Optimizing the VQ scan for image quality and radiation exposure

Objectives To compare the performance of an initial ventilation perfusion (VQ) scan protocol with that of a data-driven modified protocol to improve diagnostic quality without increasing radiation dose to the patient.

Methods Initial VQ scan protocol consisted of ventilation (V) scan after inhalation of Tc99m-DTPA aerosol for 5 minutes followed by Tc-99m MAA perfusion (Q) scan. Interim analysis after 34 scans under an initial protocol included calculations of ventilation efficiency (VE), perfusion efficiency (QE) and perfusion/ventilation count rate ratio (Q:V). VE was defined as V count rate divided by V dose, QE as Q count rate divided by Q dose, and Q:V as Q count rate divided by V count rate. From this data, the protocol was modified to improve the Q:V ratio and applied to 60 patients. Results from the 94 scans were tabulated and comparison of VE, QE, and Q:V between the two protocols were statistically analyzed.

Results The initial protocol returned a mean VE of 7.8% (sd=4.6; range=1.4-19), mean QE of 100% (sd=31; range=39-160), and mean Q:V of 2.4 (sd=1.9; range=0.51-9.0). All three parameters displayed a wide range. 54% of these cases demonstrated an unacceptable Q:V ≤ 2 indicating that the Q dose did not overwhelm the V dose. To improve Q:V, options included decreasing V dose, increasing Q dose, or performing the V scan with a much higher dose after the Q scan. To minimize radiation, the protocol was modified to decrease the ventilation from 5 minutes to 2.5 minutes. The modified protocol yielded a mean VE of 5.1% (sd=1.8; range = 2.0-11), mean QE of 120% (sd=27; range=65-170), and mean Q:V of 3.6 (sd=1.7; range=1.2-12). Differences between protocols were statistically significant for VE, QE, and Q:V (p < 0.02). Less than 8% of cases under the modified protocol exhibited a Q:V ≤ 2.

Conclusions Initial VQ scan protocol was successfully modified to improve image quality with less radiation. By decreasing the ventilation time in half, the percentage of studies with Q:V ≤ 2 decreased from 54% to 8%. This analysis may help others to optimize their VQ protocols.