Why SPECT?

It is incumbent on the diagnostic process to treat pulmonary emboli of any size as equally important.   The fact that your patient might have only minimal symptoms revealed as an isolated peripheral lesion, could well be the "Herald" for a DVT of massive proportions that might dissect in its entirety with fatal consequences.   Remember that 10% of all PE victims do not make it to the clinic, and the accuracy of diagnosis for the survivors leads to the difference between a 2% and 22% fatal outcome ("The natural history of pulmonary embolism".  Benotti JR, Dalen JE. Clinics in Chest Medicine 5: [3]; 403-410, 1984).

Technegas, once lodged in the respiring airways, will remain there for the lifetime of the nuclide.  As part of the regulatory requirements for evaluating Technegas, we measured the clearance from the lungs of 5 volunteers.   Over a 24h period, less than 3% of the activity (after allowing for decay) had moved from the lung field.

 Indeed, the transport characteristics of Technegas, make it superior to Krypton-81m in terms of truly reflecting quantitatively the ventilated volume of the lung.  The 13 second half-life of Krypton-81m leads to 2 artefacts.  One is that the activity in the 150mL of conducting airways can be a significant component of the lung image as measured by the "Penetration Index"[PI]  ("Airways penetration of inhaled radio-aerosol: an index to small airways function?" Agnew JE, Pavia D, Clarke SW. Eur J Respir Dis 62: 239-255; 1981).    The PI for Technegas is routinely higher than Krypton-81m for this reason.  This artefact comes about because the patient has to inhale the agent constantly during the imaging procedure.  The second short-coming of Krypton-81m is that as there is only partial gas exchange per breath, it can take over a half-life for the gas to reach equilibrium in the lung bases.  Thus there is a preferential signal in the apices. A practical sequela from this is that a patient will tend to breathe more deeply with a mask, enhancing the movement artefact in the images.   After Technega s administration, the patient can be helped to lie quietly and completely relax on the camera bed.  A mild sedative may assist the achievement of quiescent tidal breathing, minimising lung movement .

Although it was logical to explore tomographic imaging techniques, including a demonstration of breatholding vs quiescent during the clinical development of Technegas applications, it has taken many years for the imaging technology itself to percolate into sufficient number of departments to enable its use to become more widespread.

The initial limitations of single head cameras and slow processing times, made any general use of the technique not cost effective, and thus all but a few devotees of the art developed their Technegas experience on the back of pre-existing knowledge with planar aerosols, MAA and noble gases.

As a consequence, although there was almost universal agreement in the published literature that Technegas reduced the "indeterminate" diagnosis through increased confidence brought about by image quality, there was still a significant proportion of uncertainty in the diagnostic reports.

There are now many published reports on the value of Technegas in SPECT (SPET) imaging listed in the bibliography (#’s 1, 9, 40, 70, 91, 94, 96, 100, 108, 110, 111, 113, 117, 126, 127, 130,145,150,151, 164, 168, 170, 180, 184, 188, 189, 191).  Note particularly the seminal work from Dr Michael Lemb in Bremerhaven (#191 "Pulmonary thromboembolism: a retrospective study on the examination of 991 patients by ventilation/perfusion SPECT using Technegas"   Nuklearmedizin 2001; 40: 179-186).  Drs Lemb and Pohlabeln who operate a private practice in Radiology as well as Nuclear Medicine, recognised more than 12 years ago, that SPECT V/Q was the optimum imaging procedure for accurate definition of PE.   Even when the latest helical CT machines became available their "Gestalt" - to use the language of Alex Gottschalk - convinced them of the continuing value of V/Q with SPECT.

The Referral process - a shared diagnostic responsibility

Many Nuclear Medicine Physicians privately dislike reporting lung scans because they are well aware of the consequences of error,  either way. 

One of the intrinsic advantages in anatomical imaging, particularly by CT or MRI, is that it does not require the specialist skills or experience of thousands of case reports to identify abnormalities and explain them to your referring Colleague.  Properly presented, the images can look like sections straight from the text books.  Thus the burden of diagnosis is often shared directly from the images with the referring practitioner, and these days even the patient.

Planar V/Q lung images demand a range of interpretive skills and a large baseline experience to "call" many of the studies.   This is what Alex Gottschalk calls "Gestalt" interpreting.    SPECT V/Q allows your referring colleague to join you in the responsibility for the call based on clearly visible patterns in the slices.  The added reassurance against artefacts of seeing a defect progress and fade in sequential slices is often helpful.   There is also available "off the shelf" subtraction software that will generate a quantitative contour map of statistically significant mismatches in a pair of congruent image slices.