|ABDOMINAL MR IMAGING:
IMPORTANT CONSIDERATIONS FOR EVALUATION OF GADOLINIUM ENHANCEMENT
|Department of Radiology University of North Carolina Hospitals Chapel Hill|
based contrast agents (GBCAs) are T1-shortening agents
that result in marked elevation of signal on T1-weighted (T1W) images.
GBCA enhancement is crucial in the detection and characterization of
first important aspect to a confident analysis of GBCA enhancement is to
obtain high quality and reproducible MR studies. In keeping with this,
we need motion-free images, devoid of substantial artifacts, especially
those due to motion. Respiration is the most important and most
troubling source of artifacts in abdominal MR
imaging. Control of these
artifacts is most critical in GBCA enhanced studies in which dynamic T1W
imaging is employed. Motion can also adversely affect fat suppression,
reducing difference in signal intensities between enhanced tissue and
background tissue. Motion can even simulate GBCA enhancement or obscure
it . Furthermore, motion impairs the comparison between the different
phases of post-contrast imaging and pre-contrast T1W images. However,
using breathing-independent sequences and breath-hold sequences we can
obtain high-quality diagnostic MR images in the great majority of
patients. For evaluation of GBCA enhancement, we obtain T1W gradient
echo sequences (instead of spin-echo sequences): spoiled gradient echo
(2D-SGE) or three dimensional gradient echo (3D-GE). Current
“state-of-the-art” MR systems use 3D-GE as the primary technique for
dynamic contrast-enhanced imaging of the abdomen. The addition of
parallel imaging technique, especially with multi-channel coils, can
significantly shorten imaging times, rendering improved comfort and as a
result compliance for patients on breath-hold sequences.
noncooperative agitated patients, we can use magnetization-prepared
rapid-acquisition gradient echo (MPRAGE), which is a single shot
sequence, with image acquisition duration of 1 to 2 seconds, rendering
it relatively breathing independent . However, adequate quality
MPRAGE sequences (with 180° non-slice selective technique, which is
required for dynamic GBCA-enhanced acquisitions) are currently available
only in “state-of-the-art” MR systems. In this regard, the higher
intrinsic signal to noise of 3T system allow for good quality image with
MPRAGE especially when water excitation is employed .
SUBPHASE OF ENHANCEMENT
GBCA injection we routinely obtain three phases to evaluate tissue
perfusion (hepatic arterial dominant phase), blood pool venous
withdrawal (early hepatic venous phase), and interstitial space size (interstitial
phase) (Fig. 1).
most important set of images are those obtained in the hepatic arterial dominant phase (HADP), in which the contrast must
be present in the hepatic arteries and portal veins prior to appearing
in the hepatic veins [5-9] (Fig. 1). There is a relatively short time
window for this and correct timing is critical. Three different
techniques including empirical timing, test bolus and bolus tracking
methods have been employed . It has been recently reported that
arterial-phase bolus-track liver examination (ABLE) technique is a
successful method for the acquisition of hepatic arterial dominant phase
. A bolus track sequence, which produces an image approximately
every second, is used to detect the arrival of GBCA to the level of
celiac axis. After 8 seconds, the liver is scanned with standard ordered
k space for a 16-20 second sequence. During the 8 second-period, the
patient is given breathing instructions. This is also the technique that
we routinely use now at our institution.
these techniques depend on the empirical estimation of circulation time
of the contrast material from the site of injection or abdominal aorta
to the liver [5,10]. Because the circulation time of the contrast
material to the liver shows variations depending on various factors,
different subphases of enhancement, other than the HADP, may be detected
in the “early post-contrast hepatic imaging”, based on the vessel
enhancement patterns as well as the enhancement of abdominal parenchymal
organs . A recent report of our group on this subject, employing an
18-s set time-delay empiric timing scheme for the initiation of scanning
allowed us to observe five subphases of early contrast enhancement of
the liver: (a) early hepatic arterial phase (EHAP), (b) mid-hepatic
arterial phase (MHAP), (c) late hepatic arterial phase (LHAP), (d)
splenic vein only HADP (SVHADP) and (e) HADP  (Table 1).
