NEW CONTRAST AGENTS FOR IMAGING THE LIVER

Contrast agents for preclinical targeted X-ray imaging

Micro-computed tomography (micro-CT) is an X-ray based instrument that it is specifically designed for biomedical research at a preclinical stage for live imaging of small animals. This imaging modality is cost-effective, fast, and produces remarkable high-resolution images of X-ray opaque skeleton. Administration of biocompatible X-ray opaque contrast agent allows delineation of the blood vessels, and internal organs and even detection of tumor metastases as small as 300 μm. However, the main limitation of micro-CT lies in the poor efficacy or toxicity of the contrast agents. Moreover, contrast agents for micro-CT have to be stealth nanoparticulate systems, i.e. preventing their rapid renal clearance. The chemical composition and physicochemical properties will condition their uptake and elimination pathways, and therefore all the biological fluids, organs, and tissues trough this elimination route of the nanoparticles will be contrasted. Furthermore, several technologies playing on the nanoparticle properties, aim to influence these biological pathways in order to induce their accumulation onto given targeted sites, organs of tumors. In function of the methodologies carried out, taking benefit or not of the action of immune system, of the natural response of the organism like hepatocyte uptake or enhanced permeation and retention effect, or even accumulation due to ligand/receptor interactions, the technologies are called passive or active targeted imaging. The present review presents the most recent advances in the development of specific contrast agents for targeted X-ray imaging micro-CT, discussing the recent advance of in vivo targeting of nanoparticulate contrast agents, and the influence of the formulations, nature of the nanocarrier, nature and concentration of the X-ray contrasting materials, effect of the surface properties, functionalization and bioconjugation. The pharmacokinetic and versatility of nanometric systems appear particularly advantageous for addressing the versatile biomedical research needs. State of the art investigations are on going to propose contrast agents with tumor accumulating properties and will contribute for development of safer cancer medicine having detection and therapeutic modalities.

Multimodality molecular imaging of stem cells therapy for stroke

Stem cells have been proposed as a promising therapy for treating stroke. While several studies have demonstrated the therapeutic benefits of stem cells, the exact mechanism remains elusive. Molecular imaging provides the possibility of the visual representation of biological processes at the cellular and molecular level. In order to facilitate research efforts to understand the stem cells therapeutic mechanisms, we need to further develop means of monitoring these cells noninvasively, longitudinally and repeatedly. Because of tissue depth and the blood-brain barrier (BBB), in vivo imaging of stem cells therapy for stroke has unique challenges. In this review, we describe existing methods of tracking transplanted stem cells in vivo, including magnetic resonance imaging (MRI), nuclear medicine imaging, and optical imaging (OI). Each of the imaging techniques has advantages and drawbacks. Finally, we describe multimodality imaging strategies as a more comprehensive and potential method to monitor transplanted stem cells for stroke.

Dual contrast magnetic resonance imaging tracking of iron-labeled cells in vivo

Negative-contrast magnetic resonance imaging (MRI) methods utilizing magnetic susceptibility contrast agents have become one of the most widely used approaches in cellular imaging research. However, visualizing and tracking super-paramagnetic iron oxide nanoparticle (SPIO)-labeled cells on the basis of negative-contrast can limit specificity and sensitivity. Therefore, there has been a strong motivation to explore MRI methods for cellular imaging with either positive or dual contrast (both positive and negative) for identifying labeled cells; these methods offer the potential to improve significantly the sensitivity and specificity of MRI-based cell-tracking approaches. In this review, current state-of-the-art positive- and dual-contrast MRI techniques and contrast agents are described specifically for applications involving in vivo cellular tracking and imaging.

In vivo neural stem cell imaging: current modalities and future directions

Neural stem cells have been proposed as a promising therapy for treating a wide variety of neuropathologies. While several studies have demonstrated the therapeutic benefits of neural stem cells, the exact mechanism remains elusive. In order to facilitate research efforts to understand these mechanisms, and before neural stem cell-based therapies can be utilized in a clinical context, we must develop means of monitoring these cells in vivo. However, because of tissue depth and the blood-brain barrier, in vivo imaging of neural stem cells in the brain has unique challenges that do not apply to stem cells for other purposes. In this paper, we review contemporary methods for in vivo neural stem cell imaging, including MRI, PET and optical imaging techniques.

