Positive or negative contrast media

Contrast media are so called as they increased the image contrast of anatomical structures which are not normally easily visualised. Contrast media can be grouped as positive or negative, in general positive contrast media are those which have an increased absorption of x-rays and show up as white/grey and negative which have less absorption and show up as dark/grey. An example of each are: positive contrast media – iodinated compounds, negative contrast media gaseous – air or carbon dioxide.

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3 comments on “Positive or negative contrast media

  1. Peak kilovoltage (kVp) is the maximum voltage applied across an X-ray tube. It determines the kinetic energy of the electrons accelerated in the X-ray tube and the peak energy of the X-ray emission spectrum. The actual voltage across the tube may fluctuate.

    kVp controls the resulting photographic property known as “radiographic contrast” of an x-ray image (the amount of difference between the black/whites). Each body part contains a certain type of cellular composition which requires an x-ray beam with a certain kVp to penetrate it. The body part is said to have “subject contrast” (that is, different cellular make up: some dense, some not so dense tissues all within a specific body part). For example: bone to muscle to air ratios in the abdomen differ from that of the chest area. So the subject contrast is said to be higher in the chest than in the abdomen. In order to image the body so that the maximum information will result, higher subject contrast areas require a higher kVp so as to result in a low radiographic contrast image and vice versa.

    Although mAs is the primary controlling factor of radiographic density, kVp also affects the radiographic density in a round about way. As the energy (kVp) of the stream of electrons in the x-ray tube increases, the more likely the x-ray photons created from those electrons will penetrate the cells of the body and reach the image receptor (film or plate), resulting in increased radiographic density (compared to lower energy beams that may be absorbed in the body on their way to the Image Receptor). However, “scatter radiation” also contributes to increased radiographic density; in that, the higher the kVp of the beam, the more scatter will be produced. Scatter is unwanted density (that is, density created without bringing any pertinent information to the image receptor). This is why kVp is not primarily used to control density – as the density resulting from increasing kVp passes what is needed to penetrate a part, it will only add useless information to the image.

    Increasing mAs causes more photons (radiation)of the particular kVp energy, to be produced. This is helpful when larger parts are imaged, because they require more photons. The more photons you can get to pass through a particular tissue type (whose kVp is interacting at the cellular level) will result in a statistically increased amount of photons reaching the image receptor. The more photons that pass through a part, and reach the image receptor with pertinent information – the more useful the density is created on the resulting image. Conversely, lower mAs creates less photons, which will decrease density, but is helpful when you image smaller parts.

  2. Sherri Leclair was 45 when her family doctor in Mississauga suggested she have a mammogram, even though Ontario does not recommend the test until age 50 and she has no family history of the disease. It was a fateful decision.

    The physician didn’t expect anything untoward, but the test showed an abnormal mass in Leclair’s breast. The marketing specialist eventually underwent surgery, the lumpectomy removing a small but aggressive tumour.

    After chemotherapy and radiation, Leclair is now cancer-free, and eternally thankful for her doctor’s advice.

    “I feel I was the luckiest person in the world that I had that mammo­gram,” she said. “My daughter turned 10 the day after I started chemo, and I like to think my family are happy I had that mammogram, too … If I hadn’t, I felt it would have been a big mistake.”

    Women like Leclair are at the heart of a fierce debate about the merits of such tests for younger women, reignited last week when an independent task force recommended against regular mammograms for those between 40 and 49, saying they had few benefits and considerable risks.

    The report had barely made it into print when a chorus of expert voices lit into it, arguing that mammograms for younger women are far more effective than the task force suggested, and that scaling back their use would cost thousands of Canadian lives.

    The proposals “can be very dangerous, because they can cause doctors to give their patients the wrong advice,” complained Dr. Martin Yaffe, a diagnostic-imaging scientist at Toronto’s Sunnybrook Research Institute.

    Leclair said she has no position on the clash of expert opinions, but expressed a reaction that many share: “I’m confused.”

    In some ways, the dispute over when women should get mammograms comes down to a personal judgment over hard numbers — on mammography’s life-saving benefits, and the needless tests and treatments that constitute its downsides.

    Muddying the waters is a disagreement about what those numbers really are, which scientific evidence to trust and whether the discussion is being coloured by vested interests — of the radiologists paid to analyse mammograms and the industry that makes the mammography scans.

    “I think it’s really important to understand that this is not just about the emotion of breast-cancer advocates wanting more people screened,” said Sandra Palmaro, CEO of the Ontario branch of the Canadian Breast Cancer Foundation. “This is about scientific evidence.”

