• List the advantages and disadvantages of B-Flow compared to Color Coded Doppler Sonography (CCDS) and Power Doppler (PD)
• Describe the technical approach of imaging vessels with B-Flow
• Explain the physical aspect of the non-Doppler, B-Flow technique
• After reviewing clinical examples in B-Flow, define and project the appropriate clinical use of B-Flow in vascular diagnosis

Vascular ultrasound, the imaging technique most readily available for vascular disorders, can help to detect vascular affections like dissections, aneurysms or stenoses. In vascular imaging there are many different methods, invasive and non-invasive, that the clinician may choose to use. As one can imagine, non-invasive techniques are highly preferred because of their cost- and risk-effectiveness and clinical validity. In addition, the fast and reliable diagnosis of vascular disorders is of utmost importance for the early treatment of patients.

Ultrasound is a great diagnostic tool for the patient as it is non-invasive, side effect free, and available worldwide with high diagnostic significance and acceptance. The purpose of this tutorial is to show the diagnostic value of B-flow, a new vascular imaging technology, in demonstrating vascular disorders of different anatomic territories and to give a short overview of other ultrasound techniques in clinical use.

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.

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 a proven technology for imaging vascular disease, in particular large vessel dissections and hematomas with poor echo signal. With ultrasound these disease states can be detected and displayed with ease.

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. The color coding demonstrates the effective velocity vector of the erythrocytes.

In spectral Doppler sonography, the time-dependent intensity allocation is determined by 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 and then color codes them in corresponding position in the B-Mode image. Contrary 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 = the frequency with which Doppler ultrasound pulses are being transmitted) 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 ½ of the PRF. This is called the Nyquist limit). The color aliasing will 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 color coded picture information) is a detrimental artifact which can be amplified by regular arterial pulsations, deglutition (swallowing), respiration and heartbeats. This artifact results in delayed superposition / overlay of color coded and B-Mode information. As well, 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 quick image formation. One must remember though that CCDS is completely angle dependant. 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 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 [11]. 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 no display of flow direction, reduced frame rates and it is subjective to motion artifacts.

B-Flow

In 1999, B-Flow was introduced as a new technology to help image and diagnose vascular disease. B-Flow is unique in that does not use Doppler detection techniques or the Doppler principle. As in angiography, the B-Flow technique uses a morphological approach. The amplitude 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 DSA, only moving particles are imaged; stationary structures such as 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 [10].

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 compares accurately with angiography when grading vessel stenosis. [1-8].

One major advantage of B-Flow is the lack of color blooming or aliasing artifacts associated with Doppler techniques. The different characteristics of CCDS and B-Flow techniques are listed in table 1

B-Flow uses Digitally Encoded Ultrasound (DEU), a technology patented by General Electric Healthcare to provide real-time visualization of vascular hemodynamics. This is accomplished by directly visualizing blood reflectors (such as red blood cells) in a gray scale display and preferentially suppressing non-moving tissue signals, (vessel walls). It is important to remember that this technique does not rely on the Doppler principle to detect the motion of moving structures. [9, 12]. Echoes from red blood cells have an approx. 1000 times weaker signal strength, (40-60 dB) than vessel walls and the surrounding tissue. In fact, acoustic noise artifacts mask out these extremely weak echo's from blood. For this reason arterial blood flow is normally invisible in B-Mode images.

B-Flow harnesses DEU's ability to enhance weak signals and to suppress unwanted echo's. The DEU beam former consists of a transmit encoder and a receive decoder in addition to array focussing electronics. B-Flow uses a GE-patented technique called Coded Excitation (CE) to send coded pulse sequences into the body. It then decodes the returning signals to enhance sensitivity to weak signals such as blood cells by extensive elimination of acoustic noise artifacts and to suppress non-moving tissue signals.

It is well known that ultrasound signals are attenuated with increasing tissue penetration depth. The signal attenuation accumulates exponentially with the increased transmit frequency. Therefore a high frequency with a short pulse is needed to achieve a high spatial resolution. Deeper penetration with equal frequency can only be reached by using a longer pulse sequence. With incremental pulse length the excess pressure rises whereby deeper penetration is achieved but at the expense of decreased spatial resolution. This problem is solved by the Coded Excitation technique.

B-Flow is based on a subtraction principle [1-8]. A single wideband pulse is converted to a coded sequence by a transmitter that is amplitude modulated according to a special digital code. The transmitter is able to send several code sequences that are defined according to the requirements of the designated depth. Up to four encoded pulses are delivered along a vector in a defined time interval. The echo sequences obtained comprise an echo signal with certain intensity, (the echo amplitude), for each pixel. Only the coded echoes which in the decoder, match what were transmitted are analyzed. Thus, interference echo's and noise are eliminated.

