imaging, some practical issues need to be studied, such as the effect of breast compression and making sure pre- and post-injection images are properly aligned with one another so that the latter can be correctly subtracted from the former. (Paper TU-E-317-4, Tuesday, July 27, 4 PM.)

To prepare cancer patients for radiation therapy, medical physicists have developed a new tool called the "4D scan," which yields a 3D image of a tumor while tracking a patient's motions in the fourth dimension---time. A 4D scan provides a precise, stable location of a tumor--since the data from the "fourth dimension" can correct for image blurring and other distortions caused by a patient's breathing and general movements. 4D imaging has recently been introduced for CT scans, but has not been available for other very important imaging methods.

Speaking at the meeting will be two independent groups that are testing 4D versions of positron emission tomography (PET), an imaging technique particularly useful for spotting lung tumors. By using a radioactive tracer to produce images inside the body, PET distinguishes regions within a collapsed lung that are cancerous and that would otherwise appear as a uniform gray area on CT. PET also detects lymph nodes that are involved in the cancer; such "involved" nodes may be too small to detect with CT.

The two independent groups, from the Washington University School of Medicine in St. Louis (Dan Low, [email protected]) and the MD Anderson Cancer Center in Houston (Osama Mawlawi, [email protected]) use hybrid PET-CT machines. A CT scanner first maps the motion of all organs and the tumor while the patient is breathing, then a PET scanner gets detailed information on the tumor. Because the researchers know the motion of the organs and tumor from the CT scan, they can reposition the data in the PET scans to motion-correct the image. While differences exist in the two groups' approaches, the teams have together validated the 4D PET approach in phantoms (materials that simulate tissue) and in small-scale patient studies. (Papers MO-E-315-2, Monday, July 26, 4:15 PM, and TU-D-BRB-1, Tuesday, July 27, 1:30 PM.)

Oncologists have a new way to plan cancer-fighting radiation treatments: with the advent of 4D CT scans that show how a tumor moves as a patient breathes, cancer can now be targeted more precisely and efficiently. By tracking a tumor's motion, doctors may soon be able to adjust the radiation dose during treatment. A group of researchers from Massachusetts General Hospital, including Alexei Trofimov ([email protected]), developed software to create cancer treatment plans based on 4D CT, delivering the best mix of possible methods.

In one radiation treatment approach, separate doses may be created for different phases of a patient's breathing motion, synchronizing the delivery with the motion of the target tumor, so that the dose is only delivered when the tumor opens a "gate" by moving to a certain position--for example, only when the patient exhales. The disadvantage of a "gated" treatment is that it would take a significantly longer time to deliver the needed radiation. Or, organ motion could work to the patient's benefit--if the tumor's path is very well known, treatment could be adapted to the motion of the tumor. Having a moving target would actually improve the result of treatment, which is usually not the case with conventional plans.

If the treatment is based on the assumption that a tumor will move in a certain way and it does not, "the result may be just as bad as when we wrongly assume that there's no motion," said Trofimov. With motion-adaptation, the dose can become stronger if the tumor moves according to plan. "It's sort of like spray-painting in the wind -- one has to aim differently," said Trofimov, whose work has won AAPM's Jack Fowler Junior Investigator Award, given to a researcher who has been in the field less than four years.

The group's preliminary calculations show that a combination of "gating" and "motion-adaptation" might be the best approach for a physician to plan each treatment, case-by-case. (Paper TU-C-BRA-2, Tuesday, July 27, 10:10 AM.)

A highlight of every annual AAPM meeting, the President's Symposium features visionary speakers who look at future trends in medical physics. The 1982 symposium included a presentation by Paul Lauterbur, who went on to share last year's Nobel Prize for magnetic resonance imaging. This year, Andrew Maidment of the University of Pennsylvania ([email protected]) will present a talk called "Nine Orders of Magnitude: Imaging from Man to Molecules." Describing how medical imaging has shifted from the scale of the organism to the scale of the organ, Maidment will discuss how medical physicists will shift their focus from imaging cancerous lesions the size of a cubic centimeter, or a billion cells, to identifying single tumor cells. "The future of medical physics will be tied to such advances," he says. Describing the dramatic technological change over the last 10 years in how radiologists read the results of an imaging scan, Eliot Siegel of the University of Maryland ([email protected]) will explain how the shift from reading 2D films to viewing 3D computer reconstructions offers new freedoms but also contains potential challenges. For example, the flood of information from 3D imaging may make it easier to miss important parts of the image data. Finally, in a paper called "The Future of Radiotherapy," T. Rockwell Mackie of the University of Wisconsin ([email protected]) predicts that the use of protons and light ions such as carbon ions in radiation therapy will grow, as the costs of facilities with those tools is expected to be lower. (Session MO-C-BRB, Monday, July 26, 10 AM-12 PM.)


The AAPM meeting webpage (http://www.aapm.org/meetings/04AM/) contains links to the full program, plus a Virtual Pressroom with more information on the scientific program as well as announcements by the many medical-physics exhibitors at the meeting. Reporters interested in getting a complimentary press badge for the meeting should fill out a registration form by July 16 at http://www.aapm.org/meetings/04AM/documents/PressReg.pdf Even if you can't make it to Pittsburgh, the contact information and Virtual Pressroom will help you to cover meeting highlights from your desk. For assistance in contacting researchers and setting up interviews, please do not hesitate to contact Ben Stein.

Source: Eurekalert & others

Last reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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