Norbert Gritzmann and David H Evans
The introduction of multi-row technology has revolutionized computed tomography in
the past few years. Fast sequences, high SENSE factors and strong gradients have improved
MR substantially. This has led to new indications for both techniques, as for example
in cardiac imaging. Has similar progress occurred in ultrasound imaging? The answer
to this question must surely be yes, the improvements may not appear as revolutionary
as in CT or MR, but there is little doubt that giant strides have been made in ultrasound
technology and that the resultant images are constantly improving and contributing
more diagnostic information. In what follows some new applications of sonography are
evaluated.
Ultrasonic Contrast Agents (UCAs) and Contrast Enhanced Ultrasound (CEUS) CEUS and
the liver
Ultrasonic Contrast Agents (UCAs) and Contrast Enhanced Ultrasound (CEUS) CEUS and
the liver
The liver is at present the main target organ for CEUS [1], but an adequate image quality before contrast enhancement is a prerequisite. Modern
ultrasound machines with sophisticated signal processing techniques can use very low
mechanical index pulses which do not disrupt the shells of the UCAs and thus allow
excellent visualisation of the vascular system. The evaluation of blood flow kinetics
using ultrasound is at least equal to that of CT and MR since continuous visualisation
of the contrast agent is possible, and because ultrasonic contrast agents remain within
the intravascular space. Furthermore it is possible by the use of relatively high
intensity pulses (still within the diagnostic range) to destroy the agent in a particular
region and watch the reperfusion of that region. UCAs are used in many centres for
the routine differentiation of focal liver lesions [2], [3]. In the case of malignant lesions of the liver, more lesions are visualised with
than without the use of UCAs, and in the late phase an accurate differentiation of
benign and malignant focal liver lesions is possible, comparable to that obtained
with multi-detector CT [4]. Also the therapeutic effect of radiofrequency ablation of tumours can be and should
be monitored with UCAs.
CEUS and the heart
CEUS and the heart
One of the most important uses of UCAs is echocardiography. It has many roles such
as improving endocardial border definition, evaluation of shunts and regurgitation,
imaging of myocardial perfusion, and assessing coronary arteries and coronary blood
flow reserve.
CEUS and other organs
CEUS and other organs
Although mainly used in the liver, UCAs have a developing role in the pancreas, kidney
and spleen. A potential new application is in the differentiation of tumours of the
pancreas [5]. In particular, hypervascularized endocrine tumours can be recognized as being strongly
supplied with arterial blood. Even proof of avascular necrosis in pancreatitis might
be possible using UCAs. Traumatic parenchymal lesions in the liver, spleen and kidneys
can be visualised substantially more accurately than in native scans. Liver, spleen
and kidney infarcts can be diagnosed far better than with standard imaging [6], and focal nephritis can also be visualised with UCAs. Another potential application
might be the evaluation of complex cystic lesions in the kidneys. UCAs are also useful
in the assessment of urinary reflux in children.
In the arterial system the differentiation between complete occlusion and subtotal
stenoses, which can be crucial in deciding upon therapy, particularly in the carotid
arteries, will become a new indication for the use of UCAs. In small parts applications
inflammation can be visualised with high sensitivity using UCAs. Rheumatoid arthritis
may be diagnosed during the early soft tissue phase of the disease.
Particularly exciting is the prospect of molecular imaging with UCAs [7]. Work is continuing on the development of agents that target, amongst other things,
tumours, thrombus, the reticuloendothelial system, inflammation, and the lymphatic
system Perhaps even more exciting is the potential for therapy using UCAs by the
local selective release of drugs from UCAs using ultrasound pulses. Bubble destruction
can be achieved using only diagnostic intensities, and indeed it is important in many
applications to keep the ultrasound intensity relatively low so as not to inadvertently
destroy the contrast agent.
Limitations of CEUS
Limitations of CEUS
In general the skill level required to use CEUS exceeds that required for native ultrasound
examinations as the investigator requires extra knowledge of contast agents, the kinetics
of contrast agents and the vascularisation of tumours. The method is also more time
consuming, and of course to some degree negates one of the greatest advantages of
ultrasound, its complete non-invasiveness.
Another issue with UCAs has been the unwillingness of the different health care systems,
financed through the public purse or through insurance companies, to finance the additional
costs of these agents. In the liver, pancreas, liver, spleen and kidneys contrast
agents are obligatory in CT and MR studies, so it is difficult to understand why ultrasound
is singled out in this way when it can provide therapeutically relevant information
in many situations.
Broadband transducers and harmonic Imaging
Broadband transducers and harmonic Imaging
Important progress has been made in transducer design such that broadband transducers
are commonplace. They allow the ultrasonic interrogation frequency to be optimised
for the scanning depth; high frequencies close to the transducer to give the best
possible resolution, and lower frequencies from deep structures to optimise the signal
to noise ratio. Tissue harmonic imaging and its variants such as pulse inversion harmonic
imaging which require broadband transducers to receive both the fundamental and harmonic
frequencies have become standard in many applications. These techniques increase the
contrast of lesions, and some artefacts are reduced, but have the disadvantage of
being limited to shallower depths. Many high-end ultrasound machines now use pulse
encoding to provide better penetration at a given frequency, or an increase in frequency
for the same penetration depth.
