Objective, Noninvasive Wound Assessment Using B-Mode Ultrasonography
Disclosures: Dr. Wendelken is a consultant for Biosound Ultrasound Co., part owner of Hudson Diagnostic Imaging, and owner and inventor of the ultrasound device used in this study.
Developments in advanced wound care have concentrated on wound treatments and therapies. There has been little progress in the field of wound assessment and diagnostics. Clinicians have had to rely on clinical expertise and invasive methods to inspect pathology before making decisions about wound etiology and wound complications. Using diagnostic ultrasound, a novel technique has been developed to evaluate wounds objectively and noninvasively. This technique, called Wound-Mapping® (Hudson Diagnostic Imaging LLC, Elmwood Park, New Jersey) is a significant advance in the field of wound assessment and offers a sophisticated way to measure wound size, depth, undermining, edema, and abnormalities in dermis, muscle, fat, and bone surface.
The purpose of this paper is twofold. First, the authors will introduce diagnostic ultrasound as a tool for noninvasive wound assessment. Second, the authors will share their experiences using ultrasound in the examination of chronic wounds of varying etiologies.
Overview of Diagnostic Ultrasound
Ultrasonography has been an integral part of medicine for more than 30 years. It is best known for its use in the area of obstetrics and gynecology, cardiology, radiology, internal medicine, and vascular surgery. Today, sophisticated ultrasound machines can produce three-dimensional images. Ultrasound systems use relatively high-frequency sound waves that are pulsed at a specific time interval through a transducer. This same transducer listens for echoes that are produced by various tissues. After capturing these echoes, a central processing unit compiles the echoes into an image that is sent to a display (cathode ray tube or solid state display). Today’s high-speed electronics have made it possible to package a high-resolution, cost-effective imaging tool that provides accurate and reproducible ultrasound scans. Some portable ultrasound scanners can be worn on physicians’ wrists while maintaining a reasonable degree of performance.
The increased diagnostic capabilities of ultrasound technology are directly related to the availability of advanced microprocessors, which have also lowered the cost of ultrasound scanners. Practical transducers now have a frequency range from 3.0Mhz to 20Mhz with axial and spatial resolution in the hundreds of millimeters (transducer and scanner dependent). Unlike magnetic resonance imaging (MRI), a benefit of these advances is that clinicians have the option to scan soft tissue via musculoskeletal ultrasound in the office or clinic setting. This modality is readily available for physicians to scan wounds during typical patient office visits, thereby avoiding potential MRI scheduling delays and follow-up consultations. Less obvious to the patient but more important to the physician is the capability of real-time imaging of structures for immediate diagnoses or to rule out diagnoses with no delay in final report generation. Radiographs (x-rays) often are the first type of diagnostic imaging performed on wounds to rule out infection of bone and soft-tissue involvement. However, radiographic images can be misleading because pathologic changes may not appear for two weeks following the x-ray. Computerized tomography (CT) scans, like MRI, may have scheduling delays, which consume valuable time before results are obtained; this may lead to increased morbidity for patients. Of the four imaging modalities, only diagnostic ultrasound is totally noninvasive and has no contraindications, such as harmful x-rays, injected dyes, magnetic fields, or potential patient clostrophobia.
Physics of Musculoskeletal Ultrasound Imaging
A full study of ultrasound physics and related terminology is beyond the scope of this article; however, a brief review is helpful in understanding diagnostic ultrasound.
Unlike light waves, sound waves do not have the ability to be transmitted through a vacuum.4 Sound requires a physical medium through which to travel, such as a solid, liquid, or gas matter. When a sound wave is emitted from a source, it encounters a transmission medium. A sound wave can be propagated through that medium by causing molecules to vibrate or move a short distance. When the molecules move or vibrate, they encounter adjacent molecules, which cause those molecules to move or vibrate. This process continues, which allows the sound wave to travel through the medium. A frequency above 20KHz is not audible to the human ear, and because of this property, these sound waves are termed ultrasound. Ultrasound devices use gel as a contact medium during ultrasound exams. The gel permits the ultrasound wave to be transmitted from a transducer through the gel and into the body. Ultrasound transducers in medicine generate sound waves that have a frequency between 1 and 20MHz (1 hertz=1 cycle per sec: 1MHz=1 million cycles per second).