MR imaging of liver fibrosis: current state of the art

Chronic liver disease is a major public health problem worldwide. Liver fibrosis, a common feature of almost all causes of chronic liver disease, involves the accumulation of collagen, proteoglycans, and other macromolecules within the extracellular matrix. Fibrosis tends to progress, leading to hepatic dysfunction, portal hypertension, and ultimately cirrhosis. Liver biopsy, the standard of reference for diagnosing liver fibrosis, is invasive, costly, and subject to complications and sampling variability. These limitations make it unsuitable for diagnosis and longitudinal monitoring in the general population. Thus, development of a noninvasive, accurate, and reproducible test for diagnosis and monitoring of liver fibrosis would be of great value. Conventional cross-sectional imaging techniques have limited capability to demonstrate liver fibrosis. In clinical practice, imaging studies are usually reserved for evaluation of the presence of portal hypertension or hepatocellular carcinoma in cases that have progressed to cirrhosis. In response to the rising prevalence of chronic liver diseases in Western nations, a number of imaging-based methods including ultrasonography-based transient elastography, computed tomography-based texture analysis, and diverse magnetic resonance (MR) imaging-based techniques have been proposed for noninvasive diagnosis and grading of hepatic fibrosis across its entire spectrum of severity. State-of-the-art MR imaging-based techniques in current practice and in development for noninvasive assessment of liver fibrosis include conventional contrast material-enhanced MR imaging, double contrast-enhanced MR imaging, MR elastography, diffusion-weighted imaging, and MR perfusion imaging.

Initial experience on utility of gadobenate dimeglumine (Gd-BOPTA) enhanced T1-weighted MR cholangiography in diagnosis of acute cholecystitis

PURPOSE

To investigate the feasibility of the use of gadobenate dimeglumine (also known as Gd-BOPTA) -enhanced T1-weighted MR cholangiography in diagnosis of acute cholecystitis.

MATERIALS AND METHODS

This prospectively designed institutional review board-approved HIPAA-compliant study was done between January and November 2007. We included 11 consecutive patients (7 male, mean age 59 years) who presented to the emergency room with acute right upper quadrant pain and with equivocal physical examination and/or ultrasound findings. The control group included 15 patients who underwent liver MRI with Gd-BOPTA. All patients underwent contrast-enhanced (CE) MR cholangiography examinations. CE-MR cholangiography was performed on a 1.5 Tesla magnet using 3D T1-weighted high resolution isotrophic volume examination (THRIVE) obtained at the 90th min after intravenous injection of Gd-BOPTA. Imaging features detected on CE-MR cholangiography were correlated with operative and histopathologic findings.

RESULTS

In the control group, GD-BOPTA was visualized within the gallbladder in all subjects. For the study group, gallstones were present in nine patients (n = 7 both in gallbladder and cystic duct, n = 1 only in gallbladder, n = 1 only in cystic duct) on MRCP. Hydropic gallbladder was detected in seven patients, significant wall thickening in seven patients, and pericholecystic free fluid in 6 patients. On delayed phase CE cholangiography, significant enhancement of gallbladder wall was seen in 10 patients, and contrast agent excretion into gallbladder was absent in all patients. Surgery was performed in 10 patients, and cholecystostomy was done in 1 patient. Surgery and histopathology findings were consistent with cholecystitis in all patients.

CONCLUSION

In addition to anatomical assessment, Gd-BOPTA-enhanced MR cholangiography can provide functional evaluation similar to HIDA scintigraphy in diagnosing acute cholecystitis in patients with acute right upper quadrant pain and equivocal findings.

Diagnosis and treatment of benign focal liver lesions

BACKGROUND

With the routine use of improved imaging modalities, more benign liver lesions are detected nowadays. An accurate characterization of these incidental lesions may be a challenge, and frequently a biopsy or even unnecessary surgery is being performed. However, these interventions are not always to the benefit of the patient.