A signal amplitude that is adequate for imaging in B-Mode is attained using a pulse compression technique. The echo signals of an image line are subtracted one from another so that only the amplitude signal for a reflector, like that for flowing erythrocytes, are imaged in the time interval between two pulses. Only the flowing parts but not the stationary structures (e.g. connective tissue, muscles, vessel wall) are imaged. Lines in the one direction and dots in the other direction indicate the direction of flow. The signal amplitude and consequently the brightness of the echo's increases with the flow rate (brightness mode). Additionally, the tissue background can be added for better orientation, as the degree of tissue suppression is adjustable. However, flow velocities cannot be determined as the Doppler principle is not used. In essence, B-Flow images blood, which returns a B-Mode signal displayed in grayscale.

As mentioned above, B-Flow provides direct visualization of blood echo's [10].

Due to the fact that it uses a morphological approach by imaging blood flow and its surrounding tissue simultaneously, it provides a clear, unobstructed view of the vessel lumen. The usage of B-Flow is very intuitive due to the gray scale presentation of the flow data. The two main factors which affect the brightness in B-Flow images are:

1. the strength of echo's from blood
2. the blood velocity

For all ultrasound studies, multifrequency linear array probes (5-10 MHz, and 9-14 MHz) were used for the examination of carotid artery and peripheral vessels and a multifrequency convex array probe (3-5 MHz) was used for abdominal vessels. The automatic flow optimization function was set to a medium range and used for all three ultrasound modes. All vessels were examined in the cross-sectional and longitudinal planes.

For optimal Doppler detection, flow parameters must be selected depending on the Doppler shift. The wall filter must also be adjusted to the flow in the vessels. For arterial flow it is set on average 100 to 150 and less for venous flow. In addition the pulse repetition frequency (PRF) must be adjusted to the maximum blood velocity being detected. To avoid overwriting artifacts (i.e., color pixels outside the perfused lumen of the vessel) appropriate color gain is necessary. Additionally, an automatic image gain optimization may be selected.

Extracranial vessels

Common Carotid Artery Internal Jugular Vein External Carotid Artery Internal Common Artery Vertebral artery

Common carotid artery
The right common carotid artery (CCA) originates from the brachiocephalic trunk. The left CCA from the aortic arch. Usually both arteries ascend without branching through the superior mediastinum into the neck and lie medial to the jugular vein. In the neck, the carotid artery, jugular vein, and vagus nerve are enclosed in connective tissue called the carotid sheath. Occasionally the common carotid artery is the origin to the superior thyroid artery.

At the level of the superior border of the thyroid Common Carotid Artery cartilage (approximately C4) the CCA dilates usually into the carotid sinus followed by the carotid bifurcation, the origin of the internal carotid artery (ICA) and the external carotid artery (ECA). The ECA is usually located anterior medial, the ICA posterior lateral. The level of the CCA bifurcations may be asymmetrical. It has been described as low as T2 and as high as C1.

Internal Carotid Artery
The ICA is usually larger than the ECA. The ICA has no extracranial branches and can be divided into four main segments: the cervical, the petrous, the cavernous, and the cerebral segment.

External Carotid Artery
The ECA originates anterior to the ICA in the carotid triangle at the mid-cervical level and is usually smaller than the internal carotid artery. In some cases, the ECA originates lateral to the internal carotid artery. This anatomic variation occurs more frequently on the right (3:1).

At the level of the mandibulary column the ECA divides into its terminal branches: the maxillary and superficial temporal artery. All together there are eight named branches of the external carotid artery: the superior thyroid artery, the lingual artery, the facial artery, the ascending pharyngeal artery, the occipital artery, the posterior auricular artery, the superficial temporal artery, and the internal maxillary artery. The abundant number of anastomoses between the branches of the ECA and the intracranial circulation underscore its clinical significance as a collateral pathway for cerebral perfusion when significant disease is present in the ICA.

Vertebral Artery
The vertebral arteries (VA) originate as the first branch of the subclavian arteries. Atherosclerotic changes usually occur at its origin. A normal variant of the vertebral artery arises occasionally directly from the aortic arch. This is more common on the left side. Size asymmetries are common and only in about 25% are the two vertebral arteries equal. In the majority, the left vertebral artery is the dominant artery. The vertebral artery can be divided into four segments: the extravertebral, intervertebral, horizontal, and the intracranial segment. The proximal segment of the vertebral artery is approximately 4-5 cm in length and usually there are no branches. The vertebral arteries enter through the foramen magnum of the skull and the left and right vertebral arteries unite as the basilar artery. In turn the basilar artery connects with the left and right internal carotid artery and the cerebral arterial circle of Willis.

