• List the advantages and the disadvantages of B–Flow compared to Color Coded Doppler Sonography (CCDS) and Power Doppler (PD)
• Describe the incidence of local complications after transfemoral catheterization
• Recognize that the value of sonography can be enhanced by reviewing clinical case studies with B-flow imaging

In the last years percutaneous catheter procedures have become important both for diagnostic and therapeutic purposes. Here different vascular regions serve as possible interventional access courses. Besides the carotid artery, axillary artery, abdominal aorta and the popliteal artery, the brachial artery and common femoral artery and superficialis / profunda are most frequently used for arterial puncture. Since a detailed representation of all possible complications within the range of these interventional courses would take surely too long, this tutorial is only limited to the common femoral artery as a puncture site.

An arterial injection site often leads to vascular complications, with the femoral artery being the most frequent access path [1]. Generally complications happen within the range of the femoral arteries in 0,1 - 9% of cases [2-11]. Aneurysm spurium, arteriovenous fistula, dissection and hematoma as well as other complications, for example the aneurysm verum, belong to the most frequently arising complications after arterial puncture [12-13]. Complications due to puncture and catheterization depend among other things on the thickness of the catheter and/or the air-locks, accompanying anti-coagulation, the puncture direction and the access path [14]. After interventions with ultrasonic controls bleedings can occur, which compress and shift the vessels and make an evaluation of the blood flow more difficult. Thereby the detection of complications becomes more difficult as consequence of a vessel wall injury. An early proof of rare complications after a vessel intervention by ultrasonic investigations also avoids double or expensive auxiliary investigations [15].

Ultrasound represents a procedure, which is ubiquitously available, due to the compactness and mobility of the devices.

B-Mode

B-Mode (“brightness” mode) is an ultrasound technique where structures are displayed as dots. The brightness of these dots depends on the echo intensity at the interface. Each single amplitude corresponds to a brightness, e.g. from 0 to 100, on a scale ranging from white to black. State of the art ultrasound transducers contain up to 1000 or more piezoelectric crystals that can be activated electronically. The direction and delay of the impulse as well as the echo intensity and the localization of the activated crystals of the transducer are registered. In the so-called "real-time technique" a cross sectional image is generated immediately. Consequently, B-Mode is nothing more than the brightness allocations of the echo reflections. B-Mode is widely used for imaging vascular disease. It is particular useful to detect large vessel dissections and hematomas with poor echo signal.

Color Coded Duplex Sonography (CCDS)

By combining B-Mode and Doppler technologies, the direction and velocity of moving blood can be color coded and displayed. This color coding demonstrates the effective velocity vector of the erythrocytes. In spectral Doppler sonography, the time-dependent intensity allocation is determined by the use of a time-velocity-curve. For reliable results and the determination of the accurate spectral Doppler velocities, the angle between the vessel and Doppler beam direction is of utmost and critical importance.

CCDS detects Doppler shifted signals in blood vessels in their anatomical position, calculates their flow direction and velocity, which are then color coded in the corresponding position in the B-Mode image. In contrast to spectral Doppler-sonography, CCDS shows only the mean flow velocities since color cannot be angle corrected to the true direction of the vessel being interrogated. When using CCDS for flow analysis in a stenosis, the color display depicts the mean velocity in the vessel. If the detected mean velocity exceeds the predetermined velocity range of the pulse repetition frequency (PRF = frequency of the transmitted Doppler ultrasound pulses), aliasing will occur and the normal red/blue color directional display will be altered. (Aliasing occurs when the speed of the detected blood is faster than the so called Nyquist limit = ½ of the PRF). The color aliasing can indicate the region with the highest mean velocity. This area is where the pulsed Doppler (PW) samples and measurements should be made. Axial bleeding or blooming (overlay of the true vessel lumen through color coded picture information) is a detrimental artifact which can be amplified by regular arterial pulsations, swallowing, respiration and heartbeats. This artifact results in delayed superposition / overlay of color coded and B–Mode information. Furthermore, adjusting the size of the color Doppler sample volumes within the region of interest may increase or decrease the amount of axial bleeding of the color Doppler signal. Advantages of CCDS are the short duration of the examination and the real-time image formation. However, one must remember that CCDS is angle dependant. Therefore, the color display can and will change with the angle of the probe insonation beam, the actual speed of the moving blood and if the vessel bends through the color region of interest. These facts can cause confusion in interpreting CCDS displays. As well, target structures that scatter or attenuate echoes like calcified plaques, intestinal air or skin edema will also alter and affect the CCDS evaluation.

