Dynamic contrast MRI step by step: dry run, injection and arterial phase timing
A dynamic study follows contrast through an organ over time. The pictures after injection only make sense when compared with a picture taken before it, with nothing changed in between. That is why every dynamic study has the same skeleton: prepare, run the sequence once without contrast, inject, repeat the same sequence at the right moments.
The eight steps
- Prepare before the patient lies downTest the cannula with saline. Load the injector with contrast and a saline flush of 20–30 mL and remove all air. Calculate the dose from the true weight. Leave enough slack in the line for table travel. Tell the patient what they will feel, and practise the breath-hold with the exact words you will use.
- Finish everything that contrast would spoilSequences that gadolinium changes go first: in-phase and opposed-phase, pre-contrast T1, and MRCP when gadoxetate will be used (the bile turns dark on heavily T2-weighted images once it is excreted). T2 and DWI are commonly run after injection to fill a waiting period.
- Run the dynamic sequence once without contrast: the dry run (mask)Same sequence, same coverage, same breath-hold. It does three jobs. Test: you see whether the organ is covered, whether there is wrap, whether fat suppression is even, and whether the patient can hold their breath for the full scan time. Baseline: it shows what was already bright before contrast. Mask: it is subtracted from the later phases so only enhancement remains.
- Fix problems now, then freeze the set-upIf the dry run shows wrap, poor fat suppression or a breath-hold that is too long, correct it and repeat the dry run. After that change nothing: FOV, slices, matrix, coil, table position and prescan values stay as they are. Copy the mask series for the post-contrast phases so the same receiver gain, centre frequency and shim are reused. A new prescan changes image brightness and ruins subtraction and enhancement curves.
- Choose how you will time the scanFixed delay: start a set number of seconds after injection; simple, but wrong when circulation is slow. Test bolus: inject 1–2 mL and measure when it arrives. Real-time bolus detection (SmartPrep / BolusTrak / CARE Bolus): watch the vessel and start when contrast appears. Several fast arterial phases or a time-resolved sequence: acquire many, so one of them is right.
- Inject and start the clock togetherStart the injector and the timer at the same moment. Watch the injection site and the pressure during the first seconds. Give the breathing command early: it takes about 5 seconds to say and obey.
- Check each phase as it appearsLook at the vessels, not only at the organ. In a correct late arterial liver phase the hepatic artery and portal vein are bright and the hepatic veins are still dark. If a phase is mistimed or blurred, keep the following phases on schedule and tell the radiologist. A mistimed arterial phase cannot be repeated.
- Finish and documentMake the subtraction and MIP series. Label every phase with its time. Record the agent, dose, injection rate, time, and anything that happened: extravasation, movement, nausea.
How to calculate the start time
The contrast of an image is decided at the moment the centre of k-space is collected, not when the scan starts. With linear (sequential) ordering the centre is collected in the middle of the scan. With centric ordering it is collected at the beginning.
Start delay = target time − time from scan start to the centre of k-space. Begin the breathing command about 5 seconds before the start.
Example: you want the late arterial phase at 35 s. The scan lasts 16 s with linear ordering, so its centre is at 8 s. Start the scan at 35 − 8 = 27 s and begin the breathing command at about 22 s. With centric ordering the same scan would start at 35 s.
Times below are typical values from the references listed in this guide. They shift with the agent, the injection rate, the timing method and the patient's circulation. Your department's timing takes priority.
Liver: extracellular agent
| Before | Dry run (mask): 3D T1 fat-sat, same breath-hold |
| 0:00 | Inject 0.1 mmol/kg at about 2 mL/s, then saline flush |
| 0:30–0:45 | Late arterial: hepatic artery and portal vein bright, hepatic veins dark. With bolus detection, trigger from the aorta instead of using a fixed time |
| 1:00–1:15 | Portal venous: hepatic veins now bright |
| 2:00–5:00 | Delayed phase |
LI-RADS v2018 technical recommendations
Arterial phase too early
The aorta is bright, the portal vein is dark and the organ has barely enhanced.
It cannot be repeated. Keep the later phases on time and tell the radiologist. Next time use bolus detection or acquire two or three arterial phases.
Arterial phase too late
Hepatic veins are already bright; in MRA the veins overlap the arteries.
Check how the start delay was calculated: was the time to the k-space centre subtracted, and was the breathing command started early enough?
This page is one entry from the MRI Technologist Reference
The full reference has 31 protocols, 9 dynamic study timelines, 19 artifacts, 16 scanner problems and 15 safety topics, as a PDF in English and Arabic plus an interactive tool.
Educational reference. It does not replace your site protocols, the radiologist's instructions, the MR safety officer, or the manufacturer's instructions for any implant, device or contrast agent. Parameter values are typical starting points at 1.5 T and must be adapted to the scanner, coil and patient.