CEUS technique in officially recognized indications, based on our experience.
Detection of HCC's
Red links in the guidelines lead to live video samples. These are large, since they are tuned for diagnostic quality. In some cases there is a low resolution alternative.
The timer is essential to CEUS. If a timer is not used, it is practically impossible to keep track of the development of any CEUS exam. Typical metastases, for instance, show washout within a minute and microbubble degradation usually has quality consequences after 4-5 minutes. Since we prefer injecting the ultrasound contrast agent (UCA) ourselves when we examine the patients, we always make the injection 5 seconds into the timing to have time to move the left hand from the syringe to the US machine. However, for characterization of FLL’s (below) we need the arrival of the very first microbubbles into the liver image (TCA) to be the exact reference. Therefore the TCA is referred to as zero seconds in the FLL characterization chapter below, although we start the timer 5 seconds before the injection.
generation microbubbles burst immediately when exposed to conventional
US MI levels. However, with proper low MI settings they endure repeated
scanning, but any scanning has some effect on microbubble longevity. In
the average adult the choice of MI settings is pretty straightforward,
with an initial MI of ~0,13 in our ultrasound machines (Siemens Acuson
Sequoia™). However, one should regard MI as a variable
throughout exams and be prepared to alter it if necessary. In some
cases of thin or obese patients, or liver lesions in a deep or very
superficial location, the MI must be adjusted for correct
interpretations of the exams. Liver lesions with very slow circulation,
particularly haemangiomas, are sometimes subject to faster bubble
destruction than the surrounding liver parenchyma, which may give an
impression of “false washout” if the MI is not
to a very low level. Also, detection of washed-out metastases becomes
less efficient if too high an MI is applied, which partially destroys
the microbubbles in the contrasting liver parenchyma, and deeper
metastases can not be seen if MI is too low to reach all the way to the
distant surface of the liver. Repeated scans with increasing MI are
sometimes necessary for detection of metastases in order to see both
superficial and deeper parts of the liver optimally. Please read about
MI in the sections below and in "Common mistakes" in CEUS
peculiarities for further elaboration on MI settings.
Microbubble destruction is always present to some degree at exposure of the UCA to ultrasound. This sequence shows destruction even at lowest MI in the arterial phase before microbubble concentration builds up. In practice this is usually no big deal unless there is long ultrasound exposure to the same spot.
Scanning of a lesion without UCA is called a “baseline scan”. A baseline exam of the lesion prior to CEUS is recommended, so that the CEUS findings can be interpreted with reference to findings on baseline US.Frame rate in arterial phase should preferably be 15 fps or more.
ln characterization the arterial enhancement pattern is sometimes quite fast, and the details become less obvious if frame rate is too low. 15 fps or above is preferred, but there are cases when up to 50 fps within a region of interest have made a difference in for example very small FNH:s, since such a technique sometimes allows for the microbubbles to be followed as to their direction of flow also in small vessels.The lesion should preferably be less than 10 cm from the transducer.
A longer distance makes characterization less accurate. Analogous to conventional ultrasound, the closer the lesion to the transducer, the more probable it is that details of the arterial phase can be more accurately seen. It is generally better to try and reach a superficial lesion in the right liver lobe using an intercostal approach, than to try characterization from a long distance subcostally.Take care to use a proper MI setting.
An initial MI setting of ~0,13 (Sequoia) is usually a good compromise for balance between significant bubble destruction and good penetration. However, in some cases it has to be adjusted. For very shallow lesions the MI is set as low as possible (~0,07 on Sequoia), since bubble destruction can affect the interpretation of the exam. The necessary MI increases with depth, and the MI has to be high enough so that it is not compensated for by increasing gain, which causes noise artefacts and decreases image contrast. An MI of more than ~0,18 (Sequoia with CPS) is rarely beneficial, instead a closer approach to the lesion is preferable.Normal breathing if possible.
It is preferable that the patient can breathe normally during the exam. It is more difficult to “aim” at the lesion for the arterial phase, and to follow it through the vascular phases, if the patient has to hold his breath.Initial positioning of the transducer to align the centre of the lesion with any obvious feeding artery.
