Table 1.
Comparative study of imaging modalities to assess COVID-19 clinical manifestation
| HRCT / U-HRCT | DECT | CTPA | SPECT-CT | EIT | |
|---|---|---|---|---|---|
| Principle of technique | Transmission tomography | Transmission tomography | Transmission tomography | Emission (SPECT) and transmission (CT) tomography | Transmission tomography |
| Energy source | X-rays produced by single high voltage (125 kVp) tube | X-rays produced by two low and high voltages (80 and 140 kVp) tubes | X-rays produced by single high voltage (120 kVp) tube | Photons (99mTc:140 keV, 81mKr: 190 keV, 123I: 159 keV) emitted by SPECT radiotracer and X-ray from CT tube (30–80 kVp) | Low intensity alternating electrical current |
| Signal production mechanism | Attenuation of X-rays in the lungs | Attenuation of X-rays in the lungs and pulmonary blood vessels | Attenuation of X-rays in the lungs and pulmonary blood vessels | Emission of photons from SPECT radiotracer and attenuation of CT X-rays in the lungs | Resistivity to the electrical current (bioimpedance) produced by air in the lungs during inspiration and expiration |
| Radiation exposure | Exposure to X-rays | Exposure to X-rays | Exposure to X-rays | Exposure to radioactivity and X-rays | No radiation exposure |
| Effective radiation dose | 1 mSv per chest scan | 1.5–3 mSv per chest scan | 7–10 mSv per chest scan | 6–7 mSv per SPECT/CT chest scan | Not applicable |
| Contrast agent | Contrast agent not required. FRI algorithm assesses vasculature on the stack of CT images of ≤ 1-mm thickness | Iodinated contrast agents are used to visualize perfusion and pulmonary embolism in the lungs | Iodinated contrast agents are used to visualize perfusion and pulmonary embolism in the lungs | Contrast agent not required | 5–7.5% w/v hypertonic sodium chloride intravenous bolus injection serves as contrast agent to measure lung perfusion |
| Radiotracers | No radioactivity involved | No radioactivity involved | No radioactivity involved | 99mTc-MAA,99mTc-DTPA, Technegas®, and 81mKr | No radioactivity involved |
| Findings | GGO, consolidation, pleural effusion, fibrosis, and microcystic honeycombing on lung CT images. Using FRI algorithm, redistribution of blood from smaller caliber blood vessels to dilated ones in GGO was observed | GGO and consolidation on transmission scan, hyperperfusion in GGO, and hypoperfusion in apparently normal zone on iodine map; pulmonary infarction and embolism on perfusion scan were observed | GGO and plural abnormalities on transmission scan; perfusion defects in normal and abnormal lung zones and pulmonary embolism may be detected | Decreased radioactivity uptake in the infract zone on perfusion ventilation scan and decreased radioactivity distribution in COPD lungs on ventilation scan | Unequal regional bioimpedance during respiration display ventilation perfusion mismatch, distention, or % collapse of the lung in four quadrants |
| Frequency of scans | One-time scan in asymptomatic to severe patients and follow-up scans in long haulers during rehabilitation process | One time or intermittent scans in moderate to severe patients to assess lung perfusion and pulmonary embolism site | One time or intermittent scans in moderate to severe patients to assess lung perfusion and pulmonary embolism site | One time or intermittent scans in moderate to severe patients to assess lung ventilation, and perfusion | Continuous bedside examination of lung ventilation and perfusion in moderate to severe condition and evaluation of drug therapy |
| Patient group preferred | Adults and pediatric | Adults and pediatric without renal complications when using contrast agents | Adults and pediatric without renal complications when using contrast agents | Adults and only perfusion SPECT scan in pregnancy | Adults, infants, pregnant, pediatric |
HRCT high-resolution computed tomography, U-HRCT ultra high-resolution computed tomography, DECT dual energy computed tomography, CTPA conventional computed tomography pulmonary angiography SPECT-CT single photon emission tomography- computed tomography, EIT electrical impedance tomography, GGO ground-glass opacities, FRI functional respiratory imaging, COPD chronic obstructive pulmonary disease, 99mTc-MAA Technetium 99mTc macro-aggregated albumin, 99mTc-DTPA Technetium 99mTc-diethylen-tetraamino-pentaacetate, Technegas® ultra-fine dispersion of 99mTc-carbon, 81mKr 81 m krypton gas