# Quantitative Lung Perfusion Scintigraphy

## Overview

Quantitative lung perfusion scintigraphy measures the relative distribution of pulmonary blood flow to each lung and lung region. Its primary clinical application is pre-operative assessment before pneumonectomy or lobectomy. It is also used in lung transplant evaluation, radiation therapy planning, and congenital heart disease assessment.

## Radiopharmaceutical and Technique

The same agent used for diagnostic V/Q perfusion scanning, Tc-99m MAA, is employed at a dose of 2 to 4 mCi (74 to 148 MBq). The patient must be supine during injection to avoid a gravity-dependent perfusion gradient. Anterior and posterior planar images are acquired, and the geometric mean is calculated as the square root of the product of anterior and posterior counts for each lung. Differential (split) function is expressed as the percentage of total perfusion going to each lung.

### Normal Values

The right lung receives approximately 55% of total perfusion, reflecting its three lobes, while the left lung receives approximately 45%, reflecting its two lobes. Deviation from these values indicates underlying parenchymal or vascular disease.

<image>Quantitative lung perfusion scintigraphy with ROI placement on anterior and posterior views showing geometric mean calculation and split function results (right 57%, left 43%)</image>

## Pre-Pneumonectomy Assessment

### Predicted Postoperative Lung Function

Quantitative perfusion data are critical for determining surgical candidacy in lung cancer and other resectable diseases. The predicted postoperative FEV1 is calculated by multiplying the preoperative FEV1 by the fraction of perfusion going to the remaining lung. For example, if a patient has a preoperative FEV1 of 2.0 L and 60% of perfusion goes to the right lung, a planned left pneumonectomy would yield a predicted postoperative FEV1 of 1.2 L. A predicted postoperative FEV1 greater than 0.8 L or greater than 40% of the predicted normal value indicates acceptable surgical risk. The predicted postoperative DLCO is calculated similarly using split perfusion and preoperative DLCO.

### Surgical Risk Thresholds

A predicted postoperative FEV1 above 60% of predicted indicates low risk for pneumonectomy. Values between 40 and 60% represent moderate risk, and further evaluation with cardiopulmonary exercise testing should be pursued. A predicted postoperative FEV1 below 40% or predicted postoperative DLCO below 40% indicates high risk, and limited resection or non-surgical therapy should be considered. On cardiopulmonary exercise testing, a VO2max above 15 mL/kg/min supports resectability, while a value below 10 mL/kg/min indicates high risk.

| Predicted Postoperative FEV1 | Risk Category | Recommended Action |
|---|---|---|
| >60% predicted | Low risk | Proceed with surgery |
| 40–60% predicted | Moderate risk | Cardiopulmonary exercise testing |
| <40% predicted | High risk | Consider limited resection or non-surgical therapy |

| VO2max (CPET) | Interpretation |
|---|---|
| >20 mL/kg/min | Low surgical risk |
| 15–20 mL/kg/min | Acceptable risk; supports resectability |
| 10–15 mL/kg/min | Increased risk |
| <10 mL/kg/min | High risk; surgery not recommended |

### Lobectomy Assessment

Regional perfusion can be estimated by anatomic segment proportion. The right upper lobe accounts for approximately 3/10 of right lung perfusion, the right middle lobe for approximately 2/10, and the right lower lobe for approximately 5/10. The left upper lobe (including the lingula) accounts for approximately 5/8 of left lung perfusion, and the left lower lobe for approximately 3/8. SPECT-based quantification provides more accurate lobar and segmental perfusion data than planar imaging.

<image>Pre-pneumonectomy planning workflow: quantitative lung perfusion scan results combined with preoperative PFTs to calculate predicted postoperative FEV1 and DLCO with surgical risk stratification</image>

## Lung Transplant Evaluation

### Pre-Transplant Assessment

Quantitative perfusion scintigraphy documents baseline split perfusion in patients listed for lung transplantation. It identifies the more severely affected lung to guide single-lung transplant side selection. In bilateral disease, the lung with worse perfusion is typically transplanted first.

### Post-Transplant Monitoring

Serial quantitative perfusion studies monitor allograft function. Declining perfusion to the transplanted lung suggests rejection, bronchiolitis obliterans, or vascular complications. Comparing split perfusion over time is more informative than relying on a single measurement.

## Radiation Therapy Planning

Quantitative perfusion data can be incorporated into radiation treatment planning for thoracic malignancies. This allows preferential sparing of the better-perfused lung during radiation field design. Functional lung avoidance radiation therapy uses SPECT perfusion data to weight dose constraints, potentially reducing post-radiation pneumonitis by directing the dose away from functional lung tissue.

## SPECT-Based Quantification

### Advantages Over Planar

SPECT provides three-dimensional perfusion data with lobar and segmental resolution. It is more accurate for calculating differential function when significant overlap exists on planar views and correlates better with actual postoperative pulmonary function. SPECT can be combined with low-dose CT for anatomic reference.

### Technique

Standard SPECT acquisition is performed after Tc-99m MAA injection. Volumes of interest are placed on each lobe, guided by CT anatomy. Software calculates the perfusion fraction for each lobe and segment.

<image>SPECT/CT quantitative lung perfusion showing lobar perfusion percentages overlaid on CT anatomy for precise pre-lobectomy functional assessment</image>

## Congenital and Pediatric Applications

Quantitative lung perfusion scintigraphy is used to assess differential lung perfusion in congenital heart disease, such as after Fontan or Glenn procedures. It evaluates pulmonary artery stenosis or hypoplasia and monitors perfusion redistribution after pulmonary artery interventions including balloon angioplasty and stenting. Particle counts are reduced in pediatric patients: 10,000 to 50,000 in neonates and 50,000 to 100,000 in older children.

## Clinical Pearls

MAA must always be injected with the patient supine. Erect injection produces a gravity-dependent gradient that artificially increases lower lobe perfusion.

The geometric mean method corrects for depth-dependent attenuation and is essential for accurate split function calculation.

In patients with an obstructing endobronchial tumor, the quantitative perfusion scan may already show reduced perfusion to the affected lung, making the predicted postoperative function more favorable than expected.

SPECT quantification is increasingly replacing planar imaging for pre-operative assessment, especially for lobectomy planning.

Post-obstructive changes such as atelectasis and pneumonia reduce perfusion, and this should be considered when interpreting split function.

Quantitative perfusion is complementary to pulmonary function tests, not a replacement. Both are needed for comprehensive pre-surgical risk assessment.

## References

- Defined Role of Quantitative Lung Perfusion in Pre-Surgical Assessment. *Seminars in Nuclear Medicine*, 2019.
- BTS/BTOG Guidelines on Pre-operative Assessment for Lung Resection Surgery.
- ACCP Evidence-Based Clinical Practice Guidelines: Physiologic Evaluation of the Patient with Lung Cancer Being Considered for Resectional Surgery.
- Defined Role of SPECT/CT Quantitative Perfusion. *Journal of Nuclear Medicine*, 2018.