The transcatheter aortic valve implantation or TAVI planning CT protocol is used to plan for transcatheter aortic valve implantation. CT allows for the assessment of the aortic root and valve annulus in order to select an appropriate valve size and location specific to the patient. An aortic angiogram is also performed in order to determine the suitability of iliofemoral access 1.
NB: This article is intended to outline some general principles. Protocol specifics especially medications, contrast doses and decisions vs other cardiac acquisitions will vary subject to institutional protocols and patient factors as well as CT hardware and software.
The medication and contrast doses apply for cardiac CT examinations in adults.
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Indications
The main indication for TAVI CT is severe aortic stenosis 2. This can present as:
rapid or irregular heartbeat
chest pain
shortness of breath
heart palpitations
Purpose
The purpose of TAVI CT is to demonstrate the aortic valve filled with contrast without motion or step artefacts. The origins of the coronary arteries and entire aorta down to the femoral arteries should be opacified to allow for planning and measurements to be made.
This examination requires patients to maintain long breath holds and follow breathing instructions. A stable heart rate of around 60 bpm is ideal for capturing a motionless aortic valve.
Technical requirements
The minimal technical requirements for TAVI CT are the following 2:
64-slice scanner
detector element width ≤0.625 mm
option of cardiac CT and ECG-gated triggering
Patient preparation
patients should take their cardiac medications as usual
no food 3-4 hours before the scan
no caffeine for 12 hours
instructions on how to breathe
electrocardiogram signal need to be acquired
Technique
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patient position
supine with both arms above their head (as comfortable as possible)
ECG placement
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tube potential
100 kVp if patient’s weight ≤100 kg or BMI <30 kg/m2
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tube current
use automated current adjustment mode
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scout
aortic arch to femoral arteries
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scan range
calcium score to include valve only
TAVI to include the entire heart
aortogram to include aortic arch to the femoral arteries
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scan direction
craniocaudal
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contrast injection considerations
contrast agents with high iodine concentrations (270-400 mg iodine/mL)
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contrast timing
monitoring: ascending aorta
test bolus (test volume 10-20 mL at the same flow as the cardiac scan e.g. 5.0-6.0 mL/s)
bolus tracking
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contrast volume
50-70 mL contrast media at 5-7 mL/s
30-40 mL contrast media at 4-5 mL/s (for longer bolus)
50-80 mL saline chaser at 4-5 mL/s
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respiration phase
inspiration
for breath-hold consistency, a medium-sized breath is easier to reproduce throughout the examination compared to sharp deep breaths
if the aortogram and TAVI scan are performed in one breath-hold, instruct the patient to let their breath out slowly if they run out of breath
Practical points
setting the estimated heart rate as the patient's lowest heart rate will ensure the pitch is appropriate for imaging the patient's heart
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calculating scan delay for a test bolus
as the time-enhancement curve will only begin recording after the scan delay.
the widely accepted formula for calculating the scan delay is 3: peak contrast enhancement (time-enhancement curve) + scanner's diagnostic scan delay
TAVI work up
The workup is performed by either the radiographer, cardiologist, radiologist or a combination. Measurements are made for:
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valve plane
the plane just inferior to the coronary cusps
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annulus diameter
the diameter of the aorta one slice below the valve plane
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annulus size
the circular diameter of the aorta one slice below the valve plane
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right coronary artery height
from the origin of the right coronary artery to the valve plane
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left main coronary artery height
from the origin of the left main coronary artery to the valve plane
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sinus of Valsalva diameter
the diameter of the aortic valve leaflets
the largest point of the aortic bulge
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sinotubular junction diameter
one slice superior to the left and right sinus heights
just superior to the left and right coronary artery origins
where the aorta transitions to a straight wall
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left sinus height
from the top of the left sinus to the valve plane
gives rise to the left main coronary artery
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right sinus height
from the top of the right sinus to the valve plane
gives rise to the right coronary artery