Pulmonary Scintigraphy

Introduction

Pulmonary scintigraphy is a useful tool in the diagnosis of patients with suspected pulmonary problems. Of these, pulmonary embolism (PE) is the most significant indication for the use of pulmonary scintigraphy. The gold standard for diagnosis of PE is pulmonary angiography but the invasiveness of this procedure prevents its use for screening of PE. Pulmonary scintigraphy on its own is not specific enough to diagnose PE, but when used with other diagnostic tests it improves the probability of making a correct diagnosis and choosing patients suitable for angiography.

Pulmonary scintigraphy is a two part study; ventilation and perfusion, and it is commonly referred to as a Lung V/Q scan. The majority of indications for use of a lung V/Q require both parts to make a diagnosis, but ventilation and perfusion scintigraphy studies are themselves separate techniques.

Ventilation Scintigraphy

Ventilation scintigraphy can be performed using a radioactive gas or aerosol. In this section reference will only be made to radioactive aerosols. For ventilation scintigraphy to work the radioactive aerosol must reach the regions of the lungs involved in gaseous exchange. The particle size must be large enough settle in the alveolar region but small enough to pass through the large airways without settling or becoming stuck. To create the radioactive aerosol the radiopharmaceutical (Tc-99m DTPA) is placed in a special nebuliser system. The patient is asked to inhale and exhale through a mouthpiece (a nose clip is used) until sufficient aerosol reaches the lungs. This takes about five to seven minutes and depends on the patient s cooperation. Patients are instructed not to swallow but spit any saliva that builds up during the process as any activity in the gut and oesophagus will affect images. Multiple views of the lungs are taken including anterior and posterior, right and left obliques and right and left laterals. Radioactive aerosols have an effective half-life of approximately one hour, thus allowing for multiple images to be acquired. A gamma camera cannot discriminate between photons of the same energy even though they have different sources. For this reason a waiting period, to allow for decay, between studies may be required if the same isotope is used for both studies.

Perfusion Scintigraphy

In perfusion scintigraphy a map of the perfusion of the lung is produced by introducing radioactive tagged particles of diameters larger that that of a red blood cell into the bloodstream proximal to the pulmonary capillary bed. The particles lodge in the pulmonary capillaries and precapillary arterioles and provided mixing has been adequate the resulting distribution is a map of lung perfusion. The matching or mismatching of a perfusion study to a ventilation and or radiograph is key to the final diagnosis. It is therefore essential that the majority of activity from a ventilation study has decayed before commencing a perfusion study.

Clinical Applications

Pulmonary Embolism

The diagnosis of suspected PE is the single most important indication for requesting a lung V/Q scan. A lung V/Q is not specific to PE but when used with other tests its accuracy of detection improves. Patients suspected for PE come from all branches of the hospital and as a consequence many physicians who encounter the disorder have little experience or expertise in its management or diagnosis. Because both scintigraphy and angiography provide the greatest evidence, the radiologist is by far the most important member of the team. There is no set protocol to follow when diagnosing suspected PE, however every physician should be aware of non-radiographic tests or observations that aid the radiologist in making his/her diagnosis. This testing or questioning should be done before a patient is sent to a nuclear medicine department.

Predisposing Factors

Signs and Symptoms

There are no signs or symptoms specific to PE but knowledge of associated clinical findings is essential and physicians should make note of their presence or absence. The most common signs and symptoms are:

Laboratory Findings

Laboratory findings associated with but not specific to PE include abnormalities in serum enzymes, low oxygen pressure and abnormal electrocardiographs.

Radiographs

In the initial diagnosis of patients with pulmonary problems one of the first tests is a chest radiograph. A chest radiograph is a routine procedure; it can detect other disorders and may eliminate the need for further evaluation.

Interpretation

The radiologist's interpretation for a lung V/Q for suspected PE will always follow a set of specific criteria to determine the probability or likelihood of PE. Probability falls into three categories: High Probability > 80%, Intermediate 20%-80% and Low < 20%. A result of normal is given if no perfusion defects are observed. Combining the results with the other tests mentioned above aids in the diagnosis and the decision of whether to proceed to the more specific angiography.


Figure 3.1 Abnormal perfusion scan. Multiple perfusion defects found in this patient's perfusion scan. This patient was unable to perform the ventilation study, preventing any match/mismatch analysis. In the absence of a ventilation scan a chest radiograph can aid in the diagnosis.

Non-embolic Diseases and Disorders

A lung V/Q is not intended to be a screening tool for patients with pulmonary problems but the information it provides when combined with standard practice helps to improve the physician's diagnosis. Because a lung V/Q is non-specific, a patient scanned for suspected PE may be shown from the results to be more likely suffering from another disease or disorder, effectively eliminating the need for angiography. For this reason a lung V/Q scan, if available, should always precede angiography. Patchy uneven ventilation scans are associated with chronic obstructive pulmonary disease (COPD), the two main forms being emphysema and chronic bronchitis. In many cases a patient with COPD has a normal chest radiograph. Whereas the perfusion study was the most important for diagnosing PE, for COPD it is non-specific unless accompanied by a ventilation study. Many patients evaluated for PE have COPD. This fact is very important considering that without a lung V/Q scan these patients may be sent for angiography. Figure 3.2 shows the V/Q scan of a patient suspected of having PE. The scan shows defects in both ventilation and perfusion images. From this scan the probability of PE is low, however a chest radiograph should be given as well. The findings are most indicative of some form of COPD.




Figure 3.1 Abnormal perfusion scan.A patchy ventilation scan and matching perfusion defects are more indicative of COPD than PE.