Volume acquisition techniques
Volume acquisition techniques
Due to the great computing power of modern ultrasound machines it is possible to quickly
acquire 3-D and even 4-D data sets. In obstetric ultrasound, where the presence of
amniotic fluid makes segmentation very easy, 3-D is very popular, and it is becoming
more widespread in cardiology applications; in radiology however the real indications
are far rarer. The demonstration of lumps in the breast, and the reconstruction of
frontal planes can provide additional information. The optimization of ultrasound
guided biopsy in real-time using 4-D ultrasound can be advantageous as the the location
of the needle tip can sometime be better appreciated than in 2-D ultrasound. Also
volumetric scanning of the thyroid gland provides better volume estimation, and similar
techniques might be of advantage in comparing tumour volumes.
Compound scanning techniques
Compound scanning techniques
Whilst tissue boundaries do not act as perfect specular reflectors, the energy returning
from a boundary tends to fall off away from the normal, and therefore it can be advantageous
to interrogate lesions from different directions and combine the information thus
obtained. This can result in a better delineation and detection of lesions [8], but may also reduce diagnostically useable artefacts such as posterior shadowing.
Also the frame rate of the real time image is reduced.
Elastography
Elastography
Another emerging field is that of elastography, where ultrasound is used to produce
maps of the compressibility or elasticity of regions of tissue. This is being clinically
evaluated as a means of detecting and differentiating lesions in the breast, prostate,
thyroid gland and liver amongst other organs [9], [10],[11]. There is a more detailed discussion of elastography elsewhere in this newsletter.
Portable Devices
Portable Devices
A major trend in diagnostic ultrasound has been towards miniaturization, and there
are now many machines available which are comparable in size to laptop computers.
Some of these machines produce excellent images, although they do not include all
the facilities available on a standard cart-based machine, and are particularly useful
when it is better to take the ultrasound machine to the patient rather than visa versa.
Thus these machines are particularly useful in critical care and emergency departments,
but are also finding their way into many other hospital departments, where they are
used for simple tasks such as image guidance for the placement of central venous catheters,
for venous access lines and for nerve blocks, and for simple imaging tasks such as
diagnosis of cholecystolithiasis, urinary obstruction, and cysts.
One concern regarding the new small portable (and inexpensive) machines is that they
are finding their way into clinical environments where the users are not imaging experts.
It is clearly vital that proper training is given to practitioners wishing to use
such instruments even for relatively straight forward tasks.
Panoramic techniques
Panoramic techniques
These serve as a clear way of demonstrating and documenting pathologies but do not
aid the diagnostic process greatly [12].
New clinical application areas
New clinical application areas
A number of new clinical applications of sonography have been introduced in the past
few years. A particularly interesting innovation is the diagnosis of peripheral nerve
entrapment syndromes (for example carpal tunnel syndrome). Superficial nerves can
be visualized with superb resolution ultrasonically, and the modality can be useful
for assessing traumatic nerve lesions. The ultrasonic guidance of peripheral nerve
blocks is becoming routine in many centres, and the guidance of radiofrequency ablation
of tumours is frequently performed. Small parts and superficial imaging can produce
stunning images through the use of higher and higher frequencies. The dynamic investigation
of joints and tendons has suggested new indications in sports medicine and rheumatology.
In superficial tissues, ultrasound can be regarded as the gold standard for differential
diagnosis of lymophadenopathies. Detection and imaging of flow in both peripheral
and central vessel has become substantially more sensitive and artefact free.
Finally the use of high intensity focussed ultrasound (HIFU) for the destruction of
tumours is undergoing clinical trials in several centres.
Other considerations
Other considerations
One of the draw backs of ultrasound is that it remains highly examiner dependent.
The practitioner scanning the patient usually has to make a diagnosis directly during
the examination. A retrospective evaluation and/or a second opinion is much more difficult
than with other imaging procedures. Real-time scans are not usually recorded although
longer digital video loops could help to solve this problem, and the capture of complete
3-D data sets could allow examiners retrospectively to carry out virtual scans. Because
of the unusual examiner dependence of ultrasound it is particularly vital that ultrasonic
practitioners remain abreast of both clinical and technological developments, and
it is hoped that industry will continue to support education and training initiatives
to ensure ultrasound practitioners perform at the highest levels thus supporting
the continued growth of the technique.
Although the need for the practitioner to spend time with the patient at the bed-side
is often considered a disadvantage, it also has advantages in that it leads to good
intensive physician-patient communication which is highly appreciated by most patients
and can often be of considerable benefit.
Norbert E. Gritzmann and David H. Evans
eMail: norbert.gritzmann@bbsalz.at
