Standards of the Polish Ultrasound Society - update. Ultrasound examination of the aorta and arteries of the lower extremities.

A Doppler ultrasound examination has an advantage over other vessel imaging methods as it can be carried out by the patient's bedside and allows to make a diagnosis without exposing the patient to the inconveniency of transportation or an X-ray scan. The purpose of testing the lower extremities and the aorta is to objectively confirm a preliminary clinical diagnosis, localize lesions responsible for the symptoms of the disease, determine their severity and nature (e.g., if they are calcifications or soft lesions), and finally evaluate the hemodynamic criteria. In assessment of the aorta attention is paid not only to aortic diameter measurements, but also to the vascular lumen (dissections with the formation of two flow channels, detachments, balloting of atherosclerotic plaques, etc.) and the presence of atherosclerotic plaques with influx into them (PAU - penetrating ulcer in the plaque or lesions surrounding the aorta, such as retroperitoneal fibrosis or mycotic aneurysm). A correct diagnosis of an abdominal aortic aneurysm requires repeated measurements of the abdominal aorta diameter, and in particular its transverse dimension. When assessing the degree of peripheral arterial stenosis on the basis of hemodynamic parameters, degree of morphological stenosis must be taken into consideration. Collateral circulation may reduce the flow through the main vessel, and thus, the achieved systolic velocities are lower and may understate the degree of the assessed stenosis. Calf vessels are difficult to detect, which results both from the thickness of the muscle and the presence of calcifications. This article has been prepared on the basis of Standards of the Polish Ultrasound Society (2011) and updated on the basis of the latest reports from relevant literature.

Introduction

The evaluation of the abdominal aorta is one of the basic elements of abdominal imaging. Extending the examination by adding the assessment of flows in iliac vessels and lower limb arteries through color imaging and evaluation of flow velocity using spectral registration (PW Doppler) enables a fast and non-invasive diagnosis of lesions in these vessels – obstructions, stenoses, vascular anomalies. In many surgical centers an ultrasound test combined with the assessment of ankle-brachial indexes serves as a basis for the pre-qualification of patients for further proceedings.

Apparatus

The examination of the abdominal aorta and iliac arteries requires broadband 2–5 MHz convex transducers, with a minimum of 128 transceiver channels. Harmonic imaging helps to obtain reliable interpretive images. The scanner must be equipped with a color and power Doppler, and velocity measurements should be performed in a triplex mode – simultaneously with the moving image B, operating color and spectral Doppler modules.

The examination of peripheral arteries involves using linear 5 MHz, 7.5 MHz or broadband transducers with 4–7 MHz or 5–10 MHz frequencies. It is possible to use transducers of higher frequency, e.g., 5–12 MHz – but they have a smaller range of signal penetration into the body, making it difficult to examine overweight patients. From a practical point of view, it seems that the best range is 4–7 MHz.

The front of the transducer should have the length of 35–55 mm. A shorter one can be freely manipulated during the examination, while a longer one allows for a simultaneous imaging of longer segments of vessels. The Doppler module must also enable the electronic tilting of the signal recording beam. A duplex /triplex option is required. The range of recorded pulse Doppler velocities should be as wide as possible, with the minimum of 300–400 cm/s after adjusting the test angle. The color option must have a wide range of visualized velocities.

The imaging dynamics of the apparatus should be as high as possible (>140 dB), so that is can simultaneously record small echoes of homogeneous tissues and relatively large echoes of the vessel walls.

Measurement software and a system for the archiving and documentation of images (videoprinter, HDD, DVD, USB port) are also required.

Preparation for the scan

As part of the examined vascular structures are located within the abdominal cavity, the patients must follow rules applicable to this type of examination: they should not eat or drink (especially carbonated beverages) for at least 6–8 hours prior to the scan, on the preceding day they need to limit the consumption of flatulence-producing foods (raw vegetables, fruit, sweets, stale bread), and refrain from smoking for 6 hours prior to the examination. Significantly overweight patients may take preparations reducing the amount of intestinal gas.

Scanning technique

The test begins with the assessment of intra-abdominal vessels – the aorta and iliac arteries – with a 3.5 MHz convex transducer. All the vessels are examined in two planes, starting from the upper abdomen. The diameter of the entire available section of the abdominal aorta is evaluated and checked for atherosclerotic lesions, with particular emphasis on the area of the renal arteries, next the transducer is moved downwards to the division of the aorta. The aorta is scanned in the longitudinal plane in order to evaluate smaller irregularities of the lumen and atherosclerotic lesions, as well as examine the celiac trunk and the superior mesenteric artery. A closer examination of the vessels extending from the aorta is not necessary, but if any anomalies are found, it is advisable to indicate their presence. The retroperitoneal space located in the immediate vicinity of the aorta should also be evaluated. Aortic bifurcation area (splitting into common iliac arteries) is first evaluated in the transverse, later by the longitudinal plane. The color option exposes the lumen of patent vessels, the pulse Doppler assesses the spectrum and measures systolic velocity.

