CHAPTER 3 Imaging methods for guidance of aspiration cytology

Suzanne Le P. Langlois, Steve Chryssidis

Percutaneous biopsy is a well-established and routine practice in imaging departments,^1,² and is a frequently performed interventional radiographic procedure, either as an inpatient or outpatient examination, by most trained radiologists. The technique, indications and complications are extensively described in basic imaging textbooks.³ Development of expertise often occurs by ‘apprenticeship’ during undergraduate training for radiologists and for many other clinicians. The procedure is safe, inexpensive and minimally invasive. An understanding of the imaging modalities, the lesion to be biopsied and overall experience of the proceduralist contribute to the success of the procedure.

Fine needle aspiration biopsy (FNAB) typically yields a small sample for cytological assessment, with limited or no architectural information. The decision to proceed to larger-diameter needles (core biopsy) may be determined either by the results of the fine needle aspirate or, in some instances, due to the cytologist being unavailable to interpret the findings at the time of the biopsy.

Fine needle aspiration biopsy and core biopsy often complement each other in facilitating and assisting in the diagnostic process.

There have been continuous improvements in needles, biopsy guides and mechanical biopsy devices, together with technological advances in the major imaging methods of computed tomography (CT) and ultrasonography (US). The use of magnetic resonance imaging (MRI), and the development of stereotactic guidance, particularly for brain and breast biopsies, is now more readily available. Previously inaccessible lesions can be safely sampled and many more areas of the body are now routinely biopsied under guidance. Radiological guidance has allowed the development of more invasive procedures such as catheter drainage, villus biopsy, fetal blood and tissue sampling and core biopsy.⁴ This leads to a reduction in open biopsy and two-stage surgical procedures by providing a definitive diagnosis prior to primary surgical treatment.

More than one imaging modality may be required, first to localize the lesion and then to obtain biopsy material. The radiologist performing the procedure should determine the method used and will be influenced by availability of equipment, difficulty in scheduling, urgency of the procedure and perhaps operator preference and experience. Ultrasound guidance offers flexibility and speed, whereas CT often provides safer access for deeper tissue biopsy. Ultimately, the imaging modality which offers the best lesion visualization and safest route will dictate the biopsy pathway. Fluoroscopy is an alternative which may be utilized for pulmonary and bone lesions, although not as widely used as CT guidance.

All the imaging techniques have advantages and disadvantages in various parts of the body.

The portability, ease of use and relative speed of ultrasound make it a favourable modality for guided biopsy procedures, particularly for superficial and moderately deep lesions. The utility of ultrasound guided biopsy has also been recognized by specialties outside of radiology, particularly for intraoperative guided lesion assessments and biopsy.

Where practical, ultrasound is the preferred biopsy option, particularly given there is no ionizing radiation. In many instances, however, the nature and position of a lesion may mandate the use of CT guidance. Modern CT equipment allows for real-time assessment of the needle position using CT fluoroscopy, streamlining the biopsy process. This pathway involves ionizing radiation, exposing both the patient and the interventional team. The increasing body mass index of the patient population, particularly within the Western world, has led to a progressive increase in the imaging dose required to visualize relevant structures and organs. The end result is an increased radiation dose to the patient and to the interventional team members. An elevated awareness of this issue has motivated the implementation of dose minimization strategies where practicable.

Ultimately, however, the modality used will be dictated by the availability of equipment, staff and site of the lesion.

The presence of a pathologist at the biopsy generally facilitates a more efficient process. Optimal results can only be obtained by meticulous localization before biopsy and this may occupy most of the procedural time; fortunately, this can be performed prior to the arrival of the pathologist (Fig. 3.1). The pathologist may direct the radiologist to a different area of the lesion, for example to the viable periphery rather than the central necrotic area of a solid lesion, and may request additional tissue for ancillary tests, such as special stains, electron microscopy or culture, or to determine the need for a core biopsy.⁵

Fig. 3.1 Pancoast tumour biopsy

Care is required in positioning the patient for maximum comfort and access to the lesion to avoid ribs and scapula.

It matters little whether the radiologist or pathologist performs the actual aspiration. In many cases, however, the radiologist is more skilled in interpreting the images and is better able to manipulate the needle in three dimensions while viewing a two-dimensional image.

Fluoroscopy

Fluoroscopy is the traditional method for guidance of biopsies to most parts of the body. With the advent of highly technical guidance methods such as CT and US, it is used less and is often considered less accurate. However, it provides a quick alternative for those radiologists not experienced in US guidance and is most useful in guidance for small, very mobile lesions, such as focal lower zone lung lesions. In this instance, the real-time fluoroscopy may be the only method of accurately placing the needle tip within a small lesion. It also offers efficient sampling options for cortical bony lesions. Fluoroscopy is a reasonable biopsy alternative because of its low cost and general availability.

Although fluoroscopic guided biopsy may be easier with a biplane system, a single-plane system with tube tilting to provide a stereotactic type of view of the lesion and needle can also be used.

