Even with advances in technology, ultrasound can never be a gold standard for medical testing. According to the Merriam-Webster dictionary, a gold standard is something set up as an example against which others of the same type are compared.
There are multiple diagnostic imaging modalities for the diagnosis of disease. While there may be more than one modality that can diagnose a certain disease, every modality has its functions that it is best suited for.
An X-ray is a test using x-ray beams to create images. An x-ray is used to evaluate for bone fractures, lung conditions, breast cancer, bone cancer, and swallowed items. In some people a contrast is needed for clear imaging.
Computed tomography is a stronger version of an x-ray that is able to visualize all parts of the body. In a rather grotesque visual, imagine looking down on someone and cutting and removing horizontal slices from the head to feet and viewing each slice separately as an image. This is essentially what CT accomplishes. It can locate broken bones, brain hemorrhages, internal bleeding, infection, heart disease, and blood clots.
A venogram is another version of X-Ray using contrast solution to visualize the veins in the body. Ascending venography is used to diagnose deep vein thrombosis (its most common use) and descending venography is used to evaluate the function of the valves in the veins. Venography is an invasive procedure requiring an IV line for the contrast solution.
Angiography is similar to venography aside from the obvious fact: one test looks at veins and one looks at arteries. An angiography is used to locate narrowing or blockages in the arteries. Also considered invasive, an IV line must be inserted to apply the contrast solution.
Magnetic resonance imaging (MRI) creates images using a magnetic field and radio waves. It is best used to analyze the brain and spinal chord. It is also used to evaluate tendons and ligaments. The CT and MRI machines look very similar, however, they are very different in what they do. CT uses an x-ray beam whereas an MRI uses a magnetic field. They both require the patient to lie very still for long periods of time in a claustrophobic type environment. A contrast solution may be necessary for good visualization. Both CT and MRI have specific uses in which they have the best imaging quality.
Ultrasound…the final and most important part.
As stated in the above video, ultrasound uses high frequency sound waves to penetrate the body. The sound waves bounce back and create an image. This exam is typically not painful and noninvasive. There are several modalities of diagnostic ultrasound including but not limited to abdominal (evaluates organs and other areas in the abdominal region), vascular (evaluates arteries and veins throughout the entire body), cardiac (evaluates the heart, its valves, and function), and obstetric (evaluates the unborn fetus).
As far back as 1988, when diagnostic ultrasound was an up and coming technology, Roy Filly described ultrasound as “the stethoscope of the future“. The stethoscope was once used for many functions and to diagnose many diseases, however, many people in the medical field are completely unaware and untrained as to what it can all be used for. At a time when diagnostic ultrasound was uncommonly used and only for very basic things, Dr. Filly realized that it was headed in the same direction as the stethoscope had. He stated, “…sonography truly is the next stethoscope: used by many, understood by few.” Most emergency rooms and intensive care units (ICU) have small ultrasound machines for the doctors use. Unfortunately many of these doctors barely have a basic understanding of how ultrasound works and what they are looking for.
Used by many, understood by few.
While X-Ray and CT do not require the technologist to be able to diagnose the images they are acquiring, diagnostic ultrasound sonographers must be able to recognize and locate the pathology they are looking for. In the Best Practice and Research Clinical Anaesthesiology Journal, James C.R. Rippey and Alistair G. Royse discuss the use of diagnostic ultrasound in trauma. For emergency medicine residents, the amount of training includes an academic course and 200 scans. While this is adequate for doctors, the authors realized and stated that ultrasound is an operator dependent modality and to diagnose more severe diseases it should be performed by an ultrasound sonographer with extensive experience.
The training required for diagnostic ultrasound is 2 to 4 years of extensive schooling including classes such as ultrasound physics, medical terminology, hemodynamics, anatomy, and many more plus hands-on lab time. The course is finished with a seven to nine month clinical internship.
The limitations of diagnostic ultrasound are a main factor preventing it from becoming a gold standard in medical testing. Obesity is a primary restriction to ultrasound that is not easily fixed. The sound waves attenuate (lose power) the further distance they have to travel causing the object being imaged to either be poorly visualized or not visualized at all. Extensive edema (swelling) causes similar issues. Janet Cochrane Miller wrote a journal article in the Radiology Rounds newsletter in 2005 based on her research of how obesity affects diagnostic imaging modalities. The percentage of diagnostic ultrasound studies that were limited by obesity was 2%. The next highest modality affected by obesity was chest X-Ray at 0.5%.
