Abstract
This chapter provides a basic introduction to ultrasound and its use in interventional procedures. We will discuss several ways to improve image quality and optimize the features of ultrasound that make it an invaluable tool. Sonographic anatomy of the relevant spinal structures and landmarks will be reviewed. In this atlas, we will discuss “hybrid” techniques in which ultrasound and fluoroscopy are combined to enhance safety and precision, minimize discomfort, and reduce radiation exposure for interventional procedures.
Introduction
This chapter provides a basic introduction to ultrasound and its use in interventional procedures. We will discuss several ways to improve image quality and optimize the features of ultrasound that make it an invaluable tool. Sonographic anatomy of the relevant spinal structures and landmarks will be reviewed. In this atlas, we will discuss “hybrid” techniques in which ultrasound and fluoroscopy are combined to enhance safety and precision, minimize discomfort, and reduce radiation exposure for interventional procedures.
Ultrasound Equipment
Base Unit ( Fig. 4.1 )
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Comprises the screen on which the ultrasound image is displayed, as well as the keyboard and control dials or buttons.
Fig. 4.1
Example of a standard ultrasound base unit.
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Some instruments offer touch-screen capabilities and a track pad on a keyboard.
Transducers
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Transducer selection will vary depending on the type of injection and depth of the target structure. Selecting the appropriate transducer for each injection will result in improved image quality and needle guidance.
Linear ( Fig. 4.2 )
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Frequency: middle to high range (6–18 MHz).
Fig. 4.2
Example of a linear transducer.
(©FUJIFILM SonoSite, Inc. Bothell, Washington.)
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Optimal use: neck, shoulders, hips in thin/athletic patients, and shallow to medium depth structures.
Curvilinear ( Fig. 4.3 )
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Frequency: low to middle range (2–6 MHz).
Fig. 4.3
Example of a curvilinear transducer.
(©FUJIFILM SonoSite, Inc. Bothell, Washington.)
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Optimal use: lumbar spine, hips in medium/obese patients, deep structures, when a broad field of view is desired, and for steep (>45 degrees) needle approaches.
Hockey Stick ( Fig. 4.4 )
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Frequency: high range (10–18 MHz).
Fig. 4.4
Example of a hockey stick transducer.
(©FUJIFILM SonoSite, Inc. Bothell, Washington.)
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Optimal use: small superficial joints, tendons, and nerves.
Knobology
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Ultrasound knobs, dials, and buttons.
Image/Video Loop Capture
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Static images are obtained and saved for review and or documentation.
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Video loops are used to document dynamic pathologies (e.g., snapping hip) and or procedural details (e.g., aspiration of a cyst or injectate distribution).
Freeze
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Images can be “frozen” for immediate evaluation, during which on-screen structures can be labeled, measured, and if using dual screen mode, compared to contralateral structures.
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If the interventionalist/sonographer sees something interesting and would like to see it again, freezing the image and rolling the trackball or touchpad to the left typically rewinds through the last 5 to 10 seconds of scanning.
Depth/ Frequency ( Fig. 4.5A, B )
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Lower frequency transducers and settings will allow the ultrasound beam to penetrate further, thus allowing visualization of deeper structures though usually at the cost of image quality.
Fig. 4.5
Hip ultrasound: A, Insufficient depth selection. B, More appropriate depth selection. Note how the femoral head with femoral head/neck junction (arrow) is better visualized with an increased depth.
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Higher frequency settings and transducers will allow one to see more superficial structures in greater detail with generally excellent image quality usually at the cost of visualizing deeper structures.
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The depth (in centimeters or millimeters) of a particular structure can be estimated on the basis of hash marks on the side of the screen, or precisely measured using the ultrasound unit caliper feature.
Caliper
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The caliper feature is used to measure depth, length, and area and is useful for estimating needle length and angle of approach for procedures or diagnosis of nerve or tendon enlargement and response to treatment.
Gain ( Fig. 4.6A, B )
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Adjusts overall brightness of the screen, typically using a dial.
Fig. 4.6
A, Higher gain setting selected. Note the bright cortical surface. B, Lower gain. Cortical surfaces are darker, as is the surrounding tissue.
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Some units have slide controls (also known as time-gain compensation [TGC]), which adjust brightness at different depths.
Color Doppler/Power Doppler ( Fig. 4.7A, B )
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Highlights vascular structures and vascular flow.
Fig. 4.7
A, Example of color Doppler flow of a vessel in short axis, depicted as blue circle, indicating cross-section of the vessel flowing away from the transducer. B, Example of color Doppler flow of the radial artery in long axis.
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Red color indicates flow toward the transducer and blue away from the transducer.
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Lighter shades of red or blue indicate higher velocity flow.
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Power Doppler indicates flow in any direction and may be more sensitive than color Doppler depending on the ultrasound unit.
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Can be used to assess neovascularization and inflammation.
