ULTRASOUND GUIDED BIOPSY METHODS
Category: Diagnostic Radiology
Abstract : ultrasound guided biopsy methods - 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 pro
ultrasound guided biopsy methods - 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. 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.
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.
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 withdrawn containing the
required tissue core. The use of a spring-loaded gun to operate these needles is
now commonplace. 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 position 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 technique and minimizing post-biopsy
complications.
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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