Another limitation is the ability of the patient to tolerate the exam. Some patients are in extreme pain and are unable to handle the slight pushing of the ultrasound probe needed to produce clear images. In a study done by multiple doctors from the American College of Emergency Physicians on Ultrasound for Appendicitis, severe pain or abdominal guarding was a limitation of diagnostic ultrasound for the diagnosis of appendicitis. Abdominal guarding causes the patient to have a rigid abdomen preventing the sonographer from receiving optimal visualization. This limitation can sometimes be avoided by using different methods, however, it can still prevent the sonographer from getting the necessary images.
Orthopedic external fixators or dressings are also a limitation as ultrasound needs direct skin contact (aside from the gel) in order to send and receive sound waves. This limitation can also be easily avoided by removal of the dressing, however this is not always a possible solution as some surgical dressings cannot be removed. With removal of the dressing, direct contact to the wound (with the use of a sterile probe cover) may cause more pain than the patient can tolerate.
With abdominal ultrasound, bowel gas is a limitation that a very experienced sonographer may be able to avoid, however, it is difficult and sometimes impossible. Bowel gas can distort the sound waves giving an inaccurate image or completely obstruct the view.
Artery calcification in vascular ultrasound may reflect the sound waves and prevent the sonographer from receiving a complete and accurate image. A vascular ultrasound specific exam, ankle-brachial index, is especially limited by artery calcification. Ankle-brachial indices is the blood pressure at the ankle, divided by the blood pressure in the arm. This number is an indication of the arterial insufficiency in the leg. Calcification (caused mostly by diabetes) causes the artery to not compress, creating an abnormally high and inaccurate pressure.
Quantity versus quality is a problem seen in many hospitals and clinics nationwide as determined in the Journal of Diagnostic Medical Sonography in 1985. The demand for ultrasound exams is so high that the sonographers have a very limited amount of time to perform a large quantity of studies. This causes the studies to be rushed and increases the chance for mistakes. Quality Assurance is (or should be) used in all diagnostic medical testing facilities. A majority of labs will perform quality assurance monthly in that a couple random exams will be discussed and looked over. This creates an awareness for the sonographers to keep their exams up to par. It is also a good learning experience as it causes discussion and may bring up a question that someone may have had or teach him about something he had never experienced or seen. Another part of quality control is checking to make sure the ultrasound study on a patient is correlating with the results of a different test (CT, Angiogram, etc) or a previous study. When an ultrasound exam is tested against another modality for accuracy, it is often tested against the gold standard for that type of exam or diagnosis. Quality Assurance is also checked through the maintenance of the ultrasound machine.
A gold standard is something set up as an example against which others of the same type are compared.
So if diagnostic ultrasound isn’t considered a gold standard, what is? That all depends on which type of diagnosis one is looking for. In the diagnosis of abdominal aortic aneurysm, as stated by the Journal of Diagnostic Medical Sonography, computed tomography is considered the gold standard. In the diagnosis of deep vein thrombosis, venogram is considered the gold standard. In the diagnosis of appendicitis, computed tomography is the gold standard. In the diagnosis of peripheral vascular disease, angiography is the gold standard. Every known disease has its own gold standard of medical testing for diagnosis. Currently diagnostic ultrasound, while commonly used, is not a gold standard for any particular one.
As stated by Jurgen A. H. R. Claassen, “The concept of a ‘golden standard’ implies a level of perfection that can never be attained by any biological test, and will provoke criticism like that ventilated by Duggan. In contrast, a gold standard in its true meaning, derived from the monetary gold standard, merely denotes the best tool available at that time to compare different measures.”
The gold standard is not the perfect test but merely the best available test. – E Versi
Versi later goes on to say “It is the absolute truth that is never reached; gold standards are constantly challenged and superseded when appropriate.” The question is…will there ever be a day where ultrasound will supersede a gold standard to become the most accurate modality? B. Benacerraf looks into the future of ultrasound and its new role with 3D imaging. While she sees 3D imaging as being able to rival MRI (magnetic resonance imaging) there is no mention of it surpassing MRI. The future of ultrasound, however, is very exciting indeed! A new method is in progress labeled “fly through” allowing the sonographer to see a blood vessel as though going through it as one would a slide or tunnel. This amazing feature will allow for greater accuracy in the diagnosis of peripheral vascular disease. With these up and coming features ultrasound will remain a high contender for medical testing modalities and will always be the most commonly sought after exam. While these advancements are exciting, it should be remembered that ultrasound is not the only testing modality that will improve technologically. It is for this reason and the previous mentioned limitations of ultrasound that lead me to believe that though a “platinum standard” for a majority of medical tests, ultrasound can never be a gold standard for diagnostic imaging.
1. Merriam-Webster Online Dictionary copyright © 2014 by Merriam-Webster, Incorporated. Gold Standard Definition. http://www.merriam-webster.com/thesaurus/gold%20standard
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