METHODS

A Medline search of studies relevant to imaging diagnosis and management of the most common, benign, solid and non-solid liver lesions was undertaken. References from identified articles were handsearched for further relevant articles. The authors' own experiences with benign liver lesions were also taken into account.

RESULTS

Although atypical imaging features are the exception rather than the rule, it is sometimes difficult to differentiate between benign and malignant lesions, and knowledge of their imaging features is essential if unnecessary work-up is to be avoided. The use of tissue-specific contrast media, which has clearly improved the accuracy of highly advanced radiological techniques, may be helpful during differential diagnosis. Once having established an accurate diagnosis, surgery is rarely indicated for a benign liver lesion because of its asymptomatic nature.

CONCLUSION

Knowledge of imaging features and a clear management strategy during diagnostic work-up, emphasizing the indications for surgery, will minimize the number of patients who have to undergo biopsy or unnecessary surgery.

Visualization of the biliary tract using gadobenate dimeglumine: preliminary findings

OBJECTIVE

To compare contrast-enhanced magnetic resonance (MR) cholangiography (CE-MRC) performed with gadobenate dimeglumine with T2-weighted MRC (T2-MRC) for visualization of the bile ducts in nondilated biliary systems.

MATERIALS AND METHODS

Twenty consecutive patients who underwent MR imaging (MRI) of the liver and pancreas with nondilated intrahepatic ducts were included in this retrospective study. T2-weighted MRC was performed using a multislice, high-resolution fat-suppressed half-Fourier acquisition turbo spin-echo sequence. Contrast-enhanced MR cholangiography was performed using a fat-suppressed 3-dimensional fast low-angle shot sequence acquired 1 to 1.5 hour after intravenous administration of gadobenate dimeglumine. For image interpretation, the biliary system was divided into 8 segments. Two readers graded visualization of each segment on T2-MRC and CE-MRC using a 5-point scale (0, nonvisualization; 4, excellent visualization). Final opinion for each sequence was rendered by consensus. Superiority of visualization was assessed using the McNemar test and comparing adequately (visualization scores 3 and 4) and inadequately (visualization scores 0, 1, and 2) visualized segments of the ducts on both sequences. Interobserver variability was assessed with kappa statistics.

RESULTS

Overall and segment-based evaluation revealed superior visualization of biliary segments with CE-MRC compared with T2-MRC. We also found a statistically significant difference between the 2 sequences for overall and for right hepatic duct and cystic channel visualization (P < 0.05). A high concordance between readers 1 and 2 both for T2-MRC and CE-MRC was achieved (85.8% and 89.4%, respectively).

CONCLUSIONS

Gadobenate dimeglumine can be used as an alternative intrabiliary contrast agent for contrast-enhanced MR cholangiography in nondilated biliary systems in patients with normal excretory liver function tests.

Health economic evaluation of ferucarbotran-enhanced MRI in the diagnosis of liver metastases in colorectal cancer patients

PURPOSE

The objective of our study was to analyze the health economic impact of ferucarbotran-enhanced magnetic resonance imaging (MRI) in the diagnosis of hepatic colorectal cancer metastases based on observed changes in medical management.

MATERIALS AND METHODS

A decision tree simulating a patient's medical management was designed, comparing two scenarios: contrast-enhanced spiral computed tomography-based vs ferucarbotran-enhanced MRI-based (Resovist, Bayer Schering Pharma AG, Germany) diagnosis. A clinical trial in patients with presumed liver metastases (n=36) provided data on clinical decisions regarding the medical management options in relation to diagnostic outcomes: resection, chemotherapy, or best supportive care. A "gold standard" was established afterward, combining all the available clinical, imaging, laboratory, and pathology findings. A multidisciplinary panel formed by a hepatologist, a liver surgeon, and an interventional radiologist decided on the recommended medical management for each patient. Costs of medical resources associated with each management option (all expressed in Euro) were obtained from the public health insurance (average European values). Life expectancies for the different options were obtained from literature.

RESULTS

Despite an initial extra cost of 338 Euro, a significant net saving of 1,443 Euro was obtained with ferucarbotran-enhanced MRI mainly because of avoiding unnecessary surgery. There was no significant difference in the predicted life expectancy between both arms, despite the large difference in medical decision.