Ultrasound travels through various materials at different velocities. Table 1 gives the speed of sound through the various tissues in the human body. The majority of the tissues in the body transmit sound at approximately 1500m/sec. The higher the density of a material the faster the sound travels through that material. Air, for example, is a combination of gases (having low density) and conducts sound at a speed that travels at 331 meters per second. The speed of sound in bone, a highly dense structure, is 4,080 meters per second. The less dense soft tissues and bodily fluids transmit sound at an average speed of about 1540 m/sec.
Producing an image from ultrasound. Diagnostic ultrasound utilizes sound energy to produce an image. Linear array transducers contain a series of piezoelectric crystals arranged in a linear flat pattern. Piezoelectric crystals within the transducer have unique physical and electrical properties. When rapidly changing alternating voltage is applied to a crystal, it will vibrate and generate sound at a specific frequency (e.g., 8.0Mhz). The sound waves emitted from the transducer encounter various tissues and generate echoes, which are then captured by this same transducer. The captured sound waves cause the crystals in the transducer to vibrate and generate an electric current. The current is converted into an image that is viewed on the ultrasound screen.
Image resolution and the acoustic window. The resolution of the images produced by ultrasound are measured in two axes, the axial and horizontal resolutions. Axial resolution is the ability to distinguish two objects as being separate when they are directly over each other. The frequency of the transducer is the main property that helps determine the axial resolution. In general, the higher the frequency the greater the axial resolution. Horizontal resolution is the ability to distinguish two objects as separate when they are located side by side and of equal distance from the transducer.
The maximum amount of tissue that can be scanned by a transducer is limited by what is known as the acoustic window. The acoustic window is equal to the length and width of the crystal array within the end of the transducer. Figure 1 is a picture of a typical 8.0MHz linear array transducer.
Ultrasound images are best acquired by placing the probe directly over the area to be scanned. The maximum amount of echoes are captured when the acoustic window on the transducer is perpendicular to the structure scanned. The angle at which the sound waves encounter the tissue is called the angle of incidence; this angle should be as close to 90 degrees as possible. If the angle of the sound beam is greater or less than 90 degrees, the image generated will be less than optimal due to reflection of the sound wave away from the transducer.
When performing an ultrasound scan, fluid and structures comprising the approximate density of fluid will present a low acoustic impedance to the sound beam.[5,6] Accordingly, this low acoustic impedance generates few echoes, which manifest themselves as dark displays on the screen. For example, ganglion cysts, fluid-filled sacs, abscesses, or other such structures generate few echoes and will appear as black areas on the ultrasound display. At the other extreme, dense structures, such as bone, are highly reflective and deliver a high acoustic impedance in the realm of the sound beam, which makes that area of the screen bright white. The remaining soft tissue (muscles, tendons, ligaments, etc.) will fall between grades of black and white contingent upon the quality of that system’s digital scan conversion.
Diagnostic ultrasound does not have the ability to penetrate bone; however, the surface of bone or periosteum can be examined for injuries, such as fractures or other inflammatory conditions of the periosteum including osteomyelitis. Only one surface of bone can be examined at one time. This lack of echoes through the surface of bone casts an acoustic shadow as seen in Figure 2.[7,8]
Frequency, Imaging, and Modes
Transducer frequency has a direct effect on image resolution. In general, the higher the frequency the higher the resolution.[6,8] This higher frequency does, however, limit the amount of tissue penetration. Table 2 is a summary of the effect of transducer frequency. Much like a computer, the actual image produced by a scanner is the product of the transducer, the software driving the transducer, the electronic components processing the information, and the resolution of the display used to view the image.
