Marlena Jbara, MD discusses basic ultrasound principles and techniques, most common indications for the use of ultrasound in the foot and ankle, as well as reviews the American College Radiology Guidelines for imaging of soft tissue masses.
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TAPE STARTS – [00:00]
Marlena Jbara: Hi, my name is Marlena Jbara and I am an MSK radiologist at Northwell Health at Staten Island University Hospital. For the next section, we will be discussing the use of diagnostic musculoskeletal ultrasound as it applies to the foot and ankle. Disclosures: I or related party have no financial relationship to disclose. The objectives in this lecture will be to provide an overview of ultrasound principles and technique to apply to common pathologies of the foot and ankle. We will review the most common indications for the use of ultrasound in the foot and ankle conditions and I will take sometimes to review the American College of Radiology guidelines for imaging of soft tissue masses, a very important section. The power of ultrasound is that it can differentiate a cyst versus a solid lesion. In addition, it can evaluate whether or not this lesion has a vascular supply or whether it's hyperemic or inflamed. And these are going to be very useful tools to determine whether or not we have to go on to advanced imaging to prove the presence of tumor or otherwise. It can be extremely useful for image-guided biopsy and aspiration, especially in the foot and ankle given the superficial location of many of the pathologies. Of course, lesions like lipoma which are homogenously comprised of fat can be seen based on its echogenic pattern.
Lesions that have vascularity, hemangiomas, arterial venous malformations, capillary telangiectasias these can all be evaluated given the presence of Doppler to associate the vascular supply and determine whether or not there has been angiogenesis in this lesion. Furthermore, nerve sheath tumors can be identified. We can see the split fat pad sign and other signs, which I will show you in later slides. And of course, I am here to remind us all that it is operator dependent and it is dependent on the expertise of the radiologist and the technologist obtaining images. What is ultrasonography and how does it work? So ultrasound is essentially a longitudinal pressure wave. It is generated in a device called the transducer, which is the ultrasound probe. Inside the transducer, there is a spinning crystal that when electricity is applied to the crystal, the crystal moves. This movement produces sound waves and it's unique and that these sound waves will be used to identify anatomy and pathology rather than ionizing radiation as used by CAT scans and x-ray. So the transducer containing this piezoelectric crystal that spins is placed on the skin surface with a coupling gel called an ultrasound gel. And then the positioning of the probe to determine what structure we are looking at whether it's a deep structure or superficial structure, whether it can be taken in with the field of view or it has to have an extended field of view, all of that's how we begin how it works to perform an ultrasound exam.
The physics as I alluded to in the last line is that the ultrasound transducer and unit send an electrical signal to the transducer resulting in a sound wave. The electrical sound signal is converted to ultrasound energy and back again as the signal, which is sent into the patient through the transducer reflects off the different tissue densities. As this pulse reflects off the various tissue densities, it is converted back to an electrical impulse, which is captured again by the transducer, therefore producing an image, which can be viewed on the monitor. Images produced, the individual body tissue offers a natural resistance to the transmission of the ultrasound impulse. This is directly proportional to the average density of the tissue, which will be extremely useful in determining what tissue we are dealing with and various shades of gray and bright or echogenic signals and those anechoic or devoid of any contrast can become displayed on the image. Image quality, an important consideration is the frequency of the transducer used for MSK imaging. For podiatric use, the frequency of the probe is between 12 to 18 mHz. There are some important features to know regarding the frequency of the probe and that the higher the frequency the higher the resolution in the near field, but this is at expense of the depth of penetration.
Therefore a higher frequency is used for a superficial tendon or joint. An 18 mHz and a lower frequency for the evaluation of the plantar fascia at 14 mHz. Ultrasound terminology is essential if we are going to speak the same language as our referrers and our technologists. We will begin with the concept of the near field. The near fields are structures that appear in the upper half of the monitor. The far fields are scan structures that are in the position near the bottom half of the monitor. The term echogenic is very important in that those structures that generate an echo are considered echogenic. They are generally highly reflective, bright or wide on the screen like bone. In addition, anechoic refers to the opposite. Those structures that do not generate an echo and are seen as darker or blacker on the monitor like a ganglion. Terminology continued. These are different patterns that can be homogenous in their pattern, which is a uniform echo texture or heterogeneous and irregular in its echo texture. For example, a tumor that has necrotic center would have a heterogeneous echo texture. We can use terms like hyperechoic. When the echo is brighter, similar features in scar tissue or even on the surface of the bone may be hyperechoic.
