21
Imaging Techniques
suites are less desirable because of increased radiation exposure
to personnel during this procedure.1
When using fluoroscopy
for FNAB, fluoroscopic time should be kept to a minimum
and the operator should use only as much fluoroscopy as is
necessary to complete the biopsy, consistent with the as low
as reasonably achievable (ALARA) radiation safety guidelines.
Tight collimation and, when appropriate, thyroid or gonadal
shielding should be used.
computed tomography
Since its introduction in the 1970s, CT has become an indis-
pensable tool for diagnosis and has become an essential tool
for guiding FNAB and core biopsy, particularly of deep or inac-
cessible sites in the mediastinum, abdomen, and pelvis. CT uses
X-rays to create high-contrast cross-sectional images of the body
based on the differential X-ray absorption of tissues of differing
density. CT is capable of resolving physical density differences
in tissue of less than 1%, permitting differentiation of normal
and abnormal soft tissues. The CT scanner uses a rotating X-ray
source to produce radiation that passes through the body onto
an array of detectors. The attenuation of the X-ray beam by tis-
sue is continuously measured as the body is passed through the
X-ray field. These data are then processed by computer to recon-
struct the attenuation at each point in the image. CT eliminates
the superimposition of images of structures outside the area of
interest by producing a series of cross-sectional images through
the body, each image representing a slice of tissue measuring
only a few millimeters in thickness. The data acquired from
multiple contiguous or a single helical scan can be viewed as
images in the axial, coronal, or sagittal planes, depending on
the diagnostic task. With new multidetector scanners imaging of
the entire chest, abdomen, and pelvis is accomplished in a mat-
ter of seconds. Cross-sectional imaging provides 3-dimensional
spatial localization of lesion. With newer scanners providing CT
fluoroscopy, near real-time image guidance is possible.3
Compared to ultrasound, CT is more expensive and may
result in significant radiation exposure. The typical radiation
dose from a CT examination of the abdomen and pelvis may
be from 400 to 500 times greater than that from a chest radio-
graph.4 Therefore, when using CT for biopsy guidance care must
be taken to keep exposure of the patient and operator to a mini-
mum. In addition, iodinated contrast agents may be required
to provide adequate imaging of some masses. With the use of
intravenous iodinated contrast agents additional image contrast
may be achieved through the enhancement of differences in the
vascularity of tissues and lesions.
CT guidance is particularly valuable in patients with lesions
that are difficult to access or are in unusual or precarious loca-
tions, when the optimal route of biopsy requires avoidance or
transgression of vital structures, and in patients with unusual
anatomy (Fig. 21.1). CT is less operator-dependent than US and
is also preferred if an experienced US interventionalist is not
available.
Magnetic Resonance
MRI uses radiofrequency energy to generate cross-sectional
images, but, unlike CT, does not utilize ionizing radiation. Both CT
and MRI scanners can generate multiple two-dimensional cross-
sectional slices of tissue and three-dimensional reconstructions.
Tissue contrast with MRI is based on the interaction of the
magnetic field and radiofrequency pulses with protons, an
interaction fundamentally different from that used in CT and
US, and therefore may show lesions not detectable with other
methods. MRI provides excellent contrast and spatial resolu-
tion, but is more expensive than CT. MRI-guided biopsy is com-
plicated by the fact that special equipment not affected by the
magnetic field of the scanner must be used. MRI guidance is
generally limited to biopsy of lesions that cannot be adequately
imaged by CT or US, usually in the breast.
Ultrasound
US is the most versatile of imaging methods which may be used
for biopsy guidance. In addition to being readily available, the
equipment is portable and permits the use of transrectal, trans-
vaginal, and endoscopic approaches. US uses high-frequency
(3-15 MHz) acoustical energy to provide real-time, high-contrast
images throughout the body. Images are produced by the reflec-
tion of sound from tissue interfaces. No radiation is used and
there are no known risks for patients or equipment operators.
Like CT and MRI, ultrasound produces thin cross-sectional
images of the body, but unlike these methods, the ultrasound
transducer can be positioned to scan in virtually any image
plane. Because ultrasound imaging is not constrained by a gan-
try or magnet bore, angled approaches to lesions are facilitated.
Images are updated from 15 to 60 times per second, permitting
real-time imaging of needle position and target motion.
In addition to soft tissues, vascular structures may also be
imaged in real time without the need for contrast administra-
tion through the use of color or power Doppler. The combina-
tion of real-time imaging, high-contrast display of tissue and
blood flow, portability, low cost, and safety make ultrasound an
excellent method for guiding fine-needle aspiration biopsy in all
areas of the body in which imaging is not impeded by overlying
bone, gas, or excessive depth (Fig. 21.2). Adding to the value of
ultrasound is the ability to use small transducers for intraop-
erative, endoscopic, transrectal, and transvaginal imaging and
FNAB guidance.
Complications of Image-guided FNAB
FNAB is limited by imperfect success in obtaining a specific
diagnosis and by occasional complications of bleeding, infec-
tion, pneumothorax, and seeding of the needle tract by tumor.
Several variables determine both the success of image-guided
FNAB and the occurrence of complications. These include the
nature of the target lesion, the number of samples obtained,
the availability and experience of an on-site cytopathologist,
and the experience and skills of the physician performing the
biopsy. In general, the success rate for identification of malig-
nant lesions using image-guided FNAB should exceed 85% for
lung lesions and 80% for other lesions, with a serious complica-
tion rate of less than 1-2% .1
Serious complications of image-guided FNAB are uncom-
mon, but occasionally require admission to hospital,
an
unplanned increase in the level of care, or prolonged hospitali-
zation. Complications resulting in death or permanent disability
are rare. Major complications include bleeding, infection, and
unintended organ injury. A review of more than 10,000 abdomi-
nal biopsies indicated a mortality rate of 0.018% and a major
complication rate of 0.18%.5 In this study, bleeding was the
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