CONCLUSION

In this comparative medical decision analysis, it was shown that ferucarbotran-enhanced MRI has the potential to improve medical management and save health care costs.

MR-based imaging of neural stem cells

The efficacy of therapies based on neural stem cells (NSC) has been demonstrated in preclinical models of several central nervous system (CNS) diseases. Before any potential human application of such promising therapies can be envisaged, there are some important issues that need to be solved. The most relevant one is the requirement for a noninvasive technique capable of monitoring NSC delivery, homing to target sites and trafficking. Knowledge of the location and temporospatial migration of either transplanted or genetically modified NSC is of the utmost importance in analyzing mechanisms of correction and cell distribution. Further, such a technique may represent a crucial step toward clinical application of NSC-based approaches in humans, for both designing successful protocols and monitoring their outcome. Among the diverse imaging approaches available for noninvasive cell tracking, such as nuclear medicine techniques, fluorescence and bioluminescence, magnetic resonance imaging (MRI) has unique advantages. Its high temporospatial resolution, high sensitivity and specificity render MRI one of the most promising imaging modalities available, since it allows dynamic visualization of migration of transplanted cells in animal models and patients during clinically useful time periods. Different cellular and molecular labeling approaches for MRI depiction of NSC are described and discussed in this review, as well as the most relevant issues to be considered in optimizing molecular imaging techniques for clinical application.

Focal nodular hyperplasia: lesion characteristics on state-of-the-art MRI including dynamic gadolinium-enhanced and superparamagnetic iron-oxide-uptake sequences in a prospective study

PURPOSE

To image a cohort of patients with pathology-proven focal nodular hyperplasia (FNH) to assess which characteristics of state-of-the-art magnetic resonance imaging (MRI) of the liver are the most useful for improving the detection and characterization of FNH.

MATERIALS AND METHODS

In 14 patients, pathology-proven FNH (N=33) were prospectively examined using gadolinium (Gd) and superparamagnetic iron-oxide (SPIO) contrast media. All lesions were evaluated for signal intensity (SI), fatty infiltration, central scar, mode of enhancement with Gd, and uptake of SPIO. The percentage of dynamic contrast enhancement in the arterial, portal, and delayed phases was assessed. The contrast-to-noise ratio (CNR) before and after administration of SPIO contrast was calculated.

RESULTS

The SI of the lesions was low to isointense on T1-weighted (T1W) images, and intermediate to isointense on T2W images. Fatty infiltration of the lesions was present in 6%. The percentages of enhancement in the liver and lesion were 110%, 115%, and 95%, and 151%, 182%, and 160%, respectively (P<0.0001). All lesions showed uptake of SPIO with improved conspicuity of the central scar and septa. The CNR values precontrast and post-Gd/SPIO were significantly different for T1 in- and opposed-phase and black-blood echo-planar imaging (BBEPI).

CONCLUSION

Combining dynamic Gd-enhanced imaging with T1W and T2W sequences after administration of SPIO facilitates comprehensive evaluation of FNH.

3D excretory MR urography: Improved image quality with intravenous saline and diuretic administration

PURPOSE

To assess the effect of diuretic administration on the image quality of excretory magnetic resonance urography (MRU) obtained following intravenous hydration, and to determine whether intravenous hydration alone is sufficient to produce diagnostic quality studies of nondilated upper tracts.

MATERIALS AND METHODS

A total of 22 patients with nondilated upper tracts were evaluated with contrast-enhanced MRU. All patients received 250 mL of saline intravenously immediately prior to the examination. A total of 11 patients received 10-20 mg furosemide in addition to saline. Imaging was performed with a three-dimensional (3D) and two-dimensional (2D) breathhold spoiled gradient-echo sequences. Excretory MRU images were acquired five minutes after the administration of 0.1 mmol/kg gadolinium and were independently reviewed by two radiologists, who were blinded to the MRU technique. Readers evaluated the calyces, renal pelvis, and ureters qualitatively for degree of opacification, distention, and artifacts on a four-point scale. Statistical analysis was performed using a permutation test.

