Bite in the bite block – While this direction may seem simple, most patients will require some guidance as to what they are to do. The overriding principle is that the lower dentition must be stabilized by the grooves. This groove is coincident with the middle plane of the anterior portion of the focal trough and is the mechanism by which the anteroposterior position of the head is stabilized.
Stages in correct positioning of patient in bite block:
Adjust head position vertically – The vertical orientation of the head can be described according to the relative position of one of four possible lateral reference lines (Figure 47). Refer to the manufacturer’s instructions for the specific reference line for the panoramic unit being used.
While the teeth are engaged in the bite block, the panoramic unit should be raised or lowered such that the lateral reference line referred to in the manufacturer’s instructions is parallel to an external reference plane. This external reference plane can be the floor but more commonly the external plane is located on the head holding device. More sophisticated units use adjustable light beams to assist the operator in orientating the patient’s head. Once the patient’s reference line has been adjusted to coincide with the external reference plane, many units will also have a forehead support that can be moved and secured to prevent patient head movement in the vertical plane.
Adjust head position horizontally – Correct positioning with respect to the horizontal plane minimizes differential left or right distortion and can be accomplished by visual assessment of the tilt and rotation of the patient’s head from behind and correction with the assistance of lateral head side guides. In some panoramic units this is further facilitated with the use of a midsagittal reference light or line superimposed on the frontal head support. In both cases the patient is asked to assist in head positioning by looking at their reflection in the anteriorly mounted mirror and adjusting the position of their head such that the line equally divides their face.
Have the patient shuffle forward and hold the hand supports – Because of the design of the panoramic unit, the natural tendency for patients when asked to put their head in the head holder is to thrust it forward. While the head can be correctly position in the head holder in this position, this results in the extension of the cervical spine. Because of the nature of the x-ray beam projecting upwards from below the shoulder and through the back of the patient’s head, this position provides a greater depth of soft tissue of the neck with an increase in midline opacity on the resultant image. To reduce this effect, the patient should be requested to shuffle or step forward approximately half a pace. This action straightens the cervical spine in relation to the projected x-ray beam and minimizes midline image artifacts. Many panoramic units provide bilateral handgrips not only to minimize patient movement during the exposure, but also to assist in positioning the body and the neck further anteriorly prior to exposure.
Check shoulder and receptor clearance – Prior to exposure, the clinician must check to be sure that the receptor and x-ray source can clear the patient’s shoulders. If the patient has a short neck and/or heavy shoulders, contact is likely with possible stoppage of machine rotation. Steps must be taken to prevent contact. One approach is to instruct the patient to lower the right shoulder down during machine rotation to clear the receptor. Other possible strategies include instructing the patient to bend the right knee to lower the entire right side, or to hold the handlebars underhanded or cross-handed rather than over-handed or seat the patient so the hands and the arms can hang down on each side to relax the shoulders. A combination of these strategies may be necessary to clear the shoulders and facilitate smooth machine rotation. If the x-ray source and receptor fail to rotate, only a portion of the jaws will be imaged or the patient may move in response to the shoulder contact. Note that the cassette clears the shoulder in the pictures above.
Ask the patient to close lips and hold tongue on the roof of their mouth – During the previous stages the patient’s facial musculature will tend to assume a relaxed position. This results in the lips around the bite block being open and the tongue dropping to the floor of the mouth. Immediately prior to exposure the patient should be requested to close their lips and place their tongue on the roof of their mouth or slightly behind the front of their top teeth and keep it there throughout the exposure of the patient. It has been suggested that to assist in tongue placement, the patient should be asked to swallow. Unfortunately the end phase of swallowing is that the tongue drops to the floor of the mouth. Therefore this instruction is not recommended.
According to Rushton et al.,12 while failure of the patient to place their tongue in the correct position is the most common technical fault (71.9%) it rarely results in an image being unacceptable (0.1%).
Exposure and patient dismissal – According to law, when a panoramic exposure is made on a patient the operator must ensure that a) they are in a position to observe the patient throughout the entire exposure b) be either 2 meters (approximately 6 feet) from the patient or if closer than 2 meters, behind a barrier.
Immediately prior to exposure the patient should be informed that the panoramic unit will move in front of them and take approximately 20 seconds to complete the rotation. The patient should also be reminded to keep their lips together and press the tongue against the roof of their mouth for the entire exposure.
Patient exposure is achieved by depressing the exposure button and keeping it depressed throughout the entire rotation of the panoramic unit until it comes to a complete stop. The exposure control is a “deadman” switch. This means that release of the button will terminate not only the exposure of the patient to x-radiation but the rotational motion. Exposure should be terminated if the patient moves markedly at the beginning of the exposure. This can be either due to the spontaneous patient motion (e.g. cough, sneeze) or due to shoulder interference with the rotational motion of the C-arm.
Immediately after exposure, the operator should release the head supports and ask the patient to slowly back away from the head holder. The patient should then be asked to remain in the area while the film is being processed. Only after the radiograph has been determined to be diagnostically acceptable should the patient be dismissed.