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Diagnostic Testing Demystified

By Dr. Gavin Webb, Wentworth Health Partners Seacoast Orthopedics & Sports Medicine

After an injury or to evaluate a musculoskeletal problem, it is common that various diagnostic tests may be ordered. Between x-rays, MRIs, CT scans, EMGs, and U/S, it’s a bit of an alphabet soup and may be confusing to know why a certain test may be ordered. Hopefully the following information will help to clear up any questions that arise about the various tests.

X-rays: Most people are familiar with x-rays (also called radiographs). Discovered in 1895, x-rays are a form of electromagnetic radiation. To create an image, a Xray generator produces a beam of x-rays that is projected through an object. A certain amount of the x-rays are absorbed by the object and the rest that pass through it are captured on the other side by a detector. Traditionally this was on photographic film, but his has largely been replaced by digital detectors which can display the images on a screen. There is a very low dose of radiation with traditional x-rays. X-rays are usually the first test that is ordered and show bones very clearly.

CT scan  (Computed Tomography): Previously called a “CAT scan” this is a more advanced form of radiography in which the x-ray source and detectors rotate around the object producing a conical Xray beam. The  object is moved through this cone and is penetrated by many different beams which are detected and then computed to generate 2 and 3 dimensional images. Modern CT scans produces much less radiation than original units, but still significantly more than regular x-rays. CT scans give more detail about both bone and soft tissues. 

MRI (Magnetic Resonance Imaging): Scanners use strong magnetic fields to detect radio frequency energy from cells in tissues and create detailed images of both bone and soft tissues. MRIs are usually more helpful in evaluating musculoskeletal soft tissues than CT scans. It does not produce any radiation, but patients with certain implants like pacemakers cannot have MRIs. MRIs are commonly used to visualize joints to detect injuries and abnormalities. Sometimes contrast dye is injected in the joint prior to the MRI to be able to see certain structures more clearly (MR arthrogram). Metal implants may cause artifact on the study and obscure part of the joint, making it less useful. 

U/S:  Musculoskeletal ultrasound uses high frequency sound waves that are pulsed into tissue using a probe. The ultrasound waves echo off different tissues with different reflection properties and return to the probe which then displays the images on a screen. This is the same technology used by obstetricians to evaluate the fetus during pregnancy. MSK ultrasound can be used for diagnostic purposes to evaluate for abnormalities in the soft tissue. It can also be used to direct a needle for injections or aspirations and allows for a precise needle placement. It can also be used for dynamic studies that can show how tissues respond in a moving joint. 

EMG/NCV – Electromyography is a technique for evaluating the electrical activity that muscles produce. Nerve conduction velocities measure the conducting function of nerves. When done together, these tests can be helpful to determine whether the source of pain or weakness may be directly caused by a muscle or nerve injury or abnormality. EMG/NCV tests are commonly used to evaluate spine conditions and other problems like carpal tunnel syndrome where a nerve may be compressed. The electrical signals are analyzed to determine any abnormalities in either the muscles or nerves. Parts of the test involve electrical stimulations and may be somewhat uncomfortable.

Q: “I know I didn’t break a bone when I hurt my knee. Why do I need x-rays if the MRI is what is going to show the injury?”
A: X-rays also give important information about bones and joints and can show causes of pain like arthritis, calcium deposits, tumors, and alignment issues. Also, insurance usually requires obtaining x-rays before authorizing an MRI

Q: “Is getting a CT or MRI painful? 
A: Both tests involve getting the specific body part in a tube which may be uncomfortable. The MRI is generally a smaller tube, and the scans takes longer and are noisy. Both CT arthrograms and MR arthrograms involve injecting dye into the joint which can be painful. 

Q: “My doctor ordered an ultrasound guided cortisone shot. How is that different than a regular injection?”
A: The ultrasound allows the needle to be guided to the exact spot to inject the medicine. This makes it more accurate and effective and, in some cases, safer than doing to injection without guidance.

About Dr. Gavin Webb: A graduate of Williams College in Massachusetts, Dr. Webb earned his medical degree at the University of Vermont School of Medicine and awards for overall performance, research, and commitment to orthopedic education, during his orthopedic surgery residency.Webb_Gavin.jpg

While completing his sports medicine fellowship at New England Baptist Hospital, Dr. Webb was an assistant team physician for the Boston Celtics and the Northeastern Huskies.

Dr. Webb has expertise in cartilage restoration procedures and alternatives to total knee replacement surgery in younger patients. He has conducted research and lectured on cartilage injuries, presented papers at national and international meetings, and has been a reviewer for The Journal of Arthroscopic and Related Surgery.

Treating conditions affecting the shoulder and knee, Dr. Webb has a special interest in sports medicine and in the prevention and treatment of athletic injuries in patients of all ages. He is also an Affiliate Professor at the University of New Hampshire, and has been a Team Physician for the University of New Hampshire Wildcats since 2005. 

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