Approach to Musculoskeletal Ultrasound Ultrasound Machine To perform musculoskeletal US, you first need access to a US machine equipped with a high-resolution transducer. Second, you need to understand, at least, the very basics of US technology. High-resolution transducers range from 10-22 MHz and are ideal for examining the superficial soft tissues. Similar to MR and increasing field strength, the relationship between increasing US transducer frequency and resolution is not linear. Most musculoskeletal US can be performed with a linear transducer in the 10-12 MHz range. Moving from 12-17 MHz will improve resolution to a mild degree but usually does not significantly alter diagnostic capability. In other words, access to one high-frequency transducer is usually sufficient to begin musculoskeletal US. A disadvantage of high-resolution transducers is limited depth penetration; for deeper areas (gluteal region, proximal thigh) and large masses, a lower frequency transducer, such as a 5 MHz or 7.5 MHz transducer, is needed. The transducer is a delicate (and expensive) piece of equipment and must be handled with care. The transducer contains an interlinked array of piezoelectric crystals. These crystals convert an electrical signal into a US wave and vice versa. The same array of piezoelectric crystals emits and receives the US signal. This is like the transducer shouting out into tissues and then listening for a returning sound or echo. This “shouting and listening” happens many thousands of times a second (pulse repetition frequency). To improve transmission of sound waves, US gel is applied between the skin and the transducer. Sound waves are transmitted from the transducer through the US gel to the skin and deeper tissues. The ability of tissues to transmit sound waves is known as acoustic impedance. At tissue interfaces where there is a large difference in acoustic impedance, most sound waves are reflected back to the transducer, producing a bright echo. At interfaces where there is little or no difference in acoustic impedance, most sound waves are transmitted and not reflected back to the transducer, producing a dark echo. As expected, most sound waves that contact an interface parallel to the transducer face will be reflected back to the transducer. Sound waves impacting an irregular surface will be retracted away from the transducer (refraction). Sound wave attenuation also results from acoustic absorption, which is greater in tissues of higher viscosity. Getting Started The best and only way to learn musculoskeletal US is to get started. In general, the more US examinations you perform, the better you should become. Be conscious of working posture, etc., at the outset. Ensure that both you and the patient are in a comfortable and the most effective position to examine the area of interest. Take a brief clinical history from the patient and palpate any lumps or swelling, if present. This is tremendously helpful when interpreting the subsequent US findings. Warm the US gel using a dedicated US gel warmer, especially if examining the chest or abdomen. Hold the transducer between the thumb and finger. You should have either your little finger or the hypothenar eminence in contact with the patient’s skin. This helps to anchor the transducer and ensure sufficient but not excessive transducer pressure on the skin. When assessing superficial structures, and especially when assessing color flow in superficial tissues, one should actively almost lift the transducer from the skin using plenty of intervening US gel to avoid distorting the tissues or effacing the vascularity. Develop a systematic examination procedure for each joint or region that includes all critical anatomical structures. Start your examination at the most symptomatic area and at least include the immediate surrounding area in your examination. For time efficiency, it is not necessary to complete the whole joint examination in every patient. For example, if the patient has medial ankle pain, concentrate on examining the medial and ankle joint proper; there is little to be gained from routinely examining the lateral or posterior ankle structures if there are no symptoms related to these areas. In children, a full examination may not be possible before the child becomes restless, so again concentrate on the most critical area. Sedation is not usually necessary or beneficial in children. Machine Setup and Scanning Understanding proper US machine setup is crucial to conducting high-quality US. US is a more operator-dependent examination than CT or MR. One should be pedantic about trying to obtain the best possible quality US examination in every patient. There should be no such thing as a “quick US examination”; this is a recipe for missing significant pathology. Optimize image quality before and during scanning. It is helpful to begin by examining the contralateral normal side, as this allows you to both familiarize yourself with normal US anatomy and also set up the machine properly for that body part. The initial setup will often need frequent readjustment throughout the US scan. It is better to take a few high-quality images than many suboptimal images. Orient the image so that the proximal end is on the left side of the image and the distal end is on the right. Make sure there is no shadowing artifact