Imaging modes. There are three basic imaging modes used in ultrasound: B-mode, M-mode, and Duplex scan. The B-mode is an ultrasound image that is taken in two dimensions. The image is measured across the top and down the side of the screen just like a regular picture. The M-mode is a two-dimensional image that allows the recording of motion. The Duplex scan incorporates both B-Mode and color flow Doppler, which has its main applications in vascular and cardiac imaging. The ultrasound device discussed in this article utilizes the B-Mode.
Prior to scanning, the wound must be thoroughly cleansed with saline to assure the wound bed is free of debris and residues (any foreign materials may cause artifacts in imaging). The wound cavity is filled with a wound ultrasound transmission gel and then covered with an ultrasound film dressing (SonoView™ Wound Mapping Gel and Sound-Seal™ Ultrasound Film Dressing, Hudson Diagnostic Imaging, Elmwood Park, New Jersey) (Figure 3).
The ultrasound film dressing is made of urethane (0.5mm) and is coated with a medical-grade acrylic adhesive. The ultrasound transmission gel is used to fill the wound cavity and to eliminate any dead space or air pockets. The dressing seals the wound containing the gel and protects the wound from the transducer. Additional transmission gel is applied to the surface of the transducer and placed at a 90-degree angle to the wound.
Longitudinal and transverse scans are obtained by placing the probe in either direction and slowly scanning the wound from margin to margin (Figure 4). The Hudson 2020, 2040, and 2060 ultrasound scanners were used to scan the wounds in this study. Each scanner utilizes a linear array transducer with a frequency that ranges from 8.0 to 12.0Mhz. An image of a diabetic neuropathic foot ulcer (DFU) scanned in both the longitudinal and transverse axes (Figure 5) demonstrates the skin surface (SS), the ulcer (U), the base of the wound (WB), and the proximal end of the fifth metatarsal (M). In the transverse section, tunneling of the ulcer can be seen (T). Nearly exact measurements (within 0.1mm) of wound size (length, width, surface area, depth, and volume) may be calculated using the digital calipers.
Diabetic foot ulcers (DFU). The first case study involved an ischemic ulcer on the great toe in a type I insulin-dependent diabetic (IDDM) patient. Figure 6 includes a picture of the wound of the hallux and distal phalanx of the left foot. The ultrasound scan illustrates the ulcer and erosion of the distal phalanx caused by osteomyelitis. Figure 6 also includes the appearance of normal bone. Note the bright band caused by the high acoustic impedance of the cortical surface of normal bone. Below the surface of normal bone, the image appears dark due to the inability of ultrasound waves to pass through the bone. The scan of the ulcer in Figure 6 shows the distal phalanx to be discontinuous and dull. The cortical surface is uneven and pitted. Also, there is the absence of the solidly darkened area below the bone with significantly less acoustic shadowing.
The next case is an example of a diabetic neuropathic ulcer over the fifth metatarsal on the right foot. The photograph along with a longitudinal and transverse ultrasound scan can be seen in Figure 7. The scan from this fairly superficial ulcer shows a normal metatarsal head directly below. Localized edema can be visualized as the fluid space surrounding the ulcer. Localized edema is an indication that pressure and shear forces have not been adequately relieved. This signals the physician that footwear and offloading instructions need to be reviewed for the patient in order for proper healing to take place.
The third DFU case is an example of pressure and friction damage caused in an insensate foot (Figure 8). The skin over the first metatarsal phalangeal joint, although damaged, has not ulcerated. Upon scanning, deep damage can be seen to the flexor tendon and joint capsule. There is evident erosion of the metatarsal secondary to osteomyelitis.
Pressure ulcers. This case study involves a stage 3 pressure ulcer over the ischial tuberosity in a patient with spinal cord injury. Figure 9 is the photograph of the wound prior to ultrasound scanning. Included in Figure 9 are the corresponding images of this same wound. This wound was caused by a combination of pressure and shearing forces and is approximately 1cm deep. The exact size can be measured (1.55cm by .076cm) with digital calipers as demonstrated in Figure 9.