Ultrasound

Ultrasound (US) is the only real-time guidance which allows imaging in any plane and is the only suitable guidance for biopsy of fetal tissues. Its use is limited in certain areas, as ultrasound is not transmitted through air or bone. Some parts of the body,⁶ such as the chest wall and musculoskeletal system, though neglected in the past, have undergone an increase in interest for both diagnostic and interventional studies. Developments such as operative probes and vaginal and rectal transducers are now combined with portable, handheld (as compared to mobile) US units, for use in intensive care areas and operating theaters, wards and clinics, and to regional and remote areas.⁷

Real-time monitoring is a major advantage as the exact location of the needle tip can be seen during biopsy and adjustments to its position can be performed to increase the accuracy of sampling. Visibility of needles can be a problem and needles should be tested for echogenicity prior to use. Many manufacturers also provide fine needle aspiration biopsy needles with etched tips to aid in ultrasound visualization. Stylets within needles, and particularly movement of the stylet within the needle, will improve visibility. The gauge of the needle does not necessarily relate to echogenicity and in many instances a fine needle may be more highly echogenic than a subsequent core biopsy needle. Using color Doppler may enhance visibility during movement of the needle.

The choice of transducer and frequency is dictated by the area of the body to be biopsied and depth of the lesion. Intracavitary probes and developments with intravascular and intraluminal transducers allow biopsy and intervention into virtually every part of the body.

While freehand guidance is usually preferred, if a biopsy attachment is used there should be easy separation of the needle to reduce the risk of tearing tissues, particularly in areas of the body where respiratory movement may occur, for example liver and kidney. The shortest puncture route is normally chosen, though as with hepatic biopsies it is advisable that the needle traverses at least a rim of normal parenchyma to reduce the risk of hemorrhage.

While sterile water can be used as a coupling medium, there appears to be no real risk of reaction from use of sterile coupling medium, but it must not contaminate the aspirate. Sterilization of the transducer and attachment is routine, including universal precautions against infection.

Technique of ultrasound guided biopsy

The longest part of the examination is preparation – identifying the lesion, positioning the patient, and identifying the site of puncture and direction of the needle. The depth of the lesion from the skin surface is measured to determine the length of the needle required; the biopsy route is typically oblique and thus measurements need to reflect this. Continuous real-time ultrasound is used to visualize the tip of the needle entering the lesion. Scanning continues during aspiration to ensure that the needle stays within the lesion, and the aspirate may be seen moving within the needle lumen.

Maintaining sterility

Sterility of the transducer can be achieved by wiping the transducer with skin preparation, placing a layer of gel onto the end of the transducer, and carefully placing it into a sterile bag (sterilized plastic wrap, specially manufactured covers held there by a sterile rubber band), maintaining sterility at all times. Care must be taken with the transducer cord to prevent it draping over the sterile area. If coupling gel is used it should be sterile, but an alternative to gel is to use sterile saline or skin disinfectant, although these require occasional replenishment as the alcohol or water rapidly dries on the skin. The tip of the transducer merely needs to be dipped into the fluid, and enough will usually adhere to provide adequate transmission of sound waves. Coupling agents such as betadine or chlorhexidine solution may be preferred over sterile gel in some instances, as this may contaminate samples and compromise sample quality.

Biopsy procedure

The transducer is ideally held in the optimal longitudinal position to visualize the lesion. Prior to insertion of the needle, it is imperative that the alignment of the transducer is checked, so that the image on the screen is aligned with the correct orientation. Optimally, this is done by tilting the transducer slightly up and down, and assessing the areas brought into view, but should be confirmed by pressing lightly on the point of proposed skin puncture by the needle, so that the visible disturbance of the soft tissues confirms correct orientation.

Some operators have an assistant holding the transducer, maintaining its position exactly parallel to the long axis of the lesion, and ensuring that it does not slip away from the puncture site. This allows the operator to concentrate on introducing the needle exactly parallel to the transducer, and is a useful technique particularly in complex biopsy cases. Some prefer to hold both and relinquish the transducer once the needle has punctured the lesion. Only practice will determine which method is more comfortable for each individual.

Under real-time guidance the needle is introduced through the skin, 1–2 cm proximal to the transducer, and at the midpoint of the narrow side of the transducer. The needle is then advanced along the line of the transducer. The length of the needle should become visible, and the tip is seen to puncture the front wall of the lesion, then inserted a few more millimeters. If the needle does not become visible on the screen, the transducer can be used to find it, or alternatively the needle alignment can be adjusted. The angle of the needle should be changed to lie parallel to the transducer, as it is only when the lesion and the needle are in line that a successful puncture can occur.

Some operators prefer to use the transducer in an axis transverse to the lesion, once again located just distal to the proposed point of entry. This is less accurate, and does not necessarily locate the tip of the needle, merely showing a portion of the shaft.

CT scanning

There are very few areas of the body which cannot be biopsied under CT control, and extremely small lesions can be sampled. Focal masses of several millimeters within the lung and skull base (Fig. 3.2) can be biopsied and retroperitoneal biopsies are limited only by availability of needles long enough to traverse the abdomen of large patients. Traversing with fine needles offers fewer risks compared with the larger-caliber needles.⁷ CT gantry tilt also further facilitates lesion access where appropriate.

Fig. 3.2 (A) Circular low dense lesion right retropharyngeal node in a patient post right parotidectomy and radiotherapy for parotid squamous cell carcinoma. (B) Coaxial fine needle aspiration biopsy technique under CT control confirmed nodal recurrence. The utility of CT biopsy techniques and fine needle aspiration biopsy helped direct this patient’s management.

Localization of the needle tip within a lesion is very accurate with CT (Fig. 3.3). It provides detailed cross-sectional images of the body which are not limited by the same physical properties as are ultrasound images, such as interference from bowel gas and bone.