the EHAP, the renal cortex, spleen, pancreas and liver similarly
demonstrated no or slight enhancement, which significantly increased on
the MHAP. The enhancement of the pancreas on two first subphases was
significantly lower, both qualitatively and quantitatively, compared to
the other later phases (LHAP, SVHADP and HADP), where it was not
significantly different. The liver demonstrated significantly higher
enhancement on HADP and SVHADP compared to LHAP. Our results suggest
that it is too early to evaluate liver enhancement during EHAP and MHAP
(Fig. 2). In addition, EHAP is essentially identical to non-contrast
images in demonstrating liver lesions, which is thus unable to provide
distinctive enhancement of focal liver lesions. The LHAP, SVHADP and
HADP can be used to evaluate liver lesion enhancement and may be
considered as the optimal range of phases of enhancement. The pancreatic
enhancement was maximal in LHAP, and therefore pancreatic capillary
blush can be a useful surrogate for optimal timing to evaluate liver
disease (Fig. 1). These impressions are also supported by previous
reported findings in the literature [6,7,8,12,13,14]. The enhancement of
renal veins can help to determine the adequacy of liver enhancement,
since we consider an important finding of true LHAP is contrast in these
veins, in addition to visual pancreatic enhancement (Fig. 1). On the
subsequent SVHAD phase there is also the enhancement of suprarenal IVC,
splenic vein and portal vein. On HADP, contrast also appears in the
superior mesenteric vein (Table 1).
The vessel and organ enhancement patterns according to subphases
with permission from reference 5
combination of these vessel enhancement patterns with the enhancement
patterns of the renal cortex and spleen may also serve as adequate
surrogates and a guide for the optimal phase of enhancement in the case
of chronic pancreatitis or other pancreatic diseases, where pancreatic
enhancement may be minimal .
these landmarks, we can determine with confidence if the “first
pass” or capillary bed enhancement of tissues has been captured.
Optimal timing of contrast enhancement is essential not only for the
detection of liver lesions, especially hypervascular tumors such as HCC,
hypervascular liver metastases and benign hepatocellular tumors, but
also for evaluation of response to treatment (Fig. 3) [5-9,12-15]. The
vascularity of tumors, as evidenced by extent of early enhancement, has
been shown to predict the likelihood of response to certain treatment
is essential to know if the contrast-enhanced images have optimal timing
when analyzing other abdominal parenchymal organs as well. For instance,
too little pancreatic enhancement is consistent with pancreatic fibrosis
or chronic pancreatitis, and too little enhancement of renal cortex may
imply ischemic nephropathy or acute cortical necrosis. This can be
reliably judged on HADP images, based on the fixed vessels landmarks
cited above. In the EHAP, minimal enhancement of pancreas or renal
cortex may reflect an early image acquisition rather than disease
process (Fig. 2).
early hepatic venous phase
images are obtained between 45 and 90 seconds post-contrast injection
and can be recognized by the presence of GBCAs in portal and hepatic
veins (Fig. 1). Thus, the time window is relatively wide and timing is
not so critical. This phase shows maximal enhancement of the hepatic
parenchyma and is especially useful for the detection of hypo/isovascular
HCC and hypovascular metastases. It is also important in lesion
characterization, specifically hypervascular lesions such as focal
nodular hyperplasia, which shows fading and HCC which shows washout in
this phase [6,8,9,14,15] (Fig. 3).
Late hepatic-venous or
interstitial phase has a broad time range, which is from
approximately 90 seconds to 5 minutes after initiation of contrast
injection, with no exact timing requirement. During this phase, hepatic
parenchymal enhancement persists (Fig. 1). This phase also provides
additional information to characterize focal hepatic lesions, by
demonstrating their late-phase temporal handling of contrast.
