TÀI LIỆU HỘI NGHỊ KỸ THUẬT ĐIỆN QUANG VÀ Y HỌC HẠT NHÂN VIỆT NAM LẦN 7 (2019) Nguồn: https://hinhanhykhoa.com/

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Lower Limb CTA
The Alfred Hospital
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Ngon Tran
CT Supervisor Radiographer
Introduction
MDCT scanners with their isotropic
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MDCT scanners with their isotropic
resolution have made the high resolution
imaging of the lower limb vasculature
routinely possible with a single acquisition
and a single intravenous contrast injection

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Clinical Indications
• Acute and chronic peripheral arterial
disease
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disease
• Vasculitis
• Aneurysmal disease
• Post intervention follow-up (graft stent)
• Other indications: congenital
abnormalities, trauma, AVM, neoplasticabnormalities, trauma, AVM, neoplastic
disorders
Patient Positioning and Scan Range
• Patient is placed feet first, supine on the CT table
• In order to achieve the best image quality, the
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DFOV (display field of view) is kept as small as
possible by aligning the patient’s body close to the
iso-centre of the CT table
• Velcro bands are used to hold the patient’s knees
together
• Scan range is depending on the clinical indications:
Trauma: continues from CT CAPTrauma: continues from CT CAP
Others: either from top of diaphragm or above
aortic bifurcation
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Scan Technique
• Thin slices (0.625 mm or 0.5 mm) are used for this
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CT examination.
• Either 100 kVp or 120 kVp is used, depending on
the patient’s size
• Lower pitch setting is used to ensure that the
contrast has enough time to travel to the distalcontrast has enough time to travel to the distal
lower limb vasculature
Vessel Enhancement and Image
Quality
Reasons for poor image quality
•Poor timing of the contrast material bolus attenuation:
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g
defined as absent or very poor opacification of the entire
arterial tree
•Bolus outrun:
Defined as initially adequate opacification of the
proximal arterial tree which gradually fades to poor or
absent opacification distallyabsent opacification distally
This occurs when the CT scanner z-axis coverage
proceeds faster than the leading edge of the contrast
bolus (Foster et al., Radiology 2011)

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Vessel Enhancement and Image
Quality
Two techniques to monitor the arrival of IV
contrast to the region of interest:
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g
• Timing bolus
• Auto bolus tracking software:
GE: Smart-Prep
Canon: Sure-Start
Siemens: Pre-Monitoring
Principle of Timing Bolus
• The Contrast Medium Transit Time = the time
interval between the beginning of the contrast
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injection to the arrival of the bolus in the aorta is
varied among patients
• Aorto-popliteal Bolus Transit Time = the time
interval between the arrival of the contrast bolus
from the aorta to the popliteal arteries
• The Scan Delay = is decided from the results of 1st
timing run at the start location + 10 sec (fudge
factor)

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Abdominal aorta level
Popliteal artery level
Contrast Medium Injection Technique
130 -140 ml of contrast injected with a pressure
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injector (MedRad) into an antecubital vein through an
18-gauge cannula
• Timing run: 15 ml of contrast and 15 ml of
saline at 4ml/s for each timing run
• CTA run: 70-100 ml of contrast with 50 ml saline
chase at 4ml/s – depending on patient’ size

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Timing Run
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Contrast Injection Protocol

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Timing Run – Start Location
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Timing Run – below the Knees

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Lower Limb CTA Calculation Sheet (1st
part)
• Part I:
Calculate the time to peak at both locations:
A t t t t l ti
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Aorta at start location:
Number of tick mark on the graph (______× 2) + 15 = ___________ (A)
Popliteal (just below the knees):
Number of tick mark on the graph (______× 2) + 30 = ___________ (B)
• Part 2: Calculate the prep group delay
(A) + 10________ (A) + 10 sec = _________ sec
• Part 3: Calculate the time difference between the 2
timing runs (aorto-popliteal transit time)
________ (B) - ________ (A) = ________ sec (C)

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Lower Limb CTA Calculation Sheet (2nd
part)
• Part 4:
Manually enter
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- The 1st timing run location ________
- The 2nd timing run location ________
Take notes of the acquisition time between those 2
locations
Compare this acquisition time to that of (C) above. If
the acquisition time is less than (C) increase the
rotation time so that it is equal (change the rotation
time then pitch setting)
Manually enter the End location of the CTA run
GE scanner
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Siemens scanner
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Canon scanner
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Auto-bolus Tracking Software
1. Principle:
• This software allows intermittent monitoring of IV contrast
in a particular section of anatomy that is in the area of
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in a particular section of anatomy that is in the area of
interest
• These low dose scans are taken until the contrast
enhancement reaches the preferred threshold. The scan
then can be automatically triggered by the scanner or
manually by the CT radiographer
2. Applications:pp
• Trauma
• Patients with blocked distal aorta
Trauma Cases
1. Trauma: to be continued with the Trauma CT CAP
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– TL Spine scan – using a larger amount of
contrast
1. For other cases:
• Auto bolus tracking location is usually just above
the region of interest (for example: above the
facture site of the left femur)facture site of the left femur)
• Scan is manually triggered by the CT
radiographer

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Trauma Cases
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Autobolus Tracking Location
Trauma Cases with CT CAP
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Auto bolus Tracking Location
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Contrast Injection Protocol
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Patient with 46 sec scan delay

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Current Trend
1. 70 - 80 kVp has been used on some cases where patients
are small. Lowering the x-ray tube voltage to 70 - 80 kVp not
only decreases the energy of the emitted photons but also the
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only decreases the energy of the emitted photons but also the
radiation exposure to the patients. Choosing the x-ray tube
peak voltage close to the k-edge of iodine increases the
observed attenuation of iodinated contrast
1. Dual energy CTA has been used to decreases radiation
exposure to patients while maintaining or improving contrast-
to-noise ratio (Cook, T.S. ; Radiol Clin N Am 2016)
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Bibliography
1. Cook, T.S., (2016) Computed Tomography
Angiography of the Lower Extremities. Radiol Clin
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N Am 54, 115-130
2. Foster et al, (2011) Integration of 64—Detector
Lower Extremity CT Angiography into Whole-Body
Trauma Imaging: feasibility and Early Experience.
Radiology, 261: 787-795
Acknowledgement
The Alfred Radiology staff
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The Alfred Radiology staff
- Grade 3 CT supervisors
- CT radiographers