Internal Jugular Vein
The internal jugular vein composes the continuation of the sigmoidal sinus and it carries the blood from the cerebral and facial region. Together it is encased with the carotid artery and the vagal nerve in the carotid sheath. Intracerebrally the internal jugular vein lies posterior and extracerebrally laterally to the ICA respective to the CCA. Continuing into the inferior jugular vein bulb, it meets the subclavian vein and flows to the venous angle continuing as the brachiocephalic vein.

Upper Extremity

Cephalic Vein Brachial Artery Radial Artery Ulnar Artery Brachial Vein

Brachial Artery
The normal arterial anatomy of the upper extremity is depicted graphically in the figure above. The subclavian artery becomes the axillary artery at the lateral margin of the first rib. After crossing the inferolateral margin of the teres major muscle it becomes the brachial artery. It courses through the medial bicipital sulcus and bicipital aponeurosis to the cubital fossa. At the elbow the brachial artery divides, forming the radial and the ulnar artery. The interosseus branch is a branch of the ulnar artery, continuing to the wrist in some individuals. On its course the brachial artery is accompanied by the median nerve and brachial veins. Occasionally, there are normal variants of the brachial artery such as a persisting superficial brachial artery. In some individuals the brachial artery divides at the heights of upper arm.

Ulnar Artery
The ulnar artery courses from the elbow flexure along the ulnar side of the lower arm together with the ulnar nerve, now at the radial side of the ulnar carpal flexor muscle, to the hand. The termination of the ulnar artery is the superficial palmar arch.

Radial Artery
The radial artery crosses quite superficial from the radial side of the cubital fossa over the tendon of the biceps brachii muscle. Together with the superficial branch of the radial nerve it courses to the hand. The radial artery terminates in the deep palmar arch.

Superficial Veins
In the region of the hand there is an extensive palmar and dorsal venous network. From the dorsal venous rete the blood flows in two big superficial veins - the ulnary sided basilic vein and the radial cephalic vein. Both are connected by the median cubital vein in the elbow flexure. Occasionally there is a median antebrachial vein instead of the median cubital vein as a variant. This is demonstrated graphically as:

Subfascial Veins
The deep accompanying (subfascial) veins are pairs. They course with the homonymous arteries in a shared vessel sheath. The subfascial veins are connected by various small branches with the superficial veins.

Abdominal vessels

Abdominal aortic Inferior vena cava Portal venous system

Abdominal aorta
The normal abdominal aorta has a maximum infrarenal diameter average from 2 cm in adults and varies little with respect to age. The wall is clearly defined. The abdominal aorta lies adjacent to the spine, slightly to the left of midline. The first branch of the abdominal aorta after descending through the diaphragm is the celiac trunk. Followed by the superior mesenteric artery and the left and right renal artery. The last branch of the abdominal aorta before the bifurcation into the left and right iliac artery is the inferior mesenteric artery.

Inferior vena cava
The inferior vena cava (IVC) is situated anterior to the spine and to the right of the aorta. It begins at the junction of the common iliac veins and terminates in the right atrium. The mean diameter of the IVC in normal patients is about 16-17 mm, as measured just below the renal veins during quiet respiration. During quiet respiration the diameter of the IVC rages between 5 to 28 mm and can increases about 10% during deep inspiration.

Portal venous system
The portal venous system transports blood from the bowel and the spleen to the liver. The beginning of the portal venous system is at the junction of the superior mesenteric and splenic veins. It courses obliquely toward the right to terminate at the porta hepatis. There it divides into the left and right portal vein and enters the corresponding lobe of the liver.

Lower Extremity vessels

Femoral Artery Femoral Vein Saphenous Vein Popliteal artery

Arteries
The femoral artery originates beneath the inguinal ligament as a continuation of the external iliac artery. Through the vascular lacuna (beneath the inguinal ligament) the femoral artery courses to the iliopectinal fossa that is confined by the iliopsoas and pectineal muscle. In this region the artery is only covered by skin and the femoral fascia and therefore it is palpable as femoral pulse. Subsequently the femoral artery travels the length of the thigh, travels through Hunter's canal (adductor canal) and terminates at the opening of the adductor magnus muscle. Through the adductor hiatus the femoral artery leaves the canal, continues as the popliteal artery and travels behind the knee in the popliteal fossa. The popliteal vein crosses the artery and above the vein lies the popliteal nerve in the hollow of the knee. The popliteal artery terminates distally into the anterior and posterior tibial artery.