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Can be used to assess and/or confirm injectate flow, especially for deeper structures.
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Dual split screen capability facilitates the comparison of vascular and musculoskeletal structures.
Zoom ( Fig. 4.8A, B )
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Magnifies an area of interest for improved resolution and visualization.
Fig. 4.8
A, Standard image without zoom. B, Zoomed in image on an area of interest. C, Standard and D, optimized use of focal zones.
Focal Zones ( Fig. 4.8C, D )
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Focal zones optimize image quality at a particular depth. The size and location of the focal zone can be adjusted on some units with a knob or dial, while others have it set to the middle of the screen. To make appropriate adjustments, it is important to know the type of machine used. For machines with the focal zone set to the middle of the screen, it is important to adjust the depth such that the structure of interest is in the middle of the screen.
Needle Enhancement/M-Line Needle Enhancement ( Fig. 4.9A )
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The needle enhancement software incorporates ultrasound beam steering and other features to improve needle visualization. This can be a useful feature when injecting at steep angles and into deep structures.
Fig. 4.9
A, Example of needle guidance during a hip injection using a curvilinear probe. The needle is coming from the right, nearly perpendicular to the ultrasound beam depicted by the dotted line and steered using the needle enhancement software. Despite a suboptimal image, needle visualization is improved. B, M-line (center line) is used here to center the structure of interest (nerve) in the short axis. C, M-line (center line) is used here to indicate the midpoint of the transducer footprint in the long axis. D, Example of using “Extended Field of View” to visualize an Achilles tendon rupture in the long axis (arrow).
M-line ( Fig. 4.9B, C )
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To facilitate out-of-plane injections, some machines have a hash mark under the middle part of the transducer and a corresponding line, the “M-line” (center line), which can be turned on and off on the screen. Structures located under the hash mark will be seen in the middle of the ultrasound screen and marked by the “M-line.”
Note: The most important part of being able to see a needle well depends on whether the interventionalist properly aligns the needle with the transducer and ultrasound beam.
Extended Field of View ( Fig. 4.9D )
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This is a method in which the user can visualize an anatomic structure and sweep along it to obtain images that are reconstructed to form a panoramic view of the structure.
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It can be difficult to create a smooth panoramic image, as any nonlinear motion of the transducer will create a “choppy” appearance.
Ultrasound Ergonomics
Optimal Setup ( Fig. 4.10 )
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The patient and ultrasound unit are positioned close to each other, ideally with the patient positioned between the interventionalist and the monitor.
Fig. 4.10
A and B, Example of an optimal ergonomic setup for an ultrasound-guided procedure. Note the height of the ultrasound screen across from the physician, at or above the eye level, and the direct line of sight with the needle trajectory and the screen.
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A straight-line or narrow angle orientation between the interventionalist, ultrasound transducer, and screen is best. This helps minimize extraneous operator movement, as even the most subtle movement can cause misalignment of the transducer, needle, and target.
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An assistant can be helpful in capturing images, adjusting knobs, and handling supplies, allowing the interventionalist to concentrate on the procedure and maintain sterility.
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The unit can also be positioned next to the patient on the same side as the interventionalist for better access to controls and lessen the need for an assistant.
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See Chapter 5 for more details on patient safety, positioning, and procedure room setup.
Transducer Grip
Optimal
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Option 1: A firm grasp on the transducer, wrapping fingers and thumb at the base of the transducer and leaning the base of the hand and transducer on the patient ( Fig. 4.11 ).
Fig. 4.11
Example of an optimal transducer grip, with relaxed but firm grasp on the base of transducer, with hypothenar eminence resting on the patient’s skin and C grip formed by the index finger and thumb. This grip maximizes control and minimizes hand strain or fatigue.
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Option 2: The thumb and index and middle fingers hold the transducer. These fingers form a tripod, an inherently stable structure. The remaining ring and little fingers and transducer itself form a second tripod, which can be stabilized on just about any contour of the human body ( Fig. 4.12 ).
Fig. 4.12
Alternate example of an optimal transducer grip.
Suboptimal ( Fig. 4.13 )
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Lack of contact between the base of the hand (or little and ring fingers) and the patient, causing ultrasound transducer to slide around and result in a moving target on the ultrasound screen.
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Excessively firm grip on the transducer is likely to cause fatigue, making subtle movements and adjustments of the transducer more difficult.
Related posts:
Introduction: How to Use This Atlas
Caudal Epidural Steroid Injection: Ultrasound Guidance
Lumbar Zygapophysial Intraarticular Joint Injection—Posterior Approach: Fluoroscopic Guidance
Vertebral Augmentation (Vertebroplasty/Kyphoplasty): Transpedicular Approach
Cervical Zygapophysial Joint Nerve (Medial Branch) Radiofrequency Neurotomy and Nerve Injection, Posterior Approach: Fluoroscopic Guidance
Lateral Femoral Cutaneous Nerve Injection: Ultrasound Guidance
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