Ventilation Krypton-81m

5.2.1 Radiopharmaceutical

• Krypton-81m

5.2.2 Uptake mechanism / biology of the tracer

The regional ventilation distribution can be assessed with a ventilation scintigraphy after inhalation of a radioactive aerosol or gas. Krypton-81m is an inert radioactive gas that is delivered from a rubidium-81/krypton-81m generator. Krypton-81m has a short half-life (13 s) and is inhaled continuously during ventilation imaging. The gas disappears from the alveolar regions by decay faster than expiration. Thus, the regional krypton-81m concentration reflects true regional ventilation at normal breath-rate.

5.2.3 Indications

General indications for lung ventilation scintigraphy with krypton-81m often combined with a perfusion scintigraphy include, but are not limited to:

General indications for lung ventilation scintigraphy with Krypton-81m often combined with a perfusion scintigraphy include, but are not limited to:

• Diagnosis and follow-up of PE;
• Evaluate the cause of pulmonary hypertension;
• Quantify regional pulmonary function before surgery/radiation therapy for lung cancer;
• Evaluate lung transplants;
• Evaluate emphysema for lung volume reduction intervention;
• Evaluate congenital heart or lung disease such as cardiac shunts, pulmonary arterial stenosis, and arteriovenous fistulae and their treatment;
• Evaluate chronic pulmonary parenchymal disorders such as cystic fibrosis;
• Confirm the presence of bronchopleural fistula.

5.2.4 Contra-indications

• There are only relative contraindications for ventilation scintigraphy.
• Even though krypton-81m is the ventilation agent with the lowest radiation exposure, when possible the ^81mKr dose to lungs should be reduced in pregnant or potentially pregnant patients.

Regarding the ventilation study, it is not recommended to interrupt breast feeding [3,91].

5.2.5 Clinical performances

Krypton-81m is used as part of ventilation studies either performed as planar or SPECT, and the most frequent indication is diagnosis of PE. A normal perfusion scan excludes PE, and a ventilation scan is not needed. However, if the perfusion scan is abnormal, a ventilation scan is needed for the interpretation. So, in many cases V/Q scintigraphy is performed. Regarding the clinical performance of V/Q scintigraphy see details under the ^99mTc-MAA perfusion scintigraphy. Due to the different gamma energy of krypton-81m and technetium-99m, it is possible to acquire the V/Q scan (with krypton-81m/^99mTc-MAA) simultaneously. Sequential V and Q imaging is also possible.

A krypton-81m ventilation scan without a perfusion scan can be used to evaluate ventilation inhomogeneity, as often seen in obstructive lung disease, and can be used for regional lung function evaluation.

5.2.6 Dosimetry

The organ receiving the largest radiation dose is the lung (210 nGy/MBq). The ED equivalent is 27 nSv/MBq. (Reference activity 6 GBq ~ 0.16 mSv).

Caveat

“Effective Dose” is a protection quantity that provides a dose value related to the probability of health detriment to an adult reference person due to stochastic effects from exposure to low doses of ionizing radiation. It should not be used to quantify the radiation risk for a single individual associated with a particular nuclear medicine examination. It is used to characterize a certain examination in comparison to alternatives, but  it should be emphasized that if the actual risk to a certain patient population is to be assessed, it is mandatory to apply risk factors (per mSv) that are appropriate for the gender, the age distribution and the disease state of that population."

5.2.7 Interpretation criteria/major pitfalls

For interpretation of combined V/Q scintigraphy, the reader is referred to the chapter on ^99mTc-MAA perfusion scintigraphy.
Ventilation defects often occur in moderate to severe COPD, asthma, cystic fibrosis, lung fibrosis, atelectasis, lung tumour, pneumonia, pleural effusion, and pulmonary infarction.
Pitfalls include that the krypton-81m gas distribution might not reflect true regional ventilation if steady state does not occur due to very fast breathing during gas inhalation.
The cyclotron produced rubidium-81/krypton-81m generator has a short T_1/2 of 4.6 h, and this limits availability.

5.2.8 Patient preparation

No particular patient preparation is needed before ^81mKr lung ventilation scintigraphy.

A recent standard chest radiograph or a CT scan can help in the interpretation. However, this is not needed if a CT scan is performed as part of a SPECT/CT procedure.

5.2.9 Methods

The detailed recommendations are available in the EANM Pulmonary Embolism Guidelines. Further information can be found in the published literature [93].