The patient is scanned at a pre-determined time point (delay) from the contrast bolus injection
Used on all routine chest, abdomen, and/or pelvis scans, routine trauma CAP scans, and mesenteric ischemia & enterography (late arterial at 45 seconds)
Bolus Tracking
Used for vascular CTA, cardiac, liver, and pancreas
The hounsfield units of a chosen region of interest (ROI) is monitored during the contrast bolus injection. Once a certain hounsfield unit threshold is reached (ex. 130 HU), the scan is automatically initiated.
Example: A CTA Chest ROI is placed in the descending aorta. When the blood in the descending aorta reaches a density of 130 hounsfield units, the scanner automatically scans the patient.
May not scan at the right time due to the trigger HU never being reached from factors such as cardiac output, body habitus, etc.
In these situations, consider using the test bolus technique
With any bolus tracking protocol you can have a “monitoring delay” and a “diagnostic scan delay”
Monitoring delay is the delay from injection start to actual tracking of the bolus
Diagnostic scan delay is the delay between crossing the HU threshold and the actual scan initiating
Example: A protocol has a 10 second monitoring delay/80 HU threshold/15 second diagnostic scan delay
The contrast injection and bolus tracking are started at the same time, the scanner counts down 10 seconds and then begins taking bolus tracking images. Once the HU crosses 80HU, the scanner counts down 15 seconds before the scan is acquired.
If the HU unit never crosses the threshold for whatever reason, the scan won’t automatically initiate and will time out. However, the techs can manually override and hit start.
A diagnostic delay is good for venogram type protocols where you still want a tight bolus (4-5ml/s) but you have a long delay for the venous system, 90-120s before the scan is actually acquired.
The minimum diagnostic scan delay is variable due to location of bolus tracking versus scan start location and the scan mode. For instance, a non-gated flash aorta will move into scan position and acquire much quicker than a gated flash aorta due to the heart rate synchronization of gated protocols.
Test Bolus
The hounsfield units of a chosen region of interest (ROI) is monitored during a small test bolus (20-40 cc) of contrast, and the time to hounsfield unit peak is determined. Then, the scanner is set to scan the patient with the time to peak as the delay after the injection of the main contrast bolus.
Used for pulmonary embolus, cardiac scans on the Flash scanner
Ex. For a pulmonary embolism CT, an ROI is placed in the pulmonary artery and a test bolus is given. Once the time to peak is determined (ex. 6 seconds), the scan is set to initiate the full scan 6 seconds after the injection of the main contrast bolus.
Multi-Phasic Organ Based Protocols
All multiphasic organ based protocols are triggered at 100 HU in the aorta with subsequent scanning of the arterial phase ranging from 12-18 second delay after trigger depending on the scanner
Liver 4 Phase example
Arterial Phase: 18 seconds after HU trigger
Portal Venous Phase: 35 seconds after HU trigger
Final Phase: 120 seconds after HU trigger
The arterial phase on these examples will be a little bit more delayed than the arterial phase on a true arterial based examination such as an aorta study where the scanner is trigger immediately upon reaching the target HU (i.e. no 18 second delay)
In general, if you want to make sure the structure you are wanting to evaluate has fully enhanced then look at the structure that would normally enhance next and see if demonstrates enhancement
For example, to determine if you have peak liver enhancement on a well timed portal venous phase you look at the the hepatic veins to see if they are enhanced by antegrade flow
Liver Enhancement Patterns Explained
Arterial phase
Refers to the hepatic arterial phase
Hepatic artery and branches are fully enhanced
Hepatic veins not yet enhanced by antegrade flow
Two Subtypes
Early AP: Subtype of AP in which portal vein is not yet enhanced
Late AP: Subtype of AP in which portal vein is enhanced
Extracellular phase
Postcontrast phase in which liver enhancement is attributable mainly to extracellular distribution of a contrast agent.
Operationally, this refers to:
PVP and DP if an extracellular agent or gadobenate is given
PVP only if gadoxetate is given
Portal venous phase
Portal veins are fully enhanced
Hepatic veins are enhanced by antegrade flow
Liver parenchyma usually is at peak enhancement
Delayed phase
Postcontrast phase acquired with extracellular agents or gadobenate after the portal venous phase and with the following characteristics:
Portal and hepatic veins are enhanced but less than in PVP
Liver parenchyma is enhanced but usually less than in PVP
Typically acquired 2 to 5 minutes after injection
Transitional phase
Postcontrast phase acquired with a hepatobiliary agent after the extracellular phase, before the hepatobiliary phase, and with the following characteristics:
Liver vessels and hepatic parenchyma are of similar signal intensity
Both the intracellular and extracellular pools of the agent contribute substantially to parenchymal enhancement
Typically acquired 2 to 5 minutes after injection of gadoxetate
Typically not obtained with gadobenate
Hepatobiliary phase
Postcontrast phase acquired with a hepatobiliary agent where
Liver parenchyma is hyperintense to hepatic blood vessels
There is excretion of contrast into biliary system
Typically acquired about 20 minutes after injection with gadoxetate
Typically not obtained with gadobenate
If obtained, typically acquired 1-3 hours after injection with gadobenate
Suboptimal if liver is not more intense than hepatic blood vessels
Radiology Protocols 