ULTRASOUND TECHNIQUE Dr Charles B. S. Tsang, MBBS, M. Med(Surg), MS(Exp. Surg),FRCS(Ed), FRCS(Glasg), FAMS Head and Senior Consultant Surgeon, Division of Colorectal Surgery, University Surgical Cluster, National University Health System, SINGAPORE Endorectal Ultrasound We use the following equipment: 1. BK Medical Profocus® scanner with a 2052 probe. 2. Karl Storz rigid sigmoidoscope with a length of 20 cm and inner diameter of 22 mm. 3. Suction equipment 4. Normal saline to irrigate rectum if necessary 5. Boiled water to fill rectal balloon 6. 100 cc syringe with a Luer lock to connect to water standoff fitted over shaft of 2052 probe
We currently use a 2052 probe from BK Medical. This is a probe with multi-frequency transducer and in built motor puller. The transducer has a focal length of 2-5 cm and a 90 degree scanning plane. When the probe is rotated at 4-6 cycles per second, a radial scan of the rectum and surrounding structures is obtained. Frequencies available are 6, 9, 12 Mhz and 10, 13, 16 Mhz. The transducer can be pulled back manually or automatically in the situation of a 3D acquisition scan. For endorectal ultrasound, a special balloon is used to drape over the shaft of the 2052 probe and over the water standoff.
This is secured at the base with a series of rubber band. Holding the probe with the balloon in the most dependent position, the balloon is filled with 150-200 cc of water via the water standoff. Through a process of repeated aspiration and filling, any air bubbles in the system are aspirated through the syringe attached to the water standoff and expelled. Some water-soluble gel is placed on the exterior of the balloon and the probe is then ready for insertion. The patient bowels are prepared with two Fleet enemas® 30 minutes apart starting one hour before the examination.
The procedure is explained to the patient and a verbal consent is obtained. Demographic data of the patient is entered into the ultrasound computer. The examination proceeds with the patient in the left lateral position. A digital examination is performed to evaluate location, size, morphology and fixity of the rectal lesion. A rigid sigmoidoscopy is then performed. Any residual stool fluid is removed using suction equipment as it can interfere with the image. The tumor is visualized to determine its size, circumferential involvement and distance from the anal verge.
The rigid sigmoidoscope is advanced past the tumor as high up in the rectum to ensure complete imaging of the tumor and mesorectum. This is because tumor depth of invasion can vary at different portions of the growth. It is also important to visualize the mesorectum proximal to the tumor to look for enlarged metastatic lymph nodes. Whilst the endoprobe can be inserted blindly to visualize low and small rectal lesions, it is uncomfortable and extremely dangerous for mid and upper rectal lesions. Hence the use of a rigid sigmoidoscope to facilitate placement of the endosonic probe above the proximal limit of the tumor.
With the sigmoidoscope placed above the tumor, the endosonic probe is gently introduced through it. The probe is advanced thru the sigmoidoscope until resistance is felt. The sigmoidoscope is then pulled back over the probe thus exposing the transducer 6cm beyond the end of the sigmoidoscope. The balloon is then instilled with approximately 100-150 cc of water, the smallest volume required to achieve contact of the water balloon with the rectal wall. The probe is then activated by pressing a button at the proximal end or on the main console.
With the patient lying in the left lateral position and the single start/stop button on the base of the 2052 probe facing the operator , the anterior aspect of the rectum will be shown on the superior aspect of the monitor screen. The right lateral aspect of the rectum will be on the left side of the screen. Likewise, the left lateral aspect of the rectum will be on the right side of the screen. The transducer position is adjusted to maintain a central location within the rectal lumen to obtain optimal imaging of the rectal wall and perirectal structures.
The gain in the ultrasound unit is also adjusted to obtain optimal imaging. When optimal, it should be possible to visualize clearly all five layers of the rectal wall. Once this is achieved, the transducer within the 2052 probe is gradually moved caudad using the Up/Down motor buttons situated at the base of the probe. The rectal wall layers and surrounding structures are carefully observed. We also perform a 3D scan by activating the 3D button on the console. During a 3D scan, the transducer is automatically moved from the tip of the 2052 probe over a distance of 6 cm.
A series of 2D axial images are captured by the computer onboard and reconstructed into a 3D volume rendered image. The entire length of the rectal tumor is carefully examined. As such, it may be necessary to make several passes along the full length of the tumor to get all the necessary information. As the 2052 probe is a 3D probe, the transducer is moved up and down without having to physically withdraw or reinsert the probe. This makes the examination more comfortable. Once a 6 cm segment of rectum is examined, the 2052 probe can be withdrawn and the next 6 cm segment examined.
Once the 3D volume rendered images are acquired, they are carefully examined via software manipulation for uT (tumor) staging and another pass for uN (nodal) staging. For stenotic lesions, it is usually necessary to reduce the amount of fluid in the rectal balloon. If the stenosis is very tight, it may not be possible to get above the tumor. In this case, it would not be possible to stage the tumor reliably if the whole tumor cannot be imaged. Endoanal Ultrasound Endoanal ultrasounds are easier to do and does not require bowel preparation e. g. fleet enema.
The transducer is already factory sealed within the 2052 probe. We then fit a condom over the 2052 probe. Some water soluble lubricant gel is then applied to the condom and the probe gently introduced into the anal canal. The probe is then orientated as described in the technique described for endorectal ultrasound. The probe is advanced until the landmarks for the upper anal canal is identified i. e. puborectalis muscle. The transducer is then gradually withdrawn by actuating the Up/Down motor buttons and the anal sphincter complex and perianal tissues carefully visualized.
Or a 3D scan can be performed by activating the 3D button on the console. Thickness of internal and external sphincters are measured using calipers on the machine. Any sphincter defects are recorded. In females, the perineal body measurement is performed. With the gloved right index finger in the vagina, the probe is positioned in the midanal canal using the left hand. The image on the screen is then freezed and the distance between the inner aspect of the internal sphincter is measured to the hyperechoeic line representing the finger-vagina interface.
The perineal body measurement gives a good estimate of the thickness of the anal sphincter complex anteriorly. It is attenuated in the presence of sphincter deficiencies. Any hypoechoeic collection or tract representing anorectal sepsis is followed proximally and distally to identify its location and orientation according to the Park’s classification. When a fistula is suspected and an external opening is present, hydrogen peroxide is used as a contrast to help enhance the fistula tract. We place a 21 G IV cannula into the external opening and inject 1-2 cc of H2O2.
The bubbles that form within the fistula tract results in a hyperechoeic signal. This tract is then easily followed to its internal opening. Conclusion Endorectal and endoanal ultrasound provides for excellent imaging of the anorectum and surrounding tissues. Examination technique however must be meticulous. Residual stool fluid must be cleared to allow for optimal visualization. It may be necessary to perform several passes for a thorough examination. All examinations should be recorded for future reference.