If a pathology is found, documentation including the morphology and records of flow velocity changes must be prepared. When large atherosclerotic lesions are diagnosed, their thickness must be measured.

Due to poor anatomical availability, the common iliac arteries pose a diagnostic challenge, especially in overweight individuals.

Intestinal gases make the examination difficult – vascular imaging requires a lot of strength and prolonged compression (which is painful for the patient). However, it is applied only if no aneurysms in the aorta or iliac arteries were diagnosed during the preliminary evaluation.

The registration of the Doppler spectrum should be carried out while moving the transducer over the entire length of the vessel, as flow disturbances associated with stenosis may disappear 3– 4 cm away from the tapered section.

Visualization of the internal iliac artery origin is important because of three reasons. Firstly, it allows to distinguish stenoses located in the common iliac artery from those located in the external artery. Secondly, it enables the evaluation of the initial section of the external iliac artery where atherosclerotic lesions can frequently be found. Thirdly, it allows to investigate the patency and direction of blood flow in the internal iliac artery, which may be a source of blood supply to the limb in case of a distinct stenosis/occlusion of the common iliac artery.

Compression is also required when examining the proximal segments of external iliac arteries. In the section of a vessel located closer to the anterior wall of the abdomen, a good angle can be obtained between the axis of the artery and the direction of the ultrasonic wave beam. The flow velocity should be recorded for the documentation.

The examination is continued after the convex 3.5 MHz transducer is replaced with a linear-vascular one. Each extremity is examined individually, starting around the groin area from the common femoral artery. Not only the spectrum and flow velocity, but also the size, nature and thickness of atherosclerotic lesions are examined. About 4 cm below the inguinal ligament the common femoral artery bifurcates into a deep femoral artery and a superficial femoral artery (superficial segment of the femoral artery). The entire length of the superficial femoral artery is assessed and described. The flow velocity in the initial section of the deep femoral artery should be examined and included in the description.

Next, the transducer placed along the axis of the vessel above the superficial femoral artery is guided downwards in the direction of the Hunter's canal, where the vessel imaging becomes difficult. We may use a convex 3.5 MHz transducer, which due to deeper penetration enables an insight into the course of the vessels and allows recording of the spectrum.

The next step is to evaluate the popliteal artery – the examination is performed with the patient lying in a prone (or lateral) position. The vessel is evaluated starting from the lower section of the femoral artery, with the transducer moved downwards to the origin of the front tibial artery and further over the tibiofibular trunk. Velocity measurements should be made in the middle section of the vessel where the artery is positioned in a slightly oblique manner which creates a favorable angle for the registration of the Doppler spectra.

Calf arteries are examined mainly in longitudinal planes. Anterior tibial artery is easily visualized in the upper part of the calf (where it passes through the interosseous membrane) by placing the transducer longitudinally between the fibula and the tibia, approximately 5 cm below the patella. This vessel may be absent in as many as 10% of the population.

Fibular artery is examined by placing the transducer parallelly to the side edge of the fibula, and then sliding it towards the back and bending forward – this method allows to visualize both the veins and the fibular artery. Positioning the transducer in the middle section of the calf in the lateral projection by the fibula and applying pressure reveals a pulsating vessel, thus making it possible to locate it.

Posterior tibial artery is best visualized in the medial malleolus area. The transducer oriented along the long axis of the vessel is then moved upwards and the vessel can be translocating into the calf between the muscles.

Blood flow spectrum

The abdominal aorta demonstrates a three-phaseic flow spectrum. Similar spectra normally occur in the iliac, superficial femoral, popliteal, calf and peripheral arteries, but the second component of Doppler spectra disappears as the patient's age increases. Routinely, the measurements should be performed and documented in: the aorta, the lower sections of the external iliac artery and popliteal arteries.

In the case of pathological changes the following should be documented: flow changes above the stenosis, flow velocity at the stenosis site, velocities and spectral changes in the vessel below the stenosis and flow disturbances in peripheral sections of the limb. Normal flow velocities in the arteries of the lower extremities are presented in tab. 1. As the patient's age increased, the blood flow rates become reduced.