Hemangiomas reveal progressive enhancement, persistent enhancement is
observed in small-sized hemangiomas, and washout of hypervascular
metastases and HCC is also apparent during this phase [6,8,9,14,15] (Fig.
current worldwide proliferation of 3.0T MR imaging systems beyond
academic and research centers has added other important consideration to
the routine clinical practice, especially regarding GBCA-enhanced images:
3.0T versus 1.5T.
major advantage of MR imaging at 3T compared to 1.5T is the theoretical
twofold increase in signal-to-noise ratio (SNR), which can be translated
into higher spatial resolution and/or temporal resolution, particularly
with the use of parallel imaging techniques [4,16-25]. At 3.0T,
post-gadolinium T1W 3D-GE sequence can be obtained with higher quality
than at 1.5T, primarily because of the thinner section acquisition, and
is relatively resistant to the drawbacks of 3.0T MRI including specific
absorption rate constraints, prolonged T1 relaxation times and the
increase in imaging artifacts [16-25].
ability of GBCAs to reduce T1 (known as the relaxivity) is slightly
lower at 3T (5% to 10%) compared to 1.5T. However, the T1 relaxation
times of the tissues are prolonged on the order of 40% or more at 3.0T
compared to 1.5T. Therefore, an equivalent dose of GBCA at 3.0T causes
an increased contrast difference compared to 1.5T [16,18,21,23-25]. This
increased effect of GBCAs contributes to better SNR and
contrast-to-noise ratio (CNR) at 3.0T. Corroborating this and supporting
previous descriptions in the literature, one recent report of our group
has demonstrated consistent differences in the extent of enhancement of
abdominal organs between 1.5T and 3.0T on the subphases of hepatic
arterial enhancement, with achievement of higher relative enhancement at
3.0T . The benefits of this higher extent of enhancement at 3.0T may
potentially translate into better detection of lesions that have a blood
supply greater or lesser than background abdominal organs. This is
particularly important to detect hypervascular lesions in the cirrhotic
liver (such as HCC) (Fig. 3), and hypervascular metastases
addition of the newly available 32-channel coil to MRI systems will
permit significant improvement in parallel imaging, which will
accelerate 3D GE and will aid in achieving ultra-short, high-quality,
dynamic 3D imaging following bolus contrast administration. These
aspects will probably render much superior detection of lesions on
systems equipped with this functionality, raising an important
consideration to the daily clinical practice of comparing lesions
between different MR studies. For example, a MR exam for control of a
probably high-grade dysplastic nodule versus small HCC in a cirrhotic
patient or for treatment control of small liver metastases in a breast
cancer patient obtained at a 3.0T MR system. The lesions are better seen
at the present exam and seem to be enhanced more in the HADP
post-contrast imaging, when compared to the previous exam obtained at
1.5T. Is the difference a result of better lesions display at 3.0T
compared to 1.5T or disease progression?
is a rare earth metal atom and has seven unpaired electrons, which
renders it highly paramagnetic. Free gadolinium is toxic in vivo and
forms colloid particles that are phagocytized by the reticuloendothelial
system. The chelation of gadolinium to organic ligands (chelate complex)
is necessary for the atom to be used as an in vivo contrast agent in
humans. This process makes the ion chemically inert [1,2,26,27]. There
are several formulations available with different ligands, constituting
the GBCAs (Table 2). Depending on the ligand, GBCAs are classified into
linear or macrocyclic (according to the backbone structure of their
amine group) and may be further subclassified according to their charges
as ionic or non-ionic. Depending on these characteristics, GBCAs
dissociate to varying extent in solution. Following intravenous
injection, it is possible to detect gadolinium-chelate complexes, free
gadolinium ions and free ligands in human tissues. This dissociation can
be defined by the thermodynamic stability constant and dissociation
constant [28,29]. Thermodynamic stability constant determines the
concentration at which gadolinium ions will dissociate from
gadolinium-chelate complexes. The rate of this dissociation reaction is
dependent on the dissociation constant [28,29]. Taken together, these
two constants define the affinity of ligands for gadolinium ions at the
physiological pH [28,29]. Each GBCA has a different thermodynamic
stability constant and dissociation rate. Macrocyclic GBCAs create
tighter bonds with gadolinium and therefore have higher thermodynamic
stability constants and lower dissociation rates . The electrostatic
charges present in ionic GBCAs render tighter bonding than nonionic
GBCAs, and therefore ionic GBCAs are more stable than non-ionic GBCAs.
