Endoluminal ultrasound is usually performed with the patient in the left lateral decubitus position. Before the probe is inserted into the rectum, a digital rectal examination may be performed to identify the size, fixation, morphology and location of the tumour, if it is low enough. If there is a stenotic annular lesion, the finger can check to determine whether it will allow easy passage of the probe. The entire shaft of the probe is coated with a thin layer of warm gel using a paper towel. The probe tip is gently inserted through the anal canal and then angled posteriorly and advanced cephalad to as high a level as possible, with the bony sacrum used as a curved landmark. The patient should be instructed before the examination that no pain should be experienced. If pain should occur, the study should be halted until the cause of the pain is elucidated. Under no circumstances should force be used to advance the probe. The examiner should never try to push the tip through a narrow stenotic lesion. However, in most instances passage can be achieved, although the volume of the fluid in the balloon will have to be substantially reduced in order to withdraw the probe through the stenotic portion. Once the tip is advanced to as high a level as possible, usually 10-14 cm from the anal verge, the balloon can be inflated with 50 ml of water. Now the rotating transducer is activated and the rectal wall visualised. When the spigot for introducing water into the balloon is pointing towards the ceiling, by convention the anterior aspect of the rectum will be superior (12 o'clock) on the screen, right lateral will be left (9 o'clock) on the screen, left lateral will be right (3 o'clock) on the screen and posterior will be inferior (6 o'clock) on the screen (just like the image on axial CT scan). The tip of the ultrasound probe should be maintained in the centre of the rectal lumen to gain optimal imaging of the rectal wall and perirectal structures. Some adjustments may have to be made in the gain of the ultrasound unit to provide optimal imaging. Occasionally, it is possible to perfectly depict all five layers of the rectum circumferentially, but usually only a portion of the rectal wall can be optimally imaged at a time and minor adjustments will have to be made in the location of the probe relative to the rectal wall at various locations to optimally image all five layers clearly. The amount of water in the balloon may have to be increased to provide complete acoustic coupling with the rectal wall. The examiner should never distend the balloon with more than 80 ml of degassed water, as it may rupture. If this occurs, the probe must be removed from the rectum and cleaned, a new balloon installed and the whole procedure started over. If air or stool gets between the balloon and rectal wall, it will prevent visualisation of the wall. To avoid this we administer an enema 2 hours before the examination. Despite this, problems can arise and it may be necessary to remove the probe and suction out the rectum with reintroduction in order to optimise the image.
With the probe at the highest level possible and with good visualisation of the rectal wall, images are obtained at 1-cm intervals as the probe is withdrawn. The exact level of the transducer tip can be read off the metal shaft of the ultrasound probe. More closely spaced images (0.5 cm) are obtained in the area of any abnormality. The balloon may have to be deflated and reinflated to maintain good acoustic contact with the rectal wall as the probe is withdrawn down the rectum. Once the entire rectum down to the anal sphincter has been evaluated, the balloon is fully deflated and the probe is removed from the rectum. The entire length of the rectal tumour is carefully examined and it is not uncommon to require several passes along the full length of the tumour to gain all the information that is necessary. In some instances, two to six passes may be required to properly stage a rectal cancer. In most instances the use of a large bore proctoscope serves several purposes (Sapimed, Alessandria, Italy) (Fig. 5). It allows visual examination of the rectal tumour with exact determination of its location both with respect to circumferential involvement of the rectal wall and the distance from the anal verge. Secondly it allows suctioning of any residual stool or enema fluid that might interfere with the acoustic pathways of the ultrasound waves which may distort the image. Most importantly, however, it allows easy passage of the probe above the tumour to insure that the transducer is advanced above the rectal lesion to allow complete imaging of the rectal tumour. This is of extreme importance as the lower border of a rectal cancer can differ significantly in the depth of invasion to the centre or upper portions of the cancer and lymph nodes in the perirectal region are often just above the level of the tumour and will be missed if complete imaging is not obtained. Small distal lesions can be adequately imaged with the ultrasound inserted blindly and advanced above the lesion, but for most mid-rectal tumours, the use of a proctoscope will facilitate the passage of the transducer. Once the 20 cm scored mark on the shaft of the probe is at the proximal end of the proctoscope, the proctoscope is then pulled back on the probe as far as possible thus exposing the transducer for 7 cm beyond the end of the proctoscope and thus positioned above the rectal cancer.
Fig. 5a, b. Dedicated proctoscope for endorectal ultrasonography assembled with (a) 1850 probe or (b) 2050 probe
The balloon is then instilled with 30-60 cc of water, the volume of fluid usually needed to gain optimal imaging.
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