General principles of pre-intervention imaging
The main goal of diagnostic imaging in peripheral vascular disease is to determine the extent of multifocal lesions, assess the patency of individual arterial segments, and identify potential access routes. This allows creating an accurate "roadmap" (roadmap) for the surgeon.
The baseline modalities are computed tomography angiography (CTA) and magnetic resonance angiography (MRA). Due to the multiplicity of atherosclerotic plaques, scanning must encompass a vast extent: from the abdominal aorta all the way down to the pedal arteries (pedal arteries). Meanwhile, modern techniques for direct assessment of tissue ischaemia (dynamic post-contrast scanning, MR spectroscopy, or diffusion-weighted imaging) are rarely used in practice, as they seldom influence treatment management.
Computed tomography angiography (CTA)
CTA requires continuous table motion so that the scanner can cover the entire lower extremity anatomy within a single contrast pass.
- Speed technical challenge: Modern CT scanners operate so rapidly that the scan may simply "outrun" the contrast bolus travelling in the arterial phase.
- Calcification challenge: Atherosclerotically diseased vessels are frequently heavily calcified. It is extremely difficult for the physician to differentiate dense calcifications from intravascular contrast medium. This leads to false-positive or false-negative conclusions regarding segment patency, as well as overestimation or underestimation of the degree of stenosis.
- Solutions: To combat calcification artifacts, post-processing techniques are employed—multiplanar reconstructions (MPR) and volume rendering (VRT). Dual-energy CT (DECT) yields excellent results. Mask subtraction (mask subtraction) is technically complex and increases radiation dose, and is therefore used infrequently (although radiation dose is less critical in the older age group).
Magnetic resonance angiography (MRA)
Contrast-enhanced MRA using gadolinium-based agents obtained during the first pass (first pass) demonstrates the highest efficacy.
- Scanning features: Since the maximum useful field of view is limited to 40 cm, a moving-table approach (moving-table approach) is required. This allows for rapid and sequential visualization of the aortoiliac, femoropopliteal, and infrageniculate (infrageniculate) segments.
- Adipose tissue challenge: High signal intensity from bone marrow and subcutaneous fat can obscure patent arterial segments. On maximum intensity projection (MIP) images, this frequently leads to an overestimation of the severity and extent of stenosis.
- Solutions: The use of subtraction is strongly recommended. Alternatively, Dixon-type fat suppression can be utilized, which completely eliminates the fat signal and removes the need to acquire a pre-contrast mask.
- Advantages: MRA is ideally suited for multiphase imaging (multiphase imaging). This is critically important in cases of asymmetric blood flow and for visualizing the distal runoff below a hemodynamically significant (flow-limiting) stenosis.
Clinical case review (MRA)
As an example, let us consider a patient with severe left-sided intermittent claudication and a history of a contralateral above-knee amputation of the right leg (secondary to PVD). A peripheral contrast-enhanced MRA was performed at 3 table positions.
- Aortoiliac segment: Severe atheromatosis of the abdominal aorta with deep penetrating ulcers was revealed. Marked atheromatosis of the right external iliac artery and a short occlusion of the right common femoral artery are observed.
- Femoral segment: The left superficial femoral artery is occluded. Distal blood flow is reconstituted via a network of collaterals at the level of the adductor canal (adductor canal).
- Distal runoff (calf): Good runoff (run-off) is noted through all three vessels of the left calf simultaneously. There is minor venous enhancement, which, however, does not impede diagnosis.
Briefly: answers to topical questions
What clinical manifestations are included in the spectrum of peripheral vascular disease?
The clinical spectrum of peripheral vascular disease includes intermittent claudication (intermittent claudication) and ischaemic rest pain (ischaemic rest pain). Occlusive aortic disease is also characterized by diminished or absent femoral pulses, while critical limb ischaemia may present with trophic changes, ulcers, or gangrene.
What technologies are used to eliminate interfering adipose tissue signals in MR angiography?
Two main technologies are used to suppress fat signal during magnetic resonance angiography. The first technique involves classical subtraction, which is recommended for eliminating overlay artifacts. The second, modern technology is Dixon-type fat suppression, which completely eliminates the fat signal and removes the need to acquire a preliminary pre-contrast mask.
What methods for the direct assessment of tissue ischaemia exist in diagnostic imaging?
In modern diagnostic imaging, methods for assessing muscle ischaemia itself include dynamic post-contrast scanning, diffusion-weighted imaging (DWI), and spectroscopy. However, in clinical practice, these techniques have a significant limitation, as they rarely possess real clinical value when evaluating patients.
Through which specific collateral vessels is blood flow restored during superficial femoral artery occlusion?
During superficial femoral artery occlusion, blood flow is restored through an extensive network of collateral vessels. Specifically, blood flow bypasses the occlusion zone at the level of the adductor canal (adductor canal). The main source of collateral blood supply when the femoral artery is occluded below the origin of the profunda femoris artery is the descending branch of the lateral circumflex femoral artery (ramus descendens a. circumflexae femoris lateralis).
Which specific contrast agents are used for MR angiography?
For contrast-enhanced magnetic resonance angiography, gadolinium-based agents are used during the first pass (first pass).
Class and exam questions
Why is the region from the aorta to the feet scanned in peripheral vascular disease?
Atherosclerosis affects arteries in a multifocal manner. To plan treatment, the physician needs to visualize the entire vascular system in order to assess both inflow and outflow pathways.
What is the main challenge of modern fast CT scanners during lower extremity angiography?
The speed of the scanner table may exceed the blood flow velocity. As a result, the scanner "outruns" the contrast bolus, and distal vessels appear unenhanced.
How are adipose tissue artifacts that hinder vessel assessment combated in MRI?
The subtraction technique (subtracting pre-contrast images from post-contrast images) is used, or special pulse sequences with fat suppression, such as Dixon technology, are applied.
What else to study on this topic
- Technique of multiplanar reconstructions (MPR) and VRT in calcification
- Principles of dual-energy CT (DECT) in vascular imaging
- Physical principles of Dixon fat suppression
- Multiphase imaging technique in MRI
- Anatomy and assessment of adductor canal collateral blood flow
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