This randomized controlled trial evaluates a protocol optimization for Computed Tomography Pulmonary Angiography (CTPA). The study investigates the impact of repositioning the bolus-tracking region-of-interest (ROI) from the standard pulmonary trunk (PT) to the superior vena cava (SVC). The goal is to compensate for the CT scanner's inherent transit delay time (TDT) to better align the scan with the peak arterial phase. By optimizing this timing, the study assesses whether contrast media volume can be safely reduced by 40% (from 50 ml to 30 ml) while maintaining diagnostic image quality and preventing venous contamination.

Background and Rationale: Computed Tomography Pulmonary Angiography (CTPA) is the gold standard for diagnosing pulmonary embolism (PE). However, standard bolus tracking (BT) protocols that place the monitoring region-of-interest (ROI) on the main pulmonary trunk (PT) often face challenges due to the system's inherent Transit Delay Time (TDT). This delay, typically 5 seconds, can cause the diagnostic scan window to overlap with the venous phase, leading to suboptimal arterial opacification and venous contamination. This study proposes repositioning the monitoring ROI upstream to the superior vena cava (SVC) to utilize the TDT as transit time for the bolus, thereby aligning the scan window with the peak arterial phase.

Study Objectives: The primary objective is to evaluate the effect of SVC ROI repositioning on diagnostic image quality while investigating the feasibility of reducing contrast media (CM) volume by 40% (from 50 ml to 30 ml).

Methodology: This is a single-center, prospective, randomized controlled trial involving 72 adult patients. Participants are randomized into four protocol groups:

Group A (Control): ROI at PT, 50 ml contrast media. Group B: ROI at PT, 30 ml contrast media. Group C: ROI at SVC, 50 ml contrast media. Group D: ROI at SVC, 30 ml contrast media.

Protocol Details:

• Contrast Administration: Non-ionic contrast medium (370 mgI/ml) is injected at a rate of 4 ml/s, followed by a 20 ml saline flush.
• Bolus Tracking: An elliptical ROI is placed either on the PT (above the carina) or SVC depending on the group assignment.
• Triggering: The diagnostic scan is automatically initiated when enhancement reaches a threshold of 100 HU. The TDT is set to a fixed 5 seconds.
• Assessment: Diagnostic quality is assessed quantitatively by calculating arterial-venous enhancement differences (HU) and qualitatively via a 5-point Likert scale by three blinded radiologists.

Inclusion Criteria:

• All patients scheduled for a Computed Tomography Pulmonary Angiography (CTPA) examination.
• Provision of written informed consent by the subject, guardian, or medical specialist.
• Patients with an upper extremity intravenous (IV) line using a 16-20 gauge cannula.
• Patients with a stable heart rate between 60 and 120 beats per minute (bpm).

Exclusion Criteria:

• Pregnancy, breastfeeding, or use of non-reliable methods of contraception.
• Patients with impaired renal function, including acute kidney injury or chronic kidney disease (CKD) with an eGFR less than 30 mL/min/1.73m².
• Patients with a lower extremity IV line, as this bypasses the Superior Vena Cava (SVC) and results in bolus tracking technique failure.
• Patients with a heart rate lower than 60 bpm or higher than 120 bpm.
• Patients with a cannula size of 22g or smaller, as it cannot accommodate the high flow rate (4 mL/s) required by the power injector.
• Patients with severe cardiac impairment or congenital heart disease.
• Patients with a Body Mass Index (BMI) exceeding 35 kg/m², as severe obesity causes beam hardening effects that degrade image quality.

Individual participant data will be kept strictly confidential in accordance with the hospital's privacy policies and the informed consent agreement. To protect participant privacy, only aggregated results and statistical data will be reported in publications.

Three senior radiologists served as blinded outcomes assessors for this study. These assessors were blinded to all protocol details, including the anatomical location of the bolus-tracking region of interest (ROI) and the volume of contrast media administered for each participant. The CTPA images were anonymized and presented to the assessors in a randomized order to ensure objective qualitative evaluation of image quality using the 5-point Likert scale.