Power-Doppler (PD)

Power Doppler (PD), which is a variant of CCDS, has become increasingly important in recent years. PD differs from CCDS as it color encodes the echo amplitude (signal strength) and not the flow-velocity or flow-direction as CCDS does [16]. Therefore PD is virtually angle-independent and is in contrast to CCDS able to detect and display blood movement even at orthogonal angles. Thus, PD is advantageous over conventional CCDS if the examiner needs to detect a signal from an angle not conducive to Doppler interrogation angles. Moreover, PD is much more sensitive in detecting weak or small volume flows such as those found in the false lumen of a dissection. Disadvantages of PD include that it does not display flow directions, it has reduced frame rates and it is more subjective to motion artifacts than CCDS.

B-Flow

In 1999, B–Flow was introduced as a new technology to help to improve imaging and diagnosing of vascular disease. B-Flow is unique as it does not use Doppler detection techniques or the Doppler principle. Similar to angiography, the B–Flow technique uses a morphological approach. The reflected amplitudes of scattering particles, such as erythrocytes in flowing blood, is imaged by a subtraction mode of two or four image vectors along one line. Similar to digital subtraction angiography (DSA), only moving particles are imaged and stationary structures, such as the vessel walls, are subtracted (“brightness” mode of B-Flow). B-Flow data is then combined with B-Mode information enabling a significantly better amplitude visualization of the flow, independent from the angle between probe and centerline of the vessel. In “Color-Coded B-Flow,” surrounding structures, such as the vessel wall, are added for anatomic orientation. The brightness of the color signal corresponds to the flow velocity. Compared to CCDS and PD, flow direction is marked in greyscale as dots or dashes [17]. B-Flow offers simultaneous detection and display of tissue and at the same time high resolution spatial and temporal depiction of blood flow. The discrimination of blood flow and the luminal vessel outline compared to CCDS is much more accurate. The independence of the transmit angle and the almost artifact free depiction eases the flow assessment and allows accurate assessment of vessel stenosis when compared to angiography [18-25, 36-39]. One major advantage of B-Flow is the lack of color blooming or aliasing artifacts associated with Doppler techniques.

Vascular ultrasound examinations were performed in patients after percutaneous catheter procedures with access over the femoral arteries, primarily using a linear multi-frequency probe (usually 5-7 MHz or 6-9 MHz, in some cases also 9-14 MHz). For complex pathologies in the iliac arteries a convex probe (3-5 MHz) will be helpful.

The flow parameters were selected depending on the doppler shift. The wall filter was 100 to 150 Hz on average, the pulse repetition frequency (PRF) was adjusted < 1000 Hz (in most patients 500 Hz, depending on the flow characteristics). The color gain was selected as high as possible, just avoiding overwriting artifacts (i.e., color pixels outside the perfused lumen of the vessel). Additionally, an automatic image gain optimization could be selected. A transmitter output of 100 % was selected and an average image correlation and average image rate were chosen for all ultrasound modalities employed. All vessels are examined in the longitudinal and axial plane.

The ultrasound device (Logiq 9, GE Healthcare, Milwaukee, WI) features a high performance processor that allows the documentation of dynamic image sequences in cine mode by a digital frame buffer. This was used to perform a comparative assessment between CCDS, PD and B-Flow.

Aneurysm spurium:

The aneurysm spurium is marked by a tear in the vessel wall, which can result from missing closure after puncture. The leaking-out blood causes a pulsating hematoma, which isolates itself with the help of the adventitia and forms a pseudo wall. If a continuous connection remains between a hematoma and the arterial lumen, it can form an aneurysm spurium with a blood inflow and outflow. Clinically this presents by a pulsatile mass within the region of the catheters, which can be shown for example by means of Doppler sonography. Since it holds the danger of rupturing, a fast diagnosis is essential. The distinction between an aneurysm and a hypoechoic, perivascular structure (for example a hematoma) is a visual diagnosis by means of Doppler sonography. The pendulum flow in the aneurysm neck, under the changing pressure ratios, is the proof of an aneurysm spurium and makes an angiography needless. During systole blood is pressed through the aneurysm neck into the aneuryma spurium due to high intraluminal pressure with relatively high speed. During diastole there is a reversal of the pressure ratios and causes turbulent return flow from the aneurysm sac into the vessel lumen (pendulum flow). By using B-Flow vessel wall lesions can be more accurately located, the flow in the aneurysm spurium better demonstrated and in particular small remaining flow with increasing thrombosis during the compression become visible [18]. The probability of an aneurysm spurium as complication after a puncture is 0,05 - 9% [4-5, 9, 26–32].