Conversely, hypoechoic means less echogenicity and this could be seen in an area of inflammation or a partial tendon tear or you see a defect, which is hypoechoic within the substance of a tendon. Terms such as axial and horizontal resolution. Axial resolution refers to the ability to distinguish two objects when they lie directly over one another and horizontal resolution is the ability to identify two objects, which are side by side, although the same distance from the transducer. Terminology continued. We can have features of anisotropy or an artifact known as anisotropy, which is when the tissue has different optical properties apparently when you scan from different directions. And what I mean by this is that the ultrasound image will change as the angle of transducer changes. So perpendicular to the structure is preferred and as the structure changes direction for example like the posterior tibial tendon, the image will change if you don't keep the probe perpendicular to the structure of interest. You therefore may lose signal when evaluating a structure and mistake this for tear or tendon pathology. In this example of anisotropy, we can see from the schematic at the upper aspect of the slide where the transducer is directly over a tendon fragment, which is seen perpendicular 90 degrees to the transducer probe.
The entire sound beam that goes through that tissue and then has returned to the transducer returns and you have an echogenic normal tendon appearance. Conversely, if the area of interest is curved, it's possible that when you put the probe over the area of interest that there is less than 90 degrees or greater than 90 degrees and the reflective sound waves do not return to the transducer. In that case, what happens is that you can see on the bottom images in A and B. These represent transverse images of a normal Achilles tendon demonstrating the artifact of anisotropy. On the left, notice the echogenic tendon fibrils that make up the substance of the Achilles tendon. This is a transverse image where the probe is perpendicular to the central axis of the sound beam and the body structure and returns all of the sound and you can see all of the information. On the example to the right in B, you see a transverse image of a hypoechoic Achilles tendon that's not perpendicular but rather oblique to the central axis of the ultrasound beam. And we can use these features, these terms to assess common podiatric ultrasound condition. So we can see plantar fascial tears or inflammation as areas of hypoechoic heterogeneous echo texture surrounded by hypoechoic inflammation. A plantar fibroma is known to be a hypoechoic homogenous mass inseparable or within the plantar fascia.
We can look towards Achilles tendon pathology denoting tendon thickening as tendinosis, tendinitis having the presence of inflammation around the tendon. We can see partial or complete tears by noting the heterogeneous echo texture and the hypoechoic tear defect and of course, we can identify insertional exostoses and enthesophytes at the attachment of the calcaneus of the Achilles tendon. Furthermore, we can assess cystic masses like ganglion and inclusion cyst but really all soft tissue masses as we will come to see later on in this lecture. Retrocalcaneal bursitis, common entity such as Morton's neuromas can be identified easily with ultrasound, presence of intermetatarsal bursitis, of course ligament strains or tears are easily seen using podiatric ultrasound given its superficial location with relation tip to the probe. Indications continued include ankle sprain and ligament pathology. We can easily see the anterior talofibular ligament and calcaneal fibular ligament. These are very common ligaments to sprain and we can easily address them by using ultrasound. We can look for foreign bodies, assess for stress fractures by seeing hypertrophy, periosteum and of course this cannot only be diagnostic but we can offer therapeutic services through ultrasound-guided injections and even aspirations.
In terms of terminology for scanning technique, the terms long and short axis are used relative to a body structure to determine how one position to transducer relative to the foot and ankle. As the scanner, you should sit comfortably and have the monitor positioned in such a manner to make viewing the screen natural and easy. You want to use your heel of your hand and fingers to stabilize the transducer over the skin surface and be able to move slowly over the structure to best appreciate the anatomy. For ultrasound technique, when evaluating a structure you want to rock the transducer slowly from heel to toe. Length wise with the linear probe and slowly sweep the probe adjusting to stay perpendicular to the structure being studied. On the monitor, you pick the depth, the frequency, the focal point and adjust the gray zone for the best image and we will go over that in the next images. Of course, the physical exam will guide to the location and focal point of the ultrasound study. So typically when we bring the patient in for ultrasound, we generally come up to the first screen, which is the user tab and what we can do is preset buttons to select whether the structure that we are looking for is very superficial, superficial, medium or deep and we can see these presets here on this person's screen on the left but these can be put on most machinery these days.
In terms of adjusting the focus, in many cases the current technology allows you to simply touch the screen in the area that you would like to focus and this will move focal zones down so that the sound beam will be arriving at this location optimally and then the sound beams that aren't reflective within the tissues and that are reflected will be sent back to the transducer and you can see that if you want to see more echogenicity you would increase the overall gain seen on this modulator on the bottom left. Moving on the remaining presets that exist on the machine, if you want to freeze the image you found the area and the image that you like you want to freeze that so you can take a picture of that. You can touch the image to acquire still image of the frame present on the screen at the time it's touched in many instances. Of course, you want to assess in each and every case the presence of vascularity. So you will need either a color Doppler or power Doppler license and you can press those buttons if they are licensed on your machine to activate these features and you can usually use a split screen, which we can use to contrast whether say for example the plantar fascia on the left compared to the right to really demonstrate what the thickening is about and what is bilateral. The M-mode will activate the M-mode line and touching it a second time it will activate M-mode.