RESULTS

There was no significant disagreement between the two readers (P=0.14). Furosemide resulted in significant improvement in calyceal and renal pelvis distention (P<0.005), and significant artifact reduction in all upper tract segments (P<0.001) compared to the effect of saline alone.

CONCLUSION

Intravenous furosemide significantly improves the image quality of excretory MRU studies obtained following intravenous hydration. Intravenous saline alone is insufficient to produce diagnostic quality studies of the non-dilated upper tracts.

An Algorithm for the Diagnosis of Focal Liver Masses Using Microbubble Contrast-Enhanced Pulse-Inversion Sonography

OBJECTIVE

The objective of this study was to develop an algorithm for liver mass diagnosis using microbubble contrast-enhanced pulse-inversion sonography.

SUBJECTS AND METHODS

Ninety-six lesions in 92 patients were evaluated with DMP 115 (Definity)-enhanced pulse-inversion sonography, comprising 44 malignancies (29 hepatocellular carcinomas, 12 metastases, two peripheral cholangiocarcinomas, and one hepatic lymphoma) and 52 benign lesions (26 hemangiomas, 20 focal nodular hyperplasias, and six others). All had continuous low-mechanical-index imaging through the arterial and portal venous phase. A three-person blind review evaluated single images at baseline, early and peak arterial phases, and through the extended portal phases with a movie showing arterial phase wash-in. Reviewers assessed lesional vascularity and enhancement blindly but did not make a diagnosis. Combinations of answers were compared with independently determined final diagnoses to develop an algorithm for liver mass diagnosis.

RESULTS

Portal phase enhancement comprises the first step of the algorithm, with positive or sustained enhancement identifying 48 (92%) of 52 benign lesions and negative enhancement or washout present in 41 (93%) of 44 malignancies. Sustained portal phase enhancement with arterial phase peripheral nodularity and centripetal progression predicted 24 (92%) of 26 of the hemangiomas; diffuse arterial phase enhancement greater than the liver identified 19 (95%) of 20 of the focal nodular hyperplasias. With negative portal phase enhancement, arterial phase information was less effective at differentiating hepatocellular carcinoma (25 [86%] of 29 cases) from another hepatic malignancy (11 [73%] of 15 cases).

CONCLUSION

A simple diagnostic algorithm for interpretation of microbubble-enhanced sonography provides sensitive and accurate diagnosis of commonly encountered liver masses.

Mangafodipir trisodium-enhanced MRI for the detection and characterization of focal hepatic lesions: Is delayed imaging useful?

PURPOSE

To investigate the usefulness of early and delayed hepatic MRI after mangafodipir trisodium (Mn-DPDP) administration for the detection and characterization of focal hepatic lesions.

MATERIALS AND METHODS

Forty-five patients (31 males and 14 females, mean age = 61 years) with a total of 113 hepatic lesions (mean size = 3.5 cm) were included in this study (15 with hepatocellular carcinoma (HCC, N = 35), 20 with hepatic metastasis (N = 63), five with hemangioma (N = 10), three with cholangiocarcinoma (CC, N = 3), and two with liver abscess (N = 2)). T1-weighted gradient-echo MR images were obtained before and after Mn-DPDP administration, with a mean 18-hour delayed imaging. A qualitative analysis (including the size and signal intensity (SI)) and quantitative analysis (including enhancement and lesion-liver contrast-to-noise ratio (CNR)) were performed on pre- and postcontrast early and delayed MR images.

RESULTS

Compared to postcontrast early imaging, 17 (48.6%) of 35 HCCs showed higher SI, 16 (45.7%) showed no SI change, and two (5.7%) showed lower SI on delayed imaging. All 63 metastases, 10 hemangiomas, three CCs, and two abscesses showed no SI change. On delayed imaging, ring enhancement was noted in 53 metastases (84.1%), three hemangiomas (30.0%), and one abscess (50.0%), but was not seen in HCCs or CCs. Eight metastases (12.7%) also showed ring enhancement on postcontrast early imaging. No newly detected hepatic lesions were revealed on postcontrast delayed MR images compared to postcontrast early images. Regarding CNR, the HCCs showed a significant increase in CNR from postcontrast early to delayed images after administration of Mn-DPDP (P < 0.01). However, none of the metastases, hemangiomas, CCs, and abscesses showed a significant increase of CNR from postcontrast early to delayed images.