AV fistula:

A further potential dangerous complication after catherization is the AV fistula (arteriovenous fistula). It is defined as a pathological shunt between an artery and a vein. It is caused by penetrating and rarely by blunt injuries of an artery and vein at a corresponding localization. Iatrogenic AV fistulas develop when a canal with affection of the femoral artery and vein is created which dilates during catherization. After removal of the catheter a pathological shunt between these vessels remains due to this dilated canal. If there is a direct connection between the vessels it is called direct AV fistula. Indirect AV fistulas are characterized by an aneurysmatic sac between the vessels.

Large, systemic AV fistulas result in hemodynamic relevant changes, caused by shunting of the blood volume from the arterial high pressure system into the venous circulation. Due to the shunt there is an increase of blood volume and pressure in the venous system and a reduction of the peripheral vessel resistance [29]. After occlusion of the fistula these changes are reversible.

Since AV fistulas show a tendency to increase in size, there is a danger of rupture and therefore an urgent indication to treat. The method of choice is surgical repair. Intraoperatively, often a localization of puncture is found caudal of the common femoral artery. This shows how important it is to have a trained and exact technique for catherization procedures in order to prevent the patient from such complications [4]. A non-surgical alternative, especially in patients with aggressive anti-coagulant therapy, is compression by means of the ultrasound transducer. Indications for intervention are missing spontaneous occlusion of the fistula canal, increase in size and / or development of symptoms.

Doppler sonography improves diagnostics and therefore allows earlier treatment if necessary. In general, visualization of an arteriovenous blood flow and the typical noise of an AV-shunt is possible.

In color duplex sonography the characteristic perivascular vibrations can be visualized as mosaic-like "colored clouds". Due to the high flow velocities and the arterialized flow pattern of the draining vein, detection of the fistula is possible by the Doppler frequency spectrum.

Since there is potential danger of AV-fistulas, knowledge of potential factors leading to such complications might be useful. Then risk strategies might be developed prior to catheterization procedures in order to keep the 0.1 – 3.6 % rate of complications as low as possible [5, 9, 12, 28-29].

Dissection:

Manipulations with the puncturing needle, the guiding wire or the top of the catheter in the area of the vessel wall can lead to an intimal injury. In this case the direction of puncturing is important (antegrade or retrograde puncture), for further therapy. In case of a retrograde dissection the pressure of the passing blood might tend to lead to an autonomous attaching of the intimal sail on the vessel wall and therefore therapy is not primarily necessary. In case of an antegrad dissection there is the danger of an acute occlusion of the vessel by the folded down intimal sail and the possibility that an aneurysm dissecans develops. Through injury of the intimal layer, blood streams between the intimal and medial layer distally and leads to the development of a double vessel lumen which might extend over long vessel distances. In this case there is overexpanding of the outer vessel wall and occlusion of side branches (descending ischemia syndrom). A so-called re-entry, i.e. a re-entering of the blood into the true lumen from the dissection, is possible, however, this does not prevent the danger of a later occurring rupture. Therefore, in this case conventional or surgical consolidation of the defect is indicated.

Dissections may be seen in the b-image by detecting a dissection membrane or by an excentric constriction of the lumen. In case of a thombosis of the wrong lumen this often shows a somewhat less echogenic border than the true lumen. Dissection after catherization procedures are presented in the literature with a prevalence of 0.3 – 6 % [2, 34-35].

Hematoma:

Hematomas in the soft tissue of the thigh are not abnormal after catherization procedures. Local hemorrhage usually occurs as subcutaneous nonhazardous hematoma. Most of the hematomas are resorbed within a few days. However, in rare cases a compression of the femora nerve may occur, which might last for weeks to months. A surgical treatment of hematomas is generally not necessary. In big hematomas a sonographic release should be considered since these often lead to tension and pressure which is uncomfortable for the patient. However, hematomas might lead to further complications of which an aneurysm spurium is he most important. Another serious, however more rare complication is retroperitoneal haemorrhage. Femoral punctures above the inguinal band may lead to massive blood loss culminating in acute hypovolemic volume shock. According to the literature hematomas occur with a prevalence of 0.4 – 11 % after femoral puncture [1-2, 5].

B–Flow is a new and useful method which will help sonologists and sonographers enhance the diagnostic process and imaging of vascular diseases after transfemoral catheterization. Its unique advantages have become a valuable complement to CCDS and PD in peripheral vascular applications. In the future, further technical and practical developments promise an even wider clinical use of B–Flow.

The author wishes to acknowledge Dr. Sabine Weckbach, Dr. Nima Minaifar and Donald Milburn, RDCS, RVT, FSDMS for their help in preparing this article.

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