Tissue Harmonic Imaging and Doppler Ultrasound Imaging

Tissue Harmonic Imaging

Technical Aspects

Fundamental frequency is the original frequency of the acoustic beam emitted from the transducer. Harmonic wave generation is an acoustic phenomenon. Harmonic waves are integer multiples of the fundamental frequency.

The second harmonic (twice the fundamental frequency) is currently used for tissue harmonic imaging (THI). With THI, the fundamental frequency is eliminated with image processing techniques. THI advantages include improved signal-to-noise ratio and artifact reduction.

Clinical Applications

THI improves image quality and conspicuity, and has been shown to be useful in multiple clinical scenarios, including (1) obesity, (2) hollow structures (e.g., cysts, gallbladder, urinary bladder) ( Figure 4-1 ), and (3) the deep-seated major vessels (inferior vena cava [IVC] and abdominal aorta) ( Figure 4-2 ).

Figure 4-1, Comparison of images in the same location of the right upper quadrant without and with harmonic imaging. A, Image without harmonic imaging: the fundus and neck areas (arrows) of the gallbladder (GB) and intraportal venous area (PV, arrow) appear echogenic and cloudy. B, With harmonic technique, the figure shows a clear GB and portal venous structure. In addition, the tiny calcification on the anterior wall of the GB (arrowhead) is well shown on the harmonic image in B but invisible in the blurred image in A.

Figure 4-2, Comparison of image conspicuity without/with harmonic imaging in the sagittal plane of the left liver and the long axis of the inferior vena cava (IVC). The arrows point to the intra-IVC area, which is obviously cloudy and blurring in the nonharmonic image (A) compared with the harmonic image (B).

Doppler Ultrasound Imaging

Doppler ultrasonography is a noninvasive technique that provides information about the condition of blood vessels and blood flow direction. It also measures flow velocity and can be used to evaluate the vascularity of mass lesions. Color and pulsed-wave Doppler imaging provide complementary information, including spatial orientation and a time velocity spectrum, respectively.

Technical Aspects

Doppler examination requires five technical parameters (5 Ps), as follows:

Patient preparation: Fasting is required for a Doppler examination of the abdomen.
Probes: Commonly used probes are the (1) curvilinear-array probes (low frequency, 3 to 5 MHz), (2) phased-array probes (low frequency, 2 MHz), and (3) linear-array probes (high frequency, 4 to 10 MHz).
Person: The sonographer should have a considerable amount of expertise to perform a Doppler examination such as understanding of the normal anatomy, pathophysiology, and signature patterns of abdominal vessels.
Picture quality (machine): To obtain good picture quality, the radiologist should consider the following operational parameters: (1) an appropriate anatomic window, (2) depth of field, (3) frame rate, (4) flow sensitivity with adjusted gain settings, (5) image vessel of interest at a Doppler angle of 30 to 60 degrees, and (6) low wall filter settings (if these are high, significant velocity information can be lost). The recorded color flow should occupy the full anteroposterior diameter or cross-sectional area of the vessel without color flow aliasing and noise in the surrounding tissues.
Positioning: The various positions required for imaging every individual vessel.

Pros and Cons of Doppler Imaging

Pros

Noninvasive
Readily available and cost-effective
Portability: Can be done by the bedside in sick or debilitated patients
Differentiating vascular and nonvascular structures (e.g., porta hepatis) ( Figure 4-3 )
Provides information about the patency of blood vessels, direction of flow turbulence, phasicity, jet, impedance, and so on
Quantification of stenosis and direct measurement of flow lumen reduction
Tissue characterization of tumors

Cons

Operator dependency.
Doppler imaging is technically difficult to perform in obese patients and in those with overlying bowel gas or a distended abdomen, especially when desiring visualization of the mesenteric vessels, the portosplenic confluence, or renal artery origin; performing portosystemic collateral mapping; evaluating a shunt anastomosis; and so on.
Good spectral analysis cannot be achieved in patients who cannot hold their breath (e.g., acutely ill patients).
Graft surveillance at the level of the distal abdominal aorta and iliac arteries is difficult.
Abdominal aortic calcifications can be an obstacle in visualization of renal artery origin.

Normal Anatomy of Abdominal Vessels

The normal appearance and signature pattern of abdominal vessels—the portal vein ( Figure 4-4 ), hepatic vessels, mesenteric vessels ( Figures 4-5, 4-6, and 4-7 ), renal vessels ( Figure 4-8 ), abdominal aorta ( Figure 4-9 ), and IVC—are summarized in ( Table 4-1 ). Portosystemic collateral vessels ( Figures 4-10 ) and splenorenal collateral vessels ( Figure 4-11 ) are explained in detail in Table 4-2 .

Figure 4-4, Normal portal vein (PV). Pulsed Doppler image of the portal vein shows normal undulating signature pattern with phasic flow. Peak systolic velocity = 15 cm/s.

Figure 4-5, Superior mesenteric vein (SMV). Long-axis view shows a normal SMV becoming confluent with the portal vein (PV).

Figure 4-6, Superior mesenteric artery (SMA). Long-axis view shows normal high-resistance waveform patterns of artery in fasting. Peak systolic velocity = 151 cm/s; resistive index = 0.75.

Figure 4-7, Superior mesenteric artery (SMA). Postprandial Doppler image reveals low-resistance waveform pattern with increase in peak systolic velocity. Resistive index = 0.6.