Veins
In the metatarsal region is the plantar venous network. From this venous rete the blood flows in the great (medially) and small (laterally) saphenous vein. The great saphenous vein terminates at the femoral vein whereas the small saphenous vein terminates at the popliteal vein. The deep accompanying (subfascial) veins are pairs until the popliteal fossa. They course with the homonymous arteries in a shared vessel sheath.

B-Flow, which is independent of the Doppler principle, enables markedly better flow detection on the basis of amplitude imaging. [9, 12]. B-Flow is a new technique that extends the resolution, frame rate, and dynamic range of B-Mode to image blood flow and tissue simultaneously. B-Flow relies on coded excitation to boost weak signals from blood scatterers and on tissue equalisation to display flowing blood and tissue simultaneously without threshold decision and overlay. B-Flow achieves high resolution with broadband B-Mode pulses and high frame rate by using small packet sizes. B-Flow cine loops demonstrate a resolution and frame rate three times higher than color flow. This, together with over 60 dB of display dynamic range, allows B-Flow users to image, detect and display hemodynamics and vessel walls with unprecedented clarity.

B-Flow as a new and useful method will help sonographers to evolve its utility in clinical situations. Its unique advantages become a valuable complement to CCDS and PD in peripheral vascular applications. Additionally, further development promises more clinical applications for B-Flow.

The author wishes to acknowledge Dr. Med. Tobias Saam and Donald T. Milburn, RDCS, RVT, FSDMS for their help in preparing this article.

1. Jung EM, Lutz R, Clevert D-A, Rupp N. B-Flow: Sonographic assessment and therapy for femoral artery pseudoaneurysm Rofo. 2001 Sep;173(9):805-9.
2. Jung EM, Kubale R, Clevert D-A, Lutz R, Rupp N. B-flow and contrast medium-enhanced power Doppler (Optison(R))--preoperative diagnosis of high-grade stenosis of the internal carotid artery Rofo. 2003 Mar;175(3):387-92. German. Erratum in: Rofo Fortschr Geb Rontgenstr Neuen Bildgeb Verfahr. 2003 Apr;175(4):575.
3. Jung EM, Kubale R, Clevert D-A, Rupp N. Improved evaluation of stenoses of hemodialysis fistulas by B-flow ultrasound Rofo. 2003 Mar;175(3):387-92. German. Erratum in: Rofo Fortschr Geb Rontgenstr Neuen Bildgeb Verfahr. 2003 Apr;175(4):575.
4. Jung EM, Clevert D-A, Rupp N. B-flow and color-coded B-flow in sonographic diagnosis of filiform stenosis of the internal carotid artery Rofo. 2003 Sep;175(9):1251-8
5. Clevert D-A, Rupp N, Reiser M, Jung EM. Improved diagnosis of vascular dissection by ultrasound B-flow: a comparison with color-coded doppler and power Doppler sonography. Eur Radiol 2005, 15: 342-347
6. Clevert D-A, Kuable R, Strautz TI, Flach PM, Trumm C, Hoffmann RT, Reiser M, Ultrasound diagnosis of vascular complications following transfemoral puncture. Radiologe. 2005 Sep 9
7. Clevert D-A, Stickel M, Steitz H-O, Kopp R, Strautz T, Flach P, Jauch KW, Reiser M. Treatment of secondary stent-graft collapse after Endovascular Stent-Grafting for Iliac Artery Pseudoaneurysms. In press Cardiovasc Intervent Radiol. 2006
8. Jung EM, Kubale R, Ritter G, Gallegos MT, Jungius K-P, Rupp N, Clevert D-A. Diagnostics and characterisation of preocclusive stenoses and occlusions of the internal carotid artery with B-flow. In press Eur Radiol. 2006
9. Weskott HP. B-flow - a new method for detecting blood flow] Ultraschall Med. 2000 Apr;21(2):59-65
10. Noto N, Okada T, Yoshino Y, harada K. B-flow sonographic demonstration for assessing carotid atherosclerosis in young patients with heterozygous familial hypercholesterolemia. J Clin Ultrasound. 2006 Feb;34(2):43-9
11. Kubale R, Arning C. Significance of Doppler ultrasound procedures for diagnosis of carotid stenoses Radiologe. 2004 Oct;44(10):946-59.
12. Bucek RA., Reiter M., Koppensteiner I., Ahmadi R., Minar E., Lammer J. B-flow evaluation of carotid arterial stenosis: initial experience. Radiology 2002, 225: 295-299

• Take Quiz