Methods of ultrasound guidance

There are various ways of performing ultrasound guided procedures: organ/lesion localization (‘blind biopsy’), biopsy guide or freehand technique. The choice of method depends upon the procedure in question, equipment and the experience and skill of the operator.
Blind biopsy
With this method a position on the skin surface is marked overlying the organ or lesion to be biopsied, using ultrasound to localize. This remains acceptable for diffuse disease, when only a representative sample of liver tissue is required. Nevertheless, it is good practice even in these situations to visualize the needle during the procedure, and this method of biopsy is now used less frequently. 
Biopsy guidance
Most manufacturers provide a biopsy guide which fits snugly on to the transducer head and provides a rigid pathway for the needle (Fig. 11.1). These are now the commonest and preferred method of biopsy. Previously adjustable angle biopsy guides were available; however these offered no specific advantages and were prone to user error. The fixed biopsy guides contain a groove for a series of plastic inserts ranging from 14G to 22G size, depending on the size of the biopsy needle. It is often preferred to use one size greater than the needle, that is a 16G insert for an 18G needle, as the needle tends to move more freely. These guides are sterilized and fitted on to the transducer, which can either be covered by a sterile sheath or thoroughly cleaned with chlorhexidine solution. The use of a sheath is highly recommended, as it maintains the sterility of the procedure, reducing the risk of infection, with no adverse effect on the image. The needle pathway is displayed on the ultrasound monitor electronically as a line or narrow sector, through which the needle passes. The operator then scans in order to align the electronic pathway along the chosen route, the needle is
inserted and the biopsy taken. These attachments should be tested regularly to ensure the needle follows
the correct path (Fig. 11.2).

Freehand A freehand approach, in which the operator scans with one hand and introduces the needle near to
the transducer with the other, may be used for larger or more superficial lesions. This technique is commonly used for breast biopsy and biopsy in the head and neck. The needle is inserted from one end of the probe at right angles to the ultrasound beam; generally speaking the angle utilized is shallow in comparison with the fixed guide systems for deeper structures.
image
(A) Necessary component parts to perform an ultrasound-guided biopsy procedure. A series of plastic
inserts (A) range in size from 14 to 22G. The appropriate insert is inserted into a fixed biopsy guide (B). The procedure
is performed with sterile jelly (C) and a sterile probe cover (D) if required. (B) The assembled biopsy guide.
image
Testing the alignment of the biopsy guide. The electronic pathway is activated on the image, and the needle is scanned as it is passed into a jug of water.
Equipment and needles
Equipment and needles The core of tissue for histological analysis is obtained with a specially designed needle consisting of an inner needle with a chamber or recess for the tissue sample and an outer, cutting needle which moves over it—the Tru-Cut needle. The biopsy is obtained in two stages: first the inner
needle is advanced into the tissue, then the outer cutting sheath is advanced over it and the needle with drawn containing the required tissue core (Fig.below. The use of a spring-loaded gun to operate these needles is now commonplace (Fig. 11.4). Such devices are designed to operate the needle with one hand; the whole needle is advanced into the tissue, just in front of the area to be biopsied. By pressing the spring-loaded control, the inner part is rapidly advanced into the lesion, followed rapidly by the cutting sheath over it. These needles can be obtained in a variety of sizes—generally 14, 16, 18 or, less commonly, 20 gauge. Most focal lesions are biopsied with a standard 18G needle. As a general principle, as the needle advances approximately 1.5–2.0 cm during biopsy, it is advisable to posi
image
Biopsy needle closed (top) and open (bottom) the needle tip on the edge of a lesion to obtain a good histological sample as most lesion necrosis tends to be centrally located. Such biopsy guns enable the operator to scan with one hand and biopsy with the other, observing the needle within the lesion, yielding a high rate of diagnosis with a single-pass technique1 and minimizing post-biopsy complications.
image
Spring-loaded gun designed to operate the cutting needle. As an alternative to the gun/needle combination a number of ‘self-fire’ needles are available. This is essentially a single-use spring-loaded biopsy needle. Again these come in a variety of sizes but their advantage is that they are easier and lighter to use than the gun/needle combination, and therefore are easier employed in the CT situation. Most departments will tend to utilize a combination of both. In cases where the clinician is not familiar with ultrasound techniques, appropriate guidance by a sonographer, while the clinician biopsies, is highly successful, quick and avoids potential complications. Fine-needle histology, involving the use of needles of 21 gauge or less, reduces even further the possibility of postprocedure complications. These are generally not used as only small amounts of tissue are obtained for analysis and, as thin needles, they are apt to bend more easily, and are therefore more difficult to see and retain within the plane of the scan. Biopsy of deep lesions is therefore more difficult, if not impossible Fine-needle aspiration cytology Cytology is the analysis of cells rather than the core of tissue obtained for histology. This is generally more difficult to interpret pathologically, as the characteristic architecture and intercellular relationships seen in a histological sample are absent. It has the advantage, however, of allowing a finer needle to be used. This can be passed through structures, for example the stomach, blood vessels, en route to the site of interest, with no adverse effects. Fine needles for cytology are of 21 gauge or smaller. They are of a simple design with a bevelled, hollow core and no cutting mechanism. The needle is introduced under ultrasound guidance to the required position. Fragments of tissue are removed into the needle by applying negative (sucking) pressure with a syringe to the needle, while moving the needle to and fro to loosen the tissue. These can then be expelled on to a microscope slide and smeared. The main disadvantage of this technique is that it requires a highly trained and specialized pathologist to interpret the samples, whereas all trained pathologists can view histological specimens. In addition, for many conditions, histological diagnosis is required, although cytology remains a useful tool in the breast and thyroid.
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