Of course, this is what color Doppler looks like. What you would like to do is press a button on the machine that says color, it will bring up a box, which you can usually manipulate the size and dimensions of the box using your roller ball. And this will enable you to see blood flow visualization. To relocate this color region of interest box, simply touch the screen or turn the color on and off to have another box and be able to move it again. And of course the benefit of color Doppler cannot be underestimated and that this modality provides an amazing ability to not only see anatomically the image to resolution of less than 0.5 mm but also the presence of blood flow or inflammation. Therefore, we can make many diagnoses with ultrasound. By convention, the red color that you generally see on the screen is moving towards the transducer and the blue color is blood flow that's moving away from the transducer. Of course, in areas we don't expect to see blood flow, we would not want to but in tumors, tendon and nerve pathology, we may see blood flow and that will be helpful for preoperative planning. And of course in the inflammatory phase of tendon injury, which can occur about 10 to 14 days past the injury, these findings would suggest neovascularization. And just to give you an example, here we are looking at the Achilles tendon and this imager has already preset the options for us so essentially we are looking at a schematic of the way the image should look an ultrasound at the top left where number one is your Achilles tendon, number two is the soleus insertion onto the Achilles tendon.
Five would represent the flexor hallucis longus in the midline, the muscle of the deep medial flexor compartment. Number three denotes Kager's fat triangle or the pre-Achilles fat pad. And number four of course would be your calcaneus. And we can see on this image at the bottom that there is -- essentially the probe is in the long axis relative to the long axis of the Achilles and we can see on the image to the right the linear fibrosis of the Achilles tendon. Noting deep to the Achilles tendon, we have the Kager's fat triangle and we really don't see a piece of the flexor hallucis longus. We can see the echogenic bone [indecipherable] [20:53] seen here. That's the posterior aspect of the calcaneus and nice insertion of the Achilles tendon. So, all in all, just some features to remind us. It's very important to have a landmark and know the anatomy of the part you are examining. That way you can actually see abnormal anatomy. So it does take some amount of experience in separating what's normal and what's abnormal. You of course want to use dynamic motion and a combination of still images in your evaluation. In general, some people are easier to scan than others. This usually depends on water content, the patient's BMI, fat content and variations in normal anatomy that can affect the scan. Making adjustments in the gain, depth and probe frequency can help offset the variables to optimize image quality.
I would like to take a moment to turn towards our imaging tool box to see the role of ultrasound and the important assessment of soft tissue masses. The American College of Radiology provides appropriateness criteria for almost every indication of imaging and I encourage you to look up any entity that you have a question about how to image to be able to determine the best utilization practice. These are generally written by the experts in the field and they are reviewed in the timely manner and put out for American College of Radiology for nationwide guidance. So in variant one, the presence of soft tissue mass whether it's superficial and palpable, the initial studies that are usually appropriate include an x-ray and an ultrasound. An MRI may be appropriate but there is a lot of disagreement and therefore insurance companies will often not approve this. So usually the first line imaging approach is going to be x-ray and ultrasound. On the x-ray, we are going to be looking for other pathologies such as foreign body, calcification, mineralization of bony tumor and an ultrasound of course is going to give us great resolution of the near field because of its superficial location, we are going to be able to see whether or not a lesion is solid or cystic. We are going to be able to assess some degree of blood vessels and determine whether the lesion needs to go on to further imaging. In variant two, the American College of Radiology deals with soft tissue masses that aren't superficial rather deep or they are nonspecific in their clinical assessment or they are located in area that's difficult to evaluate with x-rays.
And this is where you get to see the deep soft tissues of the feet. The initial imaging study in this case of course begins with an x-ray and then whether or not what type of cross-section may be appropriate, may be an ultrasound as you can see here or it may be warranted to go directly onto a CT or MRI with or without IV contrast. So an ultrasound may be indicated as the initial imaging modality in patients who have a deep soft tissue mass because this is difficult to adequately evaluate with x-rays. In variant three, we see soft tissue masses with a nondiagnostic initial evaluation. So these are patients that have already had an ultrasound and/or an x-ray and now we are moving on to the next imaging study and usually what's appropriate in that case is an MRI and that may be with or without contrast depending on the patient and the indication of a location. Of course, CT may be appropriate certainly in those patients that have an MRI contraindication and it may be at that point to assess whether or not to get contrast. In variant four and five, these are soft tissue masses again that had a non-diagnostic initial evaluation already with an x-ray and an ultrasound and in these cases it may be appropriate to go on to advanced imaging as seen here and all of this is available to you under ACR appropriateness criteria for soft tissue masses. In summary, what we have done in this segment is to provide a general overview of ultrasound principles and technique to apply to common pathologies of foot and ankle.
We will review the most common indications for the use of ultrasound in foot and ankle conditions and further we reviewed the American College of Radiology guidelines for imaging of soft tissue masses. These references are available for your review and I thank you for the generosity of your time and attention and hope that this lecture has been of value for you. Thanks again.
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