CONCLUSION

Postcontrast delayed MR images after Mn-DPDP administration were helpful in distinguishing hepatocellular from nonhepatocellular lesions, but were not useful for lesion detection and had limited utility for lesion characterization, since benign and malignant hepatic lesions looked the same.

Effect of new manganese contrast agent on tissue intensities in human volunteers: comparison of 0.23, 0.6 and 1.5 T MRI, a part of a phase I trial

To evaluate the effect of a new oral manganese contrast agent (CMC-001) on magnetic resonance imaging (MRI) intensities at different magnetic field strengths. Twelve healthy volunteers underwent abdominal MRI 1 week before and within 2.5-4.5 h after CMC-001 (MnCl(2) and absorption promoters dissolved in water) intake at three different MR scanners of 0.23, 0.6 and 1.5 T. Image contrast and intensity enhancement of liver and pancreas were analysed relatively to muscle and fat intensities. Manganese blood levels were followed for 24 h. Whole-blood manganese concentration levels stayed within the normal range. The liver intensities on T2w images decreased about 10% for the 1/2 contrast dose and about 20% for the full contrast dose independent of the field strength. The liver intensities on T1w images increased more than 30% for 1/2 contrast dose and over 40% for full contrast dose. The maximum T1 enhancement was achieved at the highest field. Pancreas intensities were not affected. Contrast between liver, muscle and fat intensities increased with magnetic field, as well as standard errors of the volunteer-averaged intensities. Oral intake of CMC-001 influences liver intensities and does not affect pancreas intensities at different magnetic field strengths.

Comparison of Superparamagnetic Iron Oxide–Enhanced and Gadobenate Dimeglumine–Enhanced Dynamic MRI for Detection of Small Hepatocellular Carcinomas

OBJECTIVE

The purpose of this study was to compare superparamagnetic iron oxide (SPIO)-enhanced MRI with gadobenate dimeglumine-enhanced MRI for the detection of hepatocellular carcinoma using receiver operating characteristic (ROC) analysis.

MATERIALS AND METHODS

Twenty-nine consecutive patients with 35 hepatocellular carcinomas underwent gadobenate dimeglumine-enhanced MRI (unenhanced, arterial, portal, and equilibrium phases) using 3D fat-saturated volumetric interpolated imaging and SPIO-enhanced MRI on a 1.5-T unit. SPIO-enhanced T2-weighted turbo spin-echo and T2*-weighted gradient-echo sequences were performed 48 hr after completion of the dynamic study. Three observers independently interpreted the images in random order, separately, and without patient identifiers. Diagnostic accuracy was evaluated using the alternative free response receiver operating characteristic method. Sensitivity and positive predictive value were also evaluated.

RESULTS

The mean sensitivity and positive predictive value of SPIO-enhanced imaging were 81.0% and 85.0%, respectively, and those of gadobenate dimeglumine-enhanced MRI were 91.4% and 88.1%, respectively. A significant difference was seen in the sensitivity of the two MRI examinations (p < 0.05). The mean value of the area under the ROC curve (A(z)) for gadobenate dimeglumine-enhanced imaging (A(z) = 0.97 +/- 0.01) was significantly higher than that for SPIO-enhanced imaging (A(z) = 0.90 +/- 0.02) (p = 0.004).

CONCLUSION

Gadobenate dimeglumine-enhanced 3D dynamic imaging showed better diagnostic performance than SPIO-enhanced imaging for the detection of hepatocellular carcinomas.

Evaluation of [Gd(Bz-TTDA)]2- as a potential contrast agent in MR imaging of the hepatobiliary system: An animal study

PURPOSE

To evaluate the potential of a new lipophilic paramagnetic complex [Gd(Bz-TTDA)]2- [(4s)-4-benzyl-3,6,10-tri(carboxymethyl)-3,6,10-triazadodecandioic acid]2- designed for use as a hepatobiliary MR contrast agent.