Many FNH's and some HCC's have conspicuous arterial enhancement patterns. Both sometimes have an identifiable main feeding artery which preferably is positioned at the very periphery of the lesion in the initial field of view by turning the transducer to the proper angle with the aid of colour Doppler before the CEUS. In this way chances are that the further course of the artery can be detected in the arterial phase of CEUS.The scanning pattern during the very first seconds of the arterial phase is important to understand the local circulation pattern. With experience, the examiner can often adjust the initial scanning with respect to the findings during the first seconds to achieve the most information. However, basically the transducer should be placed covering the lesion, and if possible along any obvious feeding artery at TCA. A few seconds following the arrival of contrast to the region of interest it is usually quite evident whether the lesion is probably avascular, a slowly filling haemangioma or something else that is vascular and thus enhances in the very first seconds. It should also be assessed if the MI setting seems adequate. If there is very good signal from the UCA beyond the lesion then a lower MI should be considered in order to preserve microbubbles for later in the exam. On the other hand, if there is no echo from the UCA in the liver surrounding the lesion the MI is too low (see the Mechanical Index section under pitfalls)."Fanning" during the initial enhancement of characterization, then short scans at longer and longer intervals.
For characterization of FLL it is important to try and see the very first seconds of the arterial enhancement of the lesion in detail, but the scan plane can only be at one place at a time. This limitation has to be overcome by the examiner at the patient’s bedside by scanning back and forth during the arterial phase through the lesion with the scan plane. Let us call it “fanning” for simplicity. The arterial phase of CEUS is an exception from the Sonodynamics' rule of scanning in one direction only. Generally fanning back and forth begins 1-3 seconds following TCA. The speed of the fanning is roughly 2 seconds to cover a 3 cm big nodule and 3 seconds over a 5 cm nodule. As always, assessment of the exam at a workstation or the machines monitor should be possible. Try to make the first fanning motion out of the lesion in the cranial direction or to the left, so that the following complete fanning through the lesion goes craniocaudally or left-right. Such structure makes comparisons between different exams more straighforward.First scrutiny of the arterial phase, then the portal and late phases.
Ultrasound's time resolution is a very good asset, but for high frame rates the CEUS exams are limited to one scan plane with current technique. The initial enhancement is important for confident characterization of all kinds of FLL's. Unfortunately not all FLL’s display “typical” behaviour, but optimization of exam technique may sometimes be the clue to dependable characterization in those that do. Because of the very quick series of events that occur in the arterial phase, it is important to start the acquisition of the first Sonoscan at TCA. As the exam proceeds beyond the arterial phase, constant monitoring is no longer necessary, and Sonoscans can be made at longer and longer intervals. This way microbubble destruction is minimized, and the exams occupy less storage space. Make careful preparations regarding the frame rate, transducer position, region of interest, focal depth, gain, Mechanical Index (MI) and consistent Sonoscans. It is important to be able to re-evaluate the exam at a good workstation or the monitor of the US machine. Unfortunately there are HCC's which do not wash out and old FNH's and some haemangiomas that do wash out. These anomalies are reasons to optimize the arterial phase parameters as far as possible.Bubble bursts with replenishment studies can provide "additional initial enhancements" in small volumes.
Sometimes it works very well to repeat the initial enhancement
fast filling lesion by one or a few high MI burst, that momentarily
disrupt all microbubbles. Immediately after the bursts the initial
enhancement pattern of the lesion is revealed by the new microbubbles
entering the scene. The most efficient way to create bursts is by using
short colour doppler exposures to the region of interest. This
technique is not always beneficial, but it can at times be the key to
correct characterization of rapidly enhancing lesions or in cases when
the transducer was not exactly on target at TCA. Again, high framerate
and second-look capability of the Sonoexam are a must.
Bubble burst and high MI "pseudoangiogram" of small high flow haemangioma, with slowmotion workup of bubble burst (see following point).
Courtesy of Dr. Anna-Karin Siösteen-Tofte, Karolinska University Hospital, Sweden.
High MI and bubble burst replenishment in small haemangioma. Both techniques reveal the arterial characteristics of a haemangioma.
examining a fast, hypervascular lesion with a very high MI, the
microbubbles burst as they enter the field of view. In appropriate
circumstances this technique makes the arteries of the lesion stand out
against a background of less enhanced tissue, thus creating a type of
"pseudo angiogram". This has been helpful in cases of very quickly
enhancing lesions where the vessels otherwise quickly "drown" in the
tissue perfusion, giving some more time to study the arteries, mainly
in fast haemangiomas and FNH's. However, it is not easy to predict
which lesions will benefit. Sometimes a reinjection with a very small
amount of UCA (0,3 - 0,6 ml) and an initial MI of about 0,6 to 1,0
makes the process easier to control. Furthermore,
the technique is more beneficial for lesions close to the transducer
than lesions at greater depths.