from too little gel at the sides of the image, adjust the depth to the minimum needed to see all structures of interest, adjust the focal position and zone, gain (overall image gain) and time gain compensation to optimize image quality. During scanning, tissues can be compressed by the transducer to broadly gauge their stiffness. This dynamic capability of US is also helpful when differentiating between fluid and soft tissue in suspected abscess or hematoma, or between synovial proliferation and effusion. The dynamic capability of US is also helpful when assessing tendon tears, tendon subluxation, or shoulder impingement. Color or power Doppler is used routinely in the assessment of soft tissue inflammation, synovitis, and soft tissue masses. The relative sensitivity of color or power Doppler does vary from machine to machine. Color Doppler is flow-direction sensitive, whereas power Doppler is not flow-direction sensitive, though it is considered more sensitive overall to detecting slow flow and flow in small vessels. Low-frequency transducers reveal slow velocity flow better. Reducing the pulse repetition frequency and lowering the wall filter will also improve detection of slow vascular flow. Supplementary Imaging Techniques With conventional US, the transducer emits and receives a sound pulse of a specific frequency at a single angle of insonation. The returning signal is of the same frequency though weaker with the transducer “listening” specifically for that frequency. Several techniques over and above standard grayscale imaging may enhance image resolution, depiction, and interrogation. Spatial compound US amalgamates sonographic data from several different angles of insonation into a single image. By averaging image data from multiple angles of insonation, spatial compound imaging improves definition of soft tissue planes, reduces speckle and other noise, and improves image detail. With tissue harmonic imaging, the transducer listens for not only returning signals of the transmitted frequency, but also returning signals of harmonic frequencies (generated by passage of the sound wave through tissue) that are twice the transmitted frequency. Benefits of tissue harmonic imaging are most apparent in the midfield region, helping to improve tissue contrast and edge demarcation of soft tissue masses or tendon tears, though with loss of some spatial resolution. Extended field-of-view imaging allows panoramic view, which is helpful for measuring large masses, comparing tissue echogenicity in confluent areas, and perceiving anatomical relationships. Only gold members can continue reading. 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Approach to Musculoskeletal Ultrasound Ultrasound Machine To perform musculoskeletal US, you first need access to a US machine equipped with a high-resolution transducer. Second, you need to understand, at least, the very basics of US technology. High-resolution transducers range from 10-22 MHz and are ideal for examining the superficial soft tissues. Similar to MR and increasing field strength, the relationship between increasing US transducer frequency and resolution is not linear. Most musculoskeletal US can be performed with a linear transducer in the 10-12 MHz range. Moving from 12-17 MHz will improve resolution to a mild degree but usually does not significantly alter diagnostic capability. In other words, access to one high-frequency transducer is usually sufficient to begin musculoskeletal US. A disadvantage of high-resolution transducers is limited depth penetration; for deeper areas (gluteal region, proximal thigh) and large masses, a lower frequency transducer, such as a 5 MHz or 7.5 MHz transducer, is needed. The transducer is a delicate (and expensive) piece of equipment and must be handled with care. The transducer contains an interlinked array of piezoelectric crystals. These crystals convert an electrical signal into a US wave and vice versa. The same array of piezoelectric crystals emits and receives the US signal. This is like the transducer shouting out into tissues and then listening for a returning sound or echo. This “shouting and listening” happens many thousands of times a second (pulse repetition frequency). To improve transmission of sound waves, US gel is applied between the skin and the transducer. Sound waves are transmitted from the transducer through the US gel to the skin and deeper tissues. The ability of tissues to transmit sound waves is known as acoustic impedance. At tissue interfaces where there is a large difference in acoustic impedance, most sound waves are reflected back to the transducer, producing a bright echo. At interfaces where there is little or no difference in acoustic impedance, most sound waves are transmitted and not reflected back to the transducer, producing a dark echo. As expected, most sound waves that contact an interface parallel to the transducer face will be reflected back to the transducer. Sound waves impacting an irregular surface will be retracted away from the transducer (refraction). Sound wave attenuation also results from acoustic absorption, which is greater in tissues of higher viscosity. Getting Started The best and only way to learn musculoskeletal US is to get started. In general, the more US examinations you perform, the better you should become. Be conscious