Another pressure ulcer found over the heel of the foot can be seen in Figure 10. The resulting scan, taken along the retrocalcaneal surface, reveals a large heel spur immediately below the ulcer. The bone spur makes it difficult to provide adequate pressure relief.
Lower leg ulcers and chronic venous insufficiency. A venous ulcer on the medial aspect of the malleolus is presented in Figure 11. The ulcer had a six-year history of nonhealing. The wound measured 3.24cm in length, 1.6cm in width, and was 48mm deep. The distal tibia appeared intact and showed no evidence of osteomyelitis. Most venous ulcers are not deep, averaging less than 4mm in depth.
Figure 12 illustrates a patient with chronic venous insufficiency and associated symptomatology (hemosiderosis, stasis dermatitis, and lipodermatosclerosis). Ultrasound scans were obtained over an area of lipodermatosclerosis and an area of normal skin (proximal). Hypoechoic regions can be observed in the scan of the skin hardened by lipodermatosclerosis. These hypoechoic areas are caused by superficial veracosities with pooled venous blood.
The next case is a venous ulcer with a long-standing history of nonhealing. Figure 13 shows the ulcerated skin on the medial aspect of the left leg above the malleolus. In the scan, below the ulcer diffuse hyperechoic (brighter) areas can be seen extending deep into the tissue. These areas are fibrin deposits from leaky vessels that have calcified causing induration of the skin. The more diffuse hypoechoic (darker) areas are interstitial edema whereas the dark pockets are veracosities with pooled blood.
Pyoderma gangrenosum. Pyoderma gangrenosum (PG) is an inflammatory condition that can result in skin ulceration. The salient feature of PG is an irregular ulcer (usually painful) with a raised inflammatory border. Ulcers may be confined to the dermis but often extend into fat and even down to fascia. A halo of erythema surrounds the margin of an advancing ulceration, which may expand rapidly in one direction and more slowly in another so that a serpiginous configuration of the ulcer results.[10,11] Most consider PG a manifestation of altered immunity without consistent pattern of immunologic disturbance. The condition may be idiopathic or may be associated with inflammatory bowel disease, rheumatoid arthritis, IgA gammopathy, or myeloproliferative disorders. The pathophysiologic mechanism of PG is unknown.
Histologically, there is edema, massive neutrophilic inflammation, engorgement, thrombosis of small and medium sized vessels, necrosis, and hemorrhage.[10,11] The heavy infiltration of polymorphonuclear leukocytes may lead to abcess formation and liquefaction of the collagen.
This case study illustrates a large ulcer secondary to PG in a patient with a history of inflammatory bowel disease. The ulcer is on the posterior aspect of the right leg involving the area above the distal calf muscle. Figure 14 is a photograph of the ulcer and the resultant scan obtained. The ulcer margin was advancing laterally and the scan was confined to that area of the ulcer.
The device discussed in this article is a new application of diagnostic ultrasound. It provides a noninvasive means of imaging of wounds. Nearly exact objective measurements can be made noninvasively and without probing. Chronic wounds and surrounding tissues can be assessed for abnormalities, foreign bodies, and depth of injury. Inflammation and edema can be evaluated and measured in order to gauge the effects of therapies and offloading modalities.
Information is being gathered to compare this device with other diagnostic tools, such as MRI and CT scans. Sensitivity and specificity studies comparing this device to traditional histopathologic tests are in progress to evaluate whether this method can effectively diagnose osteomyelitis and other ulcer-causing conditions, such as vasculitis, PG, scleroderma, lupus, and sarcoidosis. The fact that this imaging can be performed without relying on injected substances holds great promise for the ischemic limb. This method may also be a viable diagnostic tool to evaluate wounds and associated soft-tissue pathology. The value of having a noninvasive imaging device that can be used in the office or clinic cannot be understated. A physician’s ability to make a diagnosis in real time during an office visit is strategically advantageous to both the clinician and the patient. The combination of a clinician’s assessment of a wound, the ability to see beyond the walls of a wound using this diagnostic assessment tool, and incorporating advanced wound care products suggest a reduction in patient morbidity and treatment costs.