High MI angiogram in FNH High MI used initially to emphasize the arterial tree of the lesion. "Spoke wheel" arterial tree, and at low MI enhancement from the vessel areas as well as a central scar. No washout.
goal is to find washed-out metastases, and the earlier CEUS phases
usually give very little additional information and are not routinely
scanned. The first US Sonoexam sometimes gives valuable additional
information, for example of simple cysts, obvious metastases, or very
bright or calcified metastases that may not be conspicuous on CEUS. The
initial Mechanical Index of the Siemens Acuson Sequoia™ is
about 0.13, but this setting may have to be altered when scanning the
right liver lobe in order to penetrate all the way to the diaphragm. If
in doubt whether the "survival time" of the microbubbles (usually 4-6
minutes) has been exceeded at some point of the protocol, it is
essential to make another injection of 2,4 ml of UCA and wait for 90
seconds before proceeding with the rest of the protocol. A second full
dose injection can be made three to four minutes after the first one,
and scanning can continue after another 90 seconds. A third injection
has never been necessary unless some equivocal finding required
characterization after detection. If this is the case, it is advisable
to wait about ten minutes before the third injection to give the UCA
time to vanish first. When using the Siemens Acuson Sequoia™,
we currently prefer using the CCI mode rather than the CPS mode for
detection of metastases, based on our experience of their respective
frame rates, spatial resolutions, contrast resolutions and impact on
microbubble longevity. For some unknown reason the UCA lasts longer in
the liver in some patients than in other. Also, the times at which the
portal and liver veins become hypoechoic also differ greatly among
individuals. Some livers maintain bright veins for the duration of the
exams, while others show darkening of the veins after only 2-3 minutes,
still leaving the parenchyma quite brightly enhanced. However, thanks
to the tubular shape of the veins it is usually not difficult to
distinguish metastases from veins, although detection is easier in
cases with isoechoic veins.
Why wait 90 seconds? Demonstration of washout in metastases over time.
average, an MI of about 0,13 (Sequoia) is sufficient for examination
through the depth of the left liver lobe, including segment 1, without
significantly destroying superficial microbubbles. However, depending
on the size of the patient and the general attenuation of the, the
initial MI may have to be adjusted up or down in some cases. Of course,
too low an MIBegin with an MI that penetrates as deeply as possible
without destroying superficial microbubbles. When reaching deep liver
areas, typically the right liver lobe covered by Sonoscans 6 and 7, the
initial MI setting is often too low to penetrate all the way to the
diaphragm, resulting in insufficient brightness in the deep parts of
the liver for sensitive perception of dark metastases. When this
happens we increase the MI as we proceed at MI increments of roughly
0,07 (Sequoia), and eventually return to a Sonoscan for a repeated
scanning in order to see through the depth of the liver. In the
repeated scans we accept that there is bubble destruction in the
superficial parts of the liver, since these have been previously
covered with a lower MI.
Detection with increasing MI At the end an example of bubble destruction due to going from higher to lower M
MI decrease with depth increase MI decreases when going from superficial to deep field of view.
For further details on our technique for detection of metastases, please see http://www.auntminnie.com/index.asp?Sec=sup&Sub=ult&Pag=dis&ItemId=70205.
Compared to metastases, HCC's do not have a reliable washout, and some may not wash out at all. On the other hand they are practically always hypervascular, and enhance earlier than the liver parenchyma. The latter is true even in cases of cirrhosis, where the liver itself generally has an earlier enhancement than normal. Regenerative nodules enhance later than the cirrhotic liver parenchyma and become isoechoic in the portal phase. Thus, the detection of HCC's and regenerative nodules has to occur in the arterial phase. HCC's may become more conspicuous if MI is raised to an otherwise "banned" level of about 0.5-0.7 (Sequoia) during this scan. This way the rapid enhancement of HCC's often stands out more clearly against the relatively less enhancing liver parenchyma. One must remember to decrease MI to a regular CEUS MI level after this initial arterial phase scan. Following eventual microbubble replenishment studies, a conventional liver Sonoexam is performed starting after 2,5 minutes (washout in HCC's is usually late).Up