These latter pharmacological characteristics have become relevant in the
radiological literature since October 2006, when it became recognized
that GBCAs could result in the condition nephrogenic systemic fibrosis (NSF)
in patients with renal impairment . High thermodynamic stability
constants and lower dissociation rates (greater affinity of ligands for
gadolinium ions) are important qualities of GBCAs to minimize risks of
clinical use GBCAs can further be classified into three types according
to their distribution in the body following intravenous injection: a)
extracellular agents, b) combined extracellular and intracellular agents,
c) blood pool agents [26,27].
Extracellular Contrast Agents
great majority of the GBCAs in clinical use are nonspecific
extracellular contrast agents, sharing similar pharmacokinetics with
iodinated contrasts in the abdomen and throughout the body. After
intravenous injection, they follow the route of blood circulation and in
the abdomen first reach the aorta and its branches; enter the splanchnic
and splenic circulation by the celiac axis, superior and inferior
mesenteric arteries, and then into their companion veins and
subsequently into the portal vein. Contrast agents enter the venous
system after the passage through the sinusoids in the liver, and after
passing the capillaries in the peripheral circulation. They are freely
redistributed from the vascular to the interstitial space [1,2,26,27].
Whereas the iodine molecule is directly imaged at computed tomography
(CT), in MR imaging it is the effect of gadolinium that is evaluated
rather than the agent itself. Gadolinium exhibits an amplification
effect, in which many adjacent water protons are relaxed by a single
gadolinium atom, shortening T1 of the tissues (effect known as
relaxivity, r1). As a result, MR imaging is orders of magnitude more
sensitive to the effect of gadolinium than is CT to the effect of iodine
. The recommended dose of the extracellular contrast agents is 0.1
mmol/kg of body weight. The recommended injection rate is 2-3 ml/sec.
All extracellular agents are eliminated by the kidneys and are not
excreted by the hepatobiliary system in patients with normal renal
function. They do not exhibit protein binding (Table 2).
agents can be used for the acquisition of hepatic arterial dominant,
early hepatic venous and interstitial phases of standard
gadolinium-enhanced MRI studies. Contrast agents with greater
T1-relaxivity may show better enhancement effects. In this regard,
Gadovist [gadobutrol, (Bayer Schering Pharma AG, Germany)], which is a
nonionic macrocylic GBCA, has higher T1 relaxivity and can be
theoretically administered in a lower dose to achieve the same imaging
effect. Due to its macrocylic structure Gadovist has higher stability in
solution and there are no cases of NSF related to its use .
[gadoterate meglumine, (Guerbet, Aulnay-sous-Bois, France)] is an ionic
macrocylic agent, and therefore combines both the predominant stability
factor of macrocyclic design with the ancillary factor of ionicity.
Hence, from a theoretical standpoint of stability, Dotarem should be one
of the best of all the GBCAs. Until the present time, there are no cases
of NSF related to its use .
Extracellular and Intracellular Contrast Agents
class of GBCAs is distributed into the extracellular space including
vascular and interstitial spaces, and intracellular spaces of
hepatocytes (hepatocyte phase). Therefore, these agents can also be
termed as combined extracellular and hepatocyte specific agents. They
can be used for the acquisition of the hepatic arterial dominant, early
hepatic venous, interstitial and hepatocyte phases of GBCA-enhanced MRI
studies. These agents are Multihance [gadobenate dimeglumine, (Bracco
Diagnostics, Milan, Italy)] and Eovist [gadoxetic acid, (Bayer
HealthCare Pharmaceuticals, Wayne, NJ, USA), in the US] / Primovist [gadoxetic
acid, (Bayer Schering Pharma AG, Germany), outside the US]. They are
taken up by hepatocytes and excreted into bile ducts. Consequently, they
have dual elimination including both renal and biliary eliminations (Table
2). The hepatocyte phase is useful to characterize lesions with biliary
structures, especially focal nodular hyperplasia (FNH) [27,31,32]. This
phase is particularly helpful for the differentiation of FNH from
adenoma. FNH contains hepatocytes and biliary canaliculi; therefore,
hepatocyte specific agents can be uptaken and excreted into the bile
ducts in FNH. However, hepatic adenomas do not contain normal
hepatocytes and biliary canaliculi; therefore, hepatocyte specific
agents do not show uptake. Thus, while FNH enhances on the hepatocyte
phase, hepatic adenomas do not [27,31]. The hepatocyte phase is also
helpful for the detection of lesions which do not contain hepatocytes,
as signal intensity difference is expanded between enhanced liver
parenchyma and non-enhanced lesions. This includes metastases, adenomas
or poorly differentiated hepatocellular carcinomas (HCCs).