MATERIALS AND METHODS

MR imaging studies for normal and hepatocellular carcinoma (HCC) rat models were performed using a 1.5-T scanner. Sequential multislice T1-weighted turbo field echo (TFE) (TR/TE/flip angle: 15 msec/6.1 msec/25 degrees) coronal images of normal rats were obtained before and after intravenous injections of 0.1 mmol/kg [Gd(Bz-TTDA)]2- in study groups (N = 12) or 0.1 mmol/kg gadopentate dimeglumine (Gd-DTPA)2- in control groups (N = 12). Similar protocols of MR imaging with additional T2-weighted images were used for the rats with implanted HCC in both study and control groups (N = 12, in each group). MR images were analyzed to evaluate the time-enhancement change (% increase of signal-to-noise ratio [SI/N]) in normal liver, renal cortex, renal medulla, and tumors. The liver-lesion contrast-to-noise ratios (CNR) were also evaluated in study and control groups. The rats were killed immediately after the last MR scan to undergo autopsy and histopathologic observation. The acute toxicity test (medial lethal dose, LD50) in mice was also done.

RESULTS

The liver enhancement in normal rats reached a plateau 5-50 minutes after injection of [Gd(Bz-TTDA)]2-, maintained for three hours, then gradually declined. Intensity of enhancement in liver, renal cortex, and medulla after injection of [Gd(Bz-TTDA)]2- was significantly higher than with Gd-DTPA. The efficacy of tumor characterization with injection of [Gd(Bz-TTDA)]2- was similar to that of Gd-DTPA at the early dynamic phase of the contrast study. However, the liver-lesion CNRs were significantly higher in the study group in the later phase, when tumor enhancement decreased and liver enhancement persisted. The dose of LD50 in acute toxicity test of [Gd(Bz-TTDA)]2- in mice was 7.5 mmol/kg.

CONCLUSION

The preliminary results in this animal study indicated that [Gd(Bz-TTDA)]2- has the potential of becoming a reliable liver MR contrast agent.

Pseudohypocalcemia after magnetic resonance imaging with gadolinium in patients with cirrhosis

Hypocalcemia in patients with cirrhosis may be due to a number of causes. We noted a relationship between injection with gadodiamide, a particular gadolinium chelate, during magnetic resonance imaging of the liver and the development of a falsely low serum total calcium level in a patient with cirrhosis. A cross-reference and retrospective chart review identified 10 additional patients in whom this phenomenon was noted. We describe the temporal relationship and clinical characteristics of these patients. Pseudohypocalcemia following magnetic resonance imaging with gadodiamide contrast should be considered in the differential diagnosis of hypocalcemia in patients with cirrhosis.

Tissue-specific MR contrast agents

The purpose of this review is to outline recent trends in contrast agent development for magnetic resonance imaging. Up to now, small molecular weight gadolinium chelates are the workhorse in contrast enhanced MRI. These first generation MR contrast agents distribute into the intravascular and interstitial space, thus allowing the evaluation of physiological parameters, such as the status or existence of the blood-brain-barrier or the renal function. Shortly after the first clinical use of paramagnetic metallochelates in 1983, compounds were suggested for liver imaging and enhancing a cardiac infarct. Meanwhile, liver specific contrast agents based on gadolinium, manganese or iron become reality. Dedicated blood pool agents will be available within the next years. These gadolinium or iron agents will be beneficial for longer lasting MRA procedures, such as cardiac imaging. Contrast enhanced lymphography after interstitial or intravenous injection will be another major step forward in diagnostic imaging. Metastatic involvement will be seen either after the injection of ultrasmall superparamagnetic iron oxides or dedicated gadolinium chelates. The accumulation of both compound classes is triggered by an uptake into macrophages. It is likely that similar agents will augment MRI of atheriosclerotic plaques, a systemic inflammatory disease of the arterial wall. Thrombus-specific agents based on small gadolinium labeled peptides are on the horizon. It is very obvious that the future of cardiovascular MRI will benefit from the development of new paramagnetic and superparamagnetic substances. The expectations for new tumor-, pathology- or receptor-specific agents are high. However, is not likely that such a compound will be available for daily routine MRI within the next decade.