    Of course, the Canadian Task Force on Preventive Health Care, the group that issued this week’s report, claims a similar evidence-over-emotion approach to defend its controversial recommendations, condemned since by the foundation.

    Appointed by the Public Health Agency of Canada to give independent advice on such questions, the panel of scientists said it perused the best evidence possible to divine the pros and cons of the mammogram, a specially designed, low-dose X-ray of the breast.

    It’s generally agreed that women over 50 should be given mammograms regularly to catch breast cancer early, when treatment is more likely to succeed. The battleground is over screening those 40 to 49.

    To determine its benefits, the panel looked at years of clinical trials in which women were chosen randomly to either receive the tests or not, considered the “gold standard” of medical evidence.

    The panel concluded that the number of deaths over 10 to 11 years from breast cancers that developed during the 40s are reduced by 15 per cent in women who are screened every two years.

    Put another way, it means 474 lives are saved for every million women screened over a decade. From yet another perspective, 2,108 women in their 40s would need to be screened every two years or so for 11 years to prevent one breast-cancer death, the task force concluded.

    At the same time, 690 of those 2,100 would get false positive results on their mammograms leading to more, unnecessary testing, with 75 having breast biopsies that are not needed. What is more, about 10 of those 2,100 will be given unwarranted lumpectomies or mastectomies, for cancers that would never have killed them. Those harm figures are gathered from less-rigorous “observational” studies, because the randomized trials tend not to pick up the negative effects, the report says.

    By comparison, 720 women from ages 50 to 69 need to be screened to save one life, with 204 false positives.

    For women in their 40s, the harms of regular screening outweigh the benefits, the panel argued.

    Still, the group is merely giving advice in a way it hopes lay people will be able to grasp, Dr. Marcello Tonelli, the task force chairman, said in an interview. Patients in provinces without screening programs for 40-somethings — about half of them — can still ask their doctor for one, Tonelli said.

    Unlike critics who work in the radiology field, the scientists on the task force had no conflicts, perceived or otherwise, he said.

    The task force is not alone. Among others, the group’s U.S. and British counterparts have come to similar conclusions in recent years. The Canadian Cancer Society, the College of Family Physicians of Canada and Health Canada also recommend regular screening only for women 50 and up.

    Critics, however, say the report was marred by a fundamental flaw — the clinical trials it considered were mostly older ones, when mammo­graphy equipment and techniques were much less exacting. More recent research, including a B.C. study published in 2006, found regular mammograms cut the death rate by about at least 25 per cent, not 15 per cent.

    Based on the number of deaths per year from breast cancers that developed in the forties, 2,000 Canadian lives could be saved over 10 years with screening, said Yaffe.

    Most of those newer experiments were observational studies — where researchers looked at the effects of real-life screening programs, rather than randomly setting up groups to compare. But randomized trials are no longer possible in the field, because woman will not now agree to possibly forgo mammograms, Yaffe said.

    As for the downsides of screening, patients do not seem to mind going through repeated testing after a positive mammogram result, said Dr. Nancy Wadden, a spokeswoman for the Canadian Association of Radiologists. “I have not met one patient who was upset,” said the St. Johns, N.L., radiologist. “On the contrary, they are more than happy to have had the level of care that in the end ruled out cancer.”

    She also rejects the notion that radiologists, whose association is partly funded by the makers of mammography and other diagnostic-imaging gear, have a natural ­bias in favour of the tests.

    Wadden said she didn’t enter medicine to make money but, regardless, could earn much more analysing CT scans or MRIs than specializing in breast imaging.

    Critics of the task force say the more recent observational studies showing more benefit from screening are reliable. Tonelli counters that they are inherently biased, partly because the type of women who conscientiously take part in screening programs tend to be more health-conscious, and less likely to die. Moreover, he said he found within three minutes a handful of recent observational studies in major journals that suggested screening younger women had, in fact, even less benefit than the task force suggested. “It’s easy to selectively highlight a few studies that support one’s point of view,” he said.

    An oncologist who specializes in treating women with breast cancer said the report seemed to him a “high-quality piece of work,” though he noted the evidence on screening is generally mixed, and even the task force called it “weak.” What would help is to better focus screening on women most at risk of cancer, including those with family histories of breast cancer or a genetic susceptibility, said Dr. Eitan Amir of Toronto’s Princess Margaret Hospital.

    “There is no doubt that breast screening works in a substantial number of patients. It also harms a substantial number of patients,” he said. “If you’re involved in screening, you’re at risk.”

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