Diagnostic ultrasound ( PDFDrive )

CHAPTER 25 Musculoskeletal Interventions 899
based on depth and local geometry. Needle selection is based
on speciic anatomic conditions (i.e., depth and size of region
of interest). We use a freehand technique in which the basic
principle is to ensure needle visualization as a specular relector. 7
his relies on orienting the needle so that it is perpendicular (or
nearly so) to the insonating beam (Fig. 25.1). he needle then
becomes a specular relector, oten having a strong ring-down
artifact. Although needle guides are available and may be of
value, a freehand technique allows greater lexibility in adjusting
needle position during a procedure. Furthermore, needle visualization
can be enhanced by injecting a small amount of anesthetic
and observing the corresponding moving echoes in either grayscale
or color low sonographic imaging. 1
N
FIG. 25.1 Needle as Specular Relector With Reverberation Artifact.
A 25-gauge needle (N) has been positioned into the retrocalcaneal bursa
deep to the Achilles tendon (T). Note that the needle is a specular
relector with a characteristic reverberation artifact (arrows).
T
Patient positioning should be assessed irst to ensure comfort
and optimal visualization of the anatomy. It is important to keep
in mind that tendons display inherent anisotropy; they will look
hypoechoic if the transducer footprint is not parallel to the
tendon. 17 herefore the transducer must be oriented to maximize
tendon echogenicity to avoid false interpretation of the tendon
as being complex luid or synovium. An ofset may be required
at the skin entry point of the needle relative to the transducer
to allow for the appropriate needle orientation. Deep structures,
such as tendons about the hip, are oten better imaged using a
curved linear or sector transducer, operating at center frequencies
of about 3.5 to 7.5 MHz. Supericial, linearly oriented structures,
such as in the wrist or ankle, are best approached using a linear
array transducer with higher center frequencies (>10 MHz).
Transducers with a small footprint (“hockey stick”) are particularly
well suited to supericial injections. hese factors should be
assessed before skin preparation.
he immiscible nature of the steroid anesthetic mixture
may likewise produce temporary contrast efect (Fig. 25.2, Video
25.1). In vitro experiments suggest that this property is caused
by alterations in acoustic impedance by the scattering material,
formed by the suspension of steroid in an aqueous background;
this results in an increase in echo intensity of about 20 dB. 19 his
contrast efect has the advantage of increasing the conspicuity
of the delivered agent during real time, enabling the operator to
better deine the distribution of delivered agent during ultrasoundguided
therapy.
INJECTION TECHNIQUE
We use a sterile technique; the area in question is cleaned with
iodine-based solution and draped with a sterile drape. he
transducer is cleaned with iodine-based or alcohol-based solutions
BASELINE EARLY LATE
FIG. 25.2 Contrast Effect. A suspension of anesthetic and triamcinolone has been injected into a cyst phantom. Baseline: Before injection,
anechoic “cyst” is shown in a scattering medium, with baseline pixel intensities listed. Early: The early mixing phase is obtained immediately after
injection. A contrast effect is evident, in which the cyst becomes almost isoechoic to the background. Late: 20 minutes after injection. In the late
phase, apparent gravitational effect results in settling of the suspension toward the dependent portions of the cyst phantom and development of
a contrast gradient.