Page 321 - Feline diagnostic imaging

20.3 Computed Tomography--uided Biopsy 327
especially useful when ultrasound is unable to identify a
lesion (due to gas‐filled lung between the lesion and the
transducer), or if the lesion is inaccessible by other means.
High contrast resolution and lack of superimposition
issues with overlying gas and bone allow unobstructed vis-
ualization of the needle pathway and target tissue.
However, unlike ultrasound‐guided needle biopsy, the nee-
dle cannot be followed through the tissues in real time; it is
followed incrementally in successive scans.

20.3.1 Technique
When using CT for needle placement, accurate needle tip
localization is essential for a safe, accurate biopsy. The
freehand method (as opposed to stereotaxic guidance sys-
tems) is most commonly used [9,10]. Pre‐ and postcon-
Figure 20.4 Transverse ultrasound image of a dog with a trast CT scans of the area are obtained initially to evaluate
peripheral pulmonary mass. The needle (arrows) enters the the extent of the lesion and the presence of adjacent vas-
hypoechoic pulmonary tissue for aspiration. It is important to
avoid entering the deeper aerated lung; pneumothorax is a cular structures. Once the target area (best site of biopsy)
complication. is determined, the exact transverse CT image and table
position are noted. The skin is marked with an indelible
marker along the transverse plane, indicated by the gan-
aspirates may have been obtained from necrotic portions of try laser light. Strips of barium paste are applied in the
the tumor, resulting in a false diagnosis of inflammation. sagittal plane; these will appear as end‐on opaque dots on
Infectious diseases can also be diagnosed cytologically, subsequent transverse images. The best skin entry site is
including bacterial and fungal pneumonia [7]. chosen, between the appropriate barium “dots.” The
Contraindications for thoracic aspirations include depth of the lesion and angle (if necessary) from vertical
coagulopathies, pulmonary bullae, severely compro- are determined. After prepping the skin, the needle is
mised pulmonary function, and pulmonary hyperten- inserted. Multiple scans can be done to follow the passage
sion. Complications include hemorrhage and of the needle, and insure correct entrance into the target
pneumothorax. Pneumothorax is more likely if pulmo- lesion (Figure 20.5). Once confirmed, the biopsy is
nary infiltrates, with intermixed air‐filled lung, are obtained. CT scanning of the area is done immediately
aspirated compared to more solid pulmonary mass after the biopsy to check for complications of hemorrhage
lesions or marked consolidation. Spreading of infection or pneumothorax.
or seeding the needle track with malignant cells appears Correct assessment of needle tip location is critical. This
to be extremely rare [7]. may be difficult due to respiratory motion or partial volume
averaging in small lesions. The smallest slice thickness
20.2.3 Aspiration/Biopsy of ( 1/3 or 1/2 the width of the mass) helps to avoid partial
Mediastinal Masses volume averaging. A false tip identification occurs when
the CT slice includes only the shaft of an angled needle
Mediastinal masses are usually well visualized, and aspira- [10]. The true needle tip has an abrupt, or square‐ended
tion/biopsy can be safely performed with ultrasound guid- tip. A low‐density artifact may be associated with both a
ance. Color Doppler evaluation is utilized to detect and false tip and a true tip, so the shape should be used as the
avoid large vessels in the cranial mediastinum. most reliable indicator [10]. Respiratory motion may be
unavoidable during the needle biopsy. In these cases, it is
safest to let the needle swing freely with respiration rather
20.3 Computed Tomography- than anchoring it in place.
Guided Biopsy The most common complications of CT‐guided thoracic
biopsy are pneumothorax and hemorrhage. These have
Computed tomographic (CT) guidance of needle biopsy been reported in 32–43% of cases, and are usually minor,
(fine needle aspiration or tissue core biopsy) can be used subclinical, and do not require treatment [8,9,11].
to sample mediastinal or pulmonary disease [8–12]. It is There is a significant correlation between complications of