Clinical Technique

The biggest variable in the administration of Technegas is the patient. The generator, through its automated control circuits, makes extremely
reproducible amounts of Technegas. The level of anxiety created by the need to use a nose clip, or the mere presence of all the Nuclear Medicine apparatus, can create hyperventilation and breathing difficulties generally, which could compromise the efficacy of the test. In addition, if the patient has acute chest pain or dyspnoea, the ability to perform the optimum deep breathing manoeuvre and breathold could be minimal. This does not of itself present a major problem, if the patient's respiratory condition is assessed before hand, since more activity may be added to the crucible to whatever level is desired (see below ).Thus rehearsal with the breathing system independent of the generator is of crucial importance in allaying the patient's concern. In training your patient, get them to imagine they are drinking via a straw. This enhances a true sucking
action from the mouth.
The nose clip has been identified as the most critical item in maintaining a good seal. Despite the temporary discomfort to the patient, take particular care to ensure that the clip is placed low enough on the nostrils to effect complete occlusion. Patients with large flared nostrils pose a real challenge in this regard.

Patient Posture

If at all possible, patients should be supine or as nearly so as they will permit, for both the inhalation and the injection phase of the V/Q procedure,
and of course the imaging phase. There are practical physiological reasons for this.

• The patient will lie still, and become far more relaxed for imaging. This will minimise organ movement and make the image quality far better for
  interpretation, in particular, the degree of congruency between ventilation and perfusion will be optimum.
• Breathing is usually diaphragmatic when supine, leading to a better registration of the lung fields on the camera during breathing, and hence
  judging the amount of Technegas to be inhaled.
• The V/Q gradient in the lung will be matched. The difference in effects of posture on the ventilation distribution for Technegas is easily measureable, underlining its gas-like property.

Best administration technique

It has become evident that the newer multi-head cameras pose some problems for breathing up the patient under the machine itself. The main one is
that even the slightest leakage of Technegas during the procedure usually from the patients lip seal or nose can lead to its intake into the cooling fans of
the camera head, or even on the collimator face, delivering a rather bizarre background pattern and increased count. Another issue is that some cameras do not allow much room for manipulating the PAS system and controlling the patients breathing.

An elegant solution originally devised by Mr Mark Butterworth of the Prince Charles Hospital in Brisbane, Australia and now practised in many other centres, is to lie the patient recumbent on a bed in the ‘hot’ laboratory or some other side room. Then take the standard radiation protection monitor as used for routine contamination surveys and sheath it in a plastic bag, and have the patient hold it against the right anterior mid lung zone of his or her chest. Breathe up the patient with Technegas in the usual way ( see below) until the activity registered on the monitor reaches a pre-determined value ( usually marked with an indelible line on the meter cover). This pre-determined value has been obtained by measuring the value obtained from a known injected dose of MAA over a series of patients as part of their normal investigation. If desired, a table relating male/female and body mass index (BMI) to a monitor reading can give more individual correlation to the activity of Technegas inhaled.

Once the inhalation manoeuvre has been completed, the patient can be returned to the waiting area until the camera is free for their imaging procedure.

Busy departments report on the practical effectiveness of this procedure and have up to five lung studies waiting at a time. The postural difference compared with lying supine for the MAA injection is a gravity gradient not related to lung segmental anatomy per se, and no confusion with interpretation for PE mismatch is reported.

The inhalation manoeuvre

If the patient is at all dyspnoeic, or has known hypoxia, it is advisable to insert a short length large bore (eg 17g.) needle, connected to the hospital oxygen line, into the PAS delivery tube and deliver a steady flow of oxygen at 6-10L/min. Alternatively, the patient may be given a few breaths of pure oxygen immediately before inhaling Technegas.

The latter method has the advantage that it does not dilute the incoming Technegas and keeps the breathing time to a minimum, but the former method is useful particularly with the frail-aged who tend to defeat the mouth seal. In these, any leakage of Technegas, being dilute, poses less of a contamination issue near the gamma camera.

Ideally, the patient should inhale as slowly as possible to minimise turbulence in any obstructions in the bronchi, from Functional Residual Capacity, to a reasonably full but comfortable inhalation maximum. Inhalation to vital capacity is not recommended any longer since it has been found that modern cameras and SPECT imaging can detect the excessive Technegas concentration apparent when the patient is breathing quiescently for imaging. Then when the perfusion injection is performed it is preferable to have the patient breath-hold at about the same level to ensure the most even V/Q distribution.

The breath should then be held for 3 seconds at least but no more than 5. This is a vital part of the procedure as it allows the convective flow to cease long enough for pure diffusion to cause the particles to migrate to the surfactant medium in the alveoli, where they stick (see Properties of Technegas for further details). As an example of the value of this part of the procedure, measurements have been done to show that breathold to no breathold can cause inhalation retention per breath to move from 80% to 20%.

The whole manoeuvre is repeated until the count rate on the gamma camera reaches the level predetermined by the department protocol. This is usually about 2,500 counts/s, the value corresponding to about 37MBq or 1mCi activity inhaled.