Tab. 1

Flow velocities in the vessels of the lower extremities

Abdominal aorta	70–100 cm/s
Common iliac arteries	70 ± 20 cm/s
External iliac arteries	120 ± 20 cm/s
Common femoral arteries	115 ± 25 cm/s
Superficial femoral arteries (proximal section)	90 ± 15 cm/s

Evaluation of the abdominal aorta

In aortic assessment, attention should be paid not only to diameter measurements, but also to the lumen of the vessel (dissections with the formation of two flow channels, detachments and the balloting of atherosclerotic plaques, etc.), the presence of atherosclerotic plaques with inflow of blood into them (PAU – penetrating ulcer in the lamina – fig. 1) or the presence of lesions around the aorta (e.g., retroperitoneal fibrosis – fig. 2; mycotic aneurysm – fig. 3).

Fig. 1

PAU – penetrating ulceration in the atherosclerotic plaque: A. visible arteriosclerotic plaque 21 × 18 × 10 mm; B. visible inflow into the plaque

Fig. 2

A, B. Retroperitoneal fibrosis. Hypoechoic “cuff ” along the aorta; C: it also comprises the inferior mesenteric artery

Fig. 3

Mycotic aneurysm. Around the echogenic reflections of the aneurysm wall a visible area of lower echogenicity can be seen (visible measurement of the thickness of the area)

Diagnosis of the lower limb stenosis

The purpose of Doppler examination of lower extremities and the aorta is to objectively confirm the preliminar y clinical diagnosis, localize lesions responsible for the symptoms of the disease, determine their severity and nature (e.g., if they are calcifications or soft lesions), and finally evaluate the hemodynamic criteria. This allows for preliminar y planning of effective revascularization (proximal vs. combined revascularization of multi-level lesions).

Locations of atherosclerotic lesions:

in iliac arteries the plaques are usually located on the back wall of the vessel;

atherosclerotic lesions are typically formed in vessel bifurcation points;

diabetic patients are often diagnosed with deep femoral artery stenosis;

calf vessels with calcifications forming in the central membrane – those require careful evaluation.

Five factors are crucial in the diagnosis of the degree of hemodynamic changes caused by stenosis:

morphological evaluation of the changes;

flow disturbances at the stenosis level;

hemodynamic implications of the stenosis expressed by disturbances in the velocity and spectrum of the flow in vessels located below the stenosis;

multilevel stenoses causing various changes in velocity and nature of the flow above, between and below the stenoses;

assessment of blood flow in the extremities in cases of single- or multi-level stenoses located in all evaluated vascular structures.

Hemodynamic criteria for diagnosing the stenosis of peripheral arterial lumen

Two methods can be used to evaluate the degree of stenosis (fig. 4).

Fig. 4

Velocity measurement in the peripheral arterial stenosis. Systolic velocity in the stenosis increased from 134 cm/s to 305 cm/s. Dilated spectral window

Criteria based on the evaluation of the peak systolic velocity ratio (modified by Kohler et al., 1987) are presented in tab. 2.

Tab. 2

Criteria based on the evaluation of the peak systolic velocity ratio (modified by Kohler et al., 1987)

1	A normal vessel – three-phase flow, no spectral dilation
2	Stenosis <20% – a three-phase flow spectrum with slight spectral dilation, a small increase in the flow not exceeding 30% compared to the proximal section. Normal spectrum of the flow proximally and distally from the stenosis
3	20–49% stenosis – a three-phase spectrum retained at the stenosis site with a retrograde wave decreasing proportionally to the increase in the numberof lesions, distinct spectral dilation with the filling of the spectral window, a 30–100% increase in PSV at the stenosis site compared to the proximal section. Normal spectrum of the flow proximally and distally from the stenosis
4	50–99% stenosis – a single-phase flow at the stenosis site with no retrograde wave, spectral dilation (with a distinct turbulence in larger stenoses), over two-fold increase in PSV compared to the proximal segment of the vessel. Peripherally, a single-phase flow with a reduced velocity depending on the degree of stenosis
5	Vascular obstruction – no color signal or flow in the vessel lumen. Slower flow with increased resistance above the vessel, slower single-phase flowin the peripheral vessels. Collateral vessels with non-physiological flow directions are often visible in the vicinity of the lower edge of the obstruction. In case of obstructed iliac vessels a reversed flow direction in larger vascular trunks (e.g., the deep femoral artery) may occur

Criteria based on the systolic velocity absolute peak value (Cosman et al., 1989) are presented in tab. 3.