has been used in Europe for number years and in the United States, it
was approved for use in December 2004 [26,27]. The agent has shown good
patient tolerance, and to date with over three million doses
administered, no cases of NSF have been associated with its use alone
(no confounded cases). It demonstrates weak and transient binding with
serum albumin in the intravascular space, remaining for a longer time
than do other gadolinium chelates. In addition, this protein-binding
characteristic results in increased T1 relaxivity compared with that
achieved with other GBCAs. Increased T1 shortening results in increased
signal intensity, which is useful for MR angiography and may yield
improvements in tumor imaging [31,32]. Serial contrast-enhanced liver
imaging can be performed with the use of Multihance after bolus
injection, in the same fashion as with other nonspecific extracellular
contrast agents [5,17,31,32]. The results are comparable with other
conventional extracellular contrast agents, particularly for the
improved visualization of hypervascular lesions [5,17,31,32]. Multihance
is approved for use at 0.1 mmol/kg; however, it has been shown that 0.05
mmol/kg (half dose) of this agent has comparable diagnostic efficacy
compared to the full dose of standard extracellular GBCAs . We
therefore routinely use half dose Multihance. One of our major
considerations is to minimize the volume of a GBCA. Multihance shows
hepatobiliary contrast enhancement peak at 60 to 120 minutes after
intravenous injection. This requires two separate imaging sessions for
has greater degree of protein binding than Multihance and a much greater
proportion of biliary excretion (Table 2). Consequently, the hepatocyte
phase is earlier, occurring at about 20 minutes (lasting till about 4
h). This permits ready acquisition of an entire post-contrast study,
including hepatic arterial dominant, early hepatic venous, and
interstitial phases, with hepatocyte-phase, in one imaging session.
Eovist/Primovist is used at ¼ dose (0.025 mmol/kg) of the standard dose
of extracellular GBCAs. However, there is not sufficient data
demonstrating the diagnostic efficacy of this dose of Eovist compared to
the standard doses of extracellular GBCAs. A recent report on this
subject  using the quantitative assessment has demonstrated that the
enhancement effect of abdominal solid organs and aorta in the arterial
phase and major in the portal phase with Gd-EOB-DTPA (Primovist) was
significantly lower compared with that of Gd-DTPA (Magnevist) in the
same subjects (healthy male volunteers), but significantly higher in the
latter phases. The authors suggest the reassessment of Gd-EOB-DTPA dose
2 - Gadolinium based contrast agents
dimeglumine, gadoxetic acid, gadofosveset and gadobutrol have higher T1
relaxivity compared to the other agents.
dimeglumine has been reported to be diagnostically effective at its half
dose (0.05 mmol/kg).
solution has double amount of gadolinium in its 1 molar (M)
concentration compared to 0.5 M concentration of the other GBCAs’
Pool Contrast Agents
pool agents predominantly stay in the vascular space. Vasovist [gadofosveset,
(Epix Pharmaceuticals, Lexington, MA, USA)] is the blood pool agent
approved by the FDA (for body MRAs). 85% of Vasovist binds serum albumin
transiently and reversibly. A small amount of Vasovist is also
distributed into the extracellular space. Binding to serum albumin
provides higher T1 relaxivity and extended intravascular enhancement,
which is even higher and longer with Vasovist compared to the other
protein binding GBCAs (Multihance and Eovist). This agent may be
advantageous for vascular imaging. However, the diagnostic role and
safety profile of this agent needs to be determined, and we have no
clinical experience with this agent.
review describes the important characteristics of GBCA-enhanced imaging.
We have emphasized the importance of breath-holding and of understanding
the significance of exact subphase of enhancement. We have described
1.5T and 3.0T imaging and advantages of 3T. The major concern with these
agents is NSF. Finally, we have described a number of the agents in
clinical use that we recommend using.
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- Dipartimento di Radiologia
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