of working posture, etc., at the outset. Ensure that both you and the patient are in a comfortable and the most effective position to examine the area of interest. Take a brief clinical history from the patient and palpate any lumps or swelling, if present. This is tremendously helpful when interpreting the subsequent US findings. Warm the US gel using a dedicated US gel warmer, especially if examining the chest or abdomen. Hold the transducer between the thumb and finger. You should have either your little finger or the hypothenar eminence in contact with the patient’s skin. This helps to anchor the transducer and ensure sufficient but not excessive transducer pressure on the skin. When assessing superficial structures, and especially when assessing color flow in superficial tissues, one should actively almost lift the transducer from the skin using plenty of intervening US gel to avoid distorting the tissues or effacing the vascularity. Develop a systematic examination procedure for each joint or region that includes all critical anatomical structures. Start your examination at the most symptomatic area and at least include the immediate surrounding area in your examination. For time efficiency, it is not necessary to complete the whole joint examination in every patient. For example, if the patient has medial ankle pain, concentrate on examining the medial and ankle joint proper; there is little to be gained from routinely examining the lateral or posterior ankle structures if there are no symptoms related to these areas. In children, a full examination may not be possible before the child becomes restless, so again concentrate on the most critical area. Sedation is not usually necessary or beneficial in children. Machine Setup and Scanning Understanding proper US machine setup is crucial to conducting high-quality US. US is a more operator-dependent examination than CT or MR. One should be pedantic about trying to obtain the best possible quality US examination in every patient. There should be no such thing as a “quick US examination”; this is a recipe for missing significant pathology. Optimize image quality before and during scanning. It is helpful to begin by examining the contralateral normal side, as this allows you to both familiarize yourself with normal US anatomy and also set up the machine properly for that body part. The initial setup will often need frequent readjustment throughout the US scan. It is better to take a few high-quality images than many suboptimal images. Orient the image so that the proximal end is on the left side of the image and the distal end is on the right. Make sure there is no shadowing artifact from too little gel at the sides of the image, adjust the depth to the minimum needed to see all structures of interest, adjust the focal position and zone, gain (overall image gain) and time gain compensation to optimize image quality. During scanning, tissues can be compressed by the transducer to broadly gauge their stiffness. This dynamic capability of US is also helpful when differentiating between fluid and soft tissue in suspected abscess or hematoma, or between synovial proliferation and effusion. The dynamic capability of US is also helpful when assessing tendon tears, tendon subluxation, or shoulder impingement. Color or power Doppler is used routinely in the assessment of soft tissue inflammation, synovitis, and soft tissue masses. The relative sensitivity of color or power Doppler does vary from machine to machine. Color Doppler is flow-direction sensitive, whereas power Doppler is not flow-direction sensitive, though it is considered more sensitive overall to detecting slow flow and flow in small vessels. Low-frequency transducers reveal slow velocity flow better. Reducing the pulse repetition frequency and lowering the wall filter will also improve detection of slow vascular flow. Supplementary Imaging Techniques With conventional US, the transducer emits and receives a sound pulse of a specific frequency at a single angle of insonation. The returning signal is of the same frequency though weaker with the transducer “listening” specifically for that frequency. Several techniques over and above standard grayscale imaging may enhance image resolution, depiction, and interrogation. Spatial compound US amalgamates sonographic data from several different angles of insonation into a single image. By averaging image data from multiple angles of insonation, spatial compound imaging improves definition of soft tissue planes, reduces speckle and other noise, and improves image detail. With tissue harmonic imaging, the transducer listens for not only returning signals of the transmitted frequency, but also returning signals of harmonic frequencies (generated by passage of the sound wave through tissue) that are twice the transmitted frequency. Benefits of tissue harmonic imaging are most apparent in the midfield region, helping to improve tissue contrast and edge demarcation of soft tissue masses or tendon tears, though with loss of some spatial resolution. Extended field-of-view imaging allows panoramic view, which is helpful for measuring large masses, comparing tissue echogenicity in confluent areas, and perceiving anatomical relationships. Only gold members can continue reading. 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