Tab. 3

Criteria based on the systolic velocity absolute peak value (Cosman et al., 1989)

	PSV cm/s	Velocity ratio
Normal	<150	<1,5:1
30–49%	150–200	1,5:1–2:1
50–75%	200–400	2:1–4:1
>75%	>400	>4:1
Occlusion	No flow

The most important factor changing the velocity at the stenosis site is collateral circulation proximal to the stenosis. Low resistance in the collateral circulation results in the underestimation of velocities reached at the stenosis site since a lot of blood flows into t hese vessels. High resistance (lack of effective collateral circulation proximal to the stenosis) resulting from stenosis is indicated by an specific spectrum record (fig. 5). An ultrasound examination is less able to accurately determine the degree of stenosis in the case of multilevel stenoses.

Fig. 5

The spectrum before the resistant stenosis suggests the lack of outflow into the collateral circulation just proximal to the stenosis. Velocity before the stenosis is 55 cm/s – the flat shape of the systolic peak suggests resistance related to the tight stenosis. The velocity inside the stenosis increased to 402 cm/s

Test description

Test description must include:

the date of the test;

patient's name, age/ date of birth;

the name of the scanner, transducer type and frequency.

Aorta

Diameter (uniform/non-uniform).

The presence of atherosclerotic lesions (including the echogenicity of major lesions).

In case of dilatations: the length of the dilated section, maximum transverse dimensions, upper neck diameter and length (the approximate distance between the upper edge of the aneurysm and the origin of renal arteries), lower neck diameter and length (the aorta located behind the aneurysm – distance from the lower edge of the aneurysm to the aortic bifurcation and the initial sections of iliac arteries).

A description of any changes in the vicinity of the aorta.

In the case of pathology, the flow velocity and the nature of the spectrum may be specified (or documented in a photo).

Iliac arteries

Vascular patency.

In case of atherosclerotic lesions – the nature and location of plaque.

In case of stenoses – the highest flow velocities at the stenosis site, the spectrum of the flow in the stenosis and below it, and changes in vascular diameter in the case of major pathologies.

Femoral arteries

Vascular patency.

The presence and nature of atherosclerotic lesions.

Superficial femoral arteries

Vascular patency.

The presence and nature of atherosclerotic lesions.

Distribution of lesions within the vessels (e.g., the initial section, a number of lesions increasing peripherally).

In case of stenoses – the spectra and velocities of the flow above the stenosis, in its vicinity and 3–4 cm below.

Popliteal arteries

Vascular patency.

The presence and nature of atherosclerotic lesions.

The nature of the spectral flow (two-phase and three-phase).

In case of pathological lesions in the vessels located above, blood flow velocities within them should be recorded (besides documenting the lesions).

In case of stenoses – velocities within the stenosis and the nature of the spectrum and flow velocity in the vessel below the stenosis should be recorded.

Calf arteries

Vascular patency.

The nature of the spectral flow (two-phase/three-phase vs. single-phase) in the dorsal foot and posterior tibial arteries.

Vascular dilatations

For vessels dilated at all levels the following should be specified: the location of the lesion, the length of the extended section, diameter measurements/transverse dimensions, the presence of blood clots and the flow spectrum below the lesion. An aneurysm is defined as dilatation of the lumen exceeding 50% of the diameter of the same vessel measured above or (preferably) below the lesion. In the case of the abdominal aorta, a 30 mm dilatation is considered an aneurysm, in the case of the common iliac artery – a 20 mm one.

In the case of lesions, each extremity is described separately for the clarity of the description.

The test description end wit h conclusions summa rizing the observed pathologies, specifying the locations of vascular occlusions or indicating the percentage degree of their constriction. It should also include recommendations for f urther diagnostics and consultations with a specialist.

Test documentation

The photographic documentation enclosed with the test results includes:

In each case:

an image of the aorta in long and transverse axes along with the aortic diameter measurement;

a record of the spectrum of the flow in external iliac arteries in their mid-lower section;

a record of the spectrum of the flow in the popliteal arteries.

In case of major pathological lesions:

an image of the pathology;

a record of the spectrum at the stenosis site and 3–4 cm below;

a record of the spectrum of the flow in the posterior tibial artery in an extremity diagnosed with lesions;

in case of aneurysms – images in the longitudinal and transverse planes + a record of the spectrum of the flow in the unchanged section of the vessel below.

Summary

A Doppler ultrasound examination of the lower extremities is used for a preliminary evaluation of blood vessels. The decision regarding the type of a possible surgery is made by a surgeon on the basis of the vascular examination (usually angio-CT).