Figure 4-8, Normal right renal artery. Right coronal oblique view with anterolateral transverse approach shows the course of the renal artery from the hilum to the origin.

Figure 4-9, Abdominal aorta (AB AO). Long-axis view of the proximal abdominal aorta shows high-resistance flow with brief flow reversal. Doppler angle = 43 degrees.

TABLE 4-1

Normal Appearance and Signature Patterns of Abdominal Vessels

Vessel	Identification	Normal Signature Pattern
Portal vein: normal caliber = 13 mm (quiet respiration)	Anechoic structure, which runs in transverse plane and converges on the porta hepatis Surrounded by a sheath of echogenic fibrous tissue	Undulating continuous waveform pattern with subtle phasic variation Hepatopetal flow (toward the liver)
Hepatic vein: normal caliber = 3 mm (measured 2 cm from inferior vena cava)	Longitudinally oriented sonolucent structures within liver parenchyma Best visualized with transverse subxiphoid approach to see the three main trunks with the inferior vena cava	Triphasic pulsatile waveform pattern with hepatofugal flow Naked margins
Hepatic artery: normal velocity = 30-60 cm/s	Vascular structure anterior to portal vein	Low-resistance flow with spectral broadening
Inferior vena cava: normal caliber = 2.5 cm	Anechoic structure in the midline to the right of the aorta and anterior to the spine Upper part best visualized using liver as an acoustic window	Pulsatile flow near the heart: “sawtooth pattern” Phasic flow distally
Abdominal aorta: Normal caliber = 2.3 cm (men), 1.9 cm (women)	Hypoechoic tubular pulsatile structure with echogenic walls best seen by longitudinal midline approach	High-resistance waveform pattern with a brief period of reversed flow (see Figure 4-9 )
Mesenteric vessels: normal caliber <10 mm	Superior mesenteric artery is surrounded by a triangular mantle of fat. It is to the right of the superior mesenteric vein, which runs parallel to the superior mesenteric artery (see Figures 4-6 and 4-7 )	Superior mesenteric artery fasting view: High-resistance waveform pattern with sharp systolic peaks with absent late diastolic flow Postprandial shows low-resistance waveform pattern.
Celiac artery	Best visualized in transverse plane, in which the T -shaped bifurcation of vessel into hepatic and splenic artery is characteristic	Low-resistance type of waveform
Renal artery and vein	Origin of artery is slightly caudad to superior mesenteric artery and best seen by transverse midline approach. Left renal vein is seen between superior mesenteric artery and aorta. Right renal vein can be traced from inferior vena cava	Artery: Low-resistance flow with broad systolic waveform and forward flow during diastole Vein: Phasic with velocity varying with respiration and cardiac activity

Figure 4-10, Portosystemic collateral vessels. Long-axis view shows large, tortuous, left gastric vein collateral vessels along the inferior border of the left lobe of the liver.

Figure 4-11, Splenorenal collateral vessels. Transverse image of the left kidney shows tortuous collateral vessels between the splenic and renal hila.

TABLE 4-2

Portosystemic Collateral Vessels: Diagnostic Criteria

Site	Portosystemic	Appearance
Gastroesophageal junction. Normal coronary vein diameter <6 mm	Between coronary/short gastric veins and systemic esophageal veins	Coronary veins >7 mm are abnormal. Prominent cephalad-directed vessel arising from portal vein opposite superior mesenteric vein
Paraumblical vein (falciform ligament) Normal = 2 mm hepatopedal flow	Between left portal vein and systemic epigastric veins near umbilicus	Solitary vein originating from left portal vein courses inferiorly through falciform ligament and anterior abdominal wall to umbilicus, demonstrating hepatofugal flow
Gastroepiploic (see Figure 4-6 )	Between gastroepiploic and esophageal/paraesophageal veins	Cephalad directed vessel along the inferior border of the left lobe
Splenorenal and gastrorenal (splenic and renal hilum)	Between splenic, coronary, short gastric, and left adrenal or renal veins	Splenorenal (see Figure 4-11 ). Tortuous, inferiorly directed vessels between spleen and upper pole of left kidney
Intestinal	Veins of ascending/descending colon, duodenum, pancreas, liver anastomosis with renal, phrenic and lumbar veins (systemic tributaries)	Collateral pathways identification on ultrasonography depends on the amount of air in the bowel at the time of study
Hemorrhoidal (perianal region)	Superior rectal vein anastomoses with systemic middle and inferior rectal veins	Rectal/pararectal varices can be detected with transvaginal or transrectal ultrasonography, cannot be visualized on transabdominal ultrasonography

Clinical Applications

Portal Hypertension

Common Causes

Prehepatic: Portal vein thrombosis (idiopathic, hypercoagulable states, pancreatitis), portal vein compression (tumor, trauma, lymphadenopathy)
Intrahepatic: Cirrhosis
Posthepatic: Budd-Chiari syndrome (idiopathic, hypercoagulable states, trauma, web, and tumor).

Diagnostic Criteria

Gray-Scale Imaging Findings

Portal vein dilatation is greater than 13 mm.
Superior mesenteric vein and splenic vein are greater than 10 mm.
Lack of caliber variation in splanchnic veins is less than 20%.
In thrombosis, there may be partial visualization or failure to visualize the portal vein (chronic) or echogenic material within distended lumen (acute) ( Figures 4-12 and 4-13 ).

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