Contrast Agents for Liver MR Imaging

CONTENTS

3.1 Introduction

3.1.1 Non-Specific Gadolinium Chelates

3.1.2 Hepatocyte-Targeted Contrast Agents

3.1.3 Agents with Combined Extracellular and Hepatocyte-Specific Distribution

3.1.4 RES-Specific Contrast Agents

3.2 Injection Schemes for Liver MRI with Different Contrast Agents

3.3 Radiologic Classification of Focal Liver Lesions on Unenhanced and Contrast-Enhanced MRI

3.4 Summary

Introduction

The imaging evaluation of patients with suspected liver masses has three principal purposes:

• lesion detection,

• lesion characterization,

• evaluation of the intra- and extrahepatic extent of tumor.

All three of the major non-invasive imaging modalities have roles to play in liver imaging.

Ultrasound (US) has traditionally been used for the primary screening of patients with abdominal pain, but the advent of contrast-enhanced US techniques now makes it a highly capable methodology for both the characterization and improved detection of liver masses [60, 93]. Computed tomography (CT) has generally been considered the imaging approach of choice for the detection of liver masses principally because of the ease of performing and interpreting large numbers of examinations, its widespread availability, and its generally acknowledged superior ability to evaluate the ex tra-hepatic abdomen [15]. However, until the advent of multi-detector CT (MDCT), the value of conventional CT for the characterization of focal liver lesions was generally considered to be inferior to that of contrast-enhanced magnetic resonance imaging (MRI) [123]. With the recent developments in MDCT technology, particularly the emergence of 16- and 64-row scanners combined with highly concentrated iodinated contrast agents, the impact of CT in both detection, and to a lesser extent, characterization of focal liver lesions has markedly improved [30, 61,117]. In particular, CT has substantially shorter acquisition times, the possibility to acquire thin sections on a routine basis within a single breath-hold, the opportunity to retrospectively calculate thinner or thicker sections from the same raw data, and improved 3D-postpro-cessing techniques. These features now permit the acquisition of similar diagnostic information to that attainable on contrast-enhanced MRI with conventional extracellular gadolinium contrast agents, where information derives from differential blood flow between the tumor and surrounding normal liver parenchyma. However, unlike the situation in MRI, there are as yet no hepatospecific contrast agents available for CT. Hence, additional information based on the functionality of liver lesions, which is attainable with MRI, is not yet attainable with MDCT. Moreover, concern over the nephrotoxicity of certain iodinated contrast agents [128] and the requisite use of ionizing radiation in CT examinations are factors which should always be considered when referring patients for diagnostic evaluation of the liver.

In contrast to the situation in CT, several different classes of contrast agent are available for routine clinical use in MRI of the liver [39,43,70,105, 122]. These include non-specific materials that distribute extracellularly in a manner similar to that of the iodinated agents used in CT, materials that are taken up specifically by hepatocytes and ex creted in part through the biliary system, and materials that are targeted specifically to the Kupffer cells of the reticuloendothelial system (RES) (Tables 1, 2). The differential use of these agents, depending on the clinical purpose, can maximize the diagnostic information available to the investigating radiologist. This chapter describes the properties and indications for each category of contrast agent for MRI of the liver.

Non-Specific Gadolinium Chelates

Chelates of the paramagnetic gadolinium ion that distribute solely to the extracellular space (i.e. do not have any tissue-specific biodistribution) have been commercially available since 1986 [113, 136,

147]. The four non-specific gadoliniumchelates approved in the USA are gadopentetate dimeglumine (Magnevist®, Gd-DTPA; Berlex Laboratories/Schering AG), gadoteridol (ProHance®, Gd-HP-DO3A; Bracco Diagnostics), gadodiamide (Omniscan®, Gd-DTPA-BMA; GE Healthcare), and gadoversetamide (Optimark®, Gd-DTPA-BMEA; Mallinckrodt). Other non-specific gadolinium agents currently approved only in Europe include gadoterate meglumine (Dotarem®, Gd-DOTA; Guerbet) and gadobutrol (Gadovist®, Gd-BT-DO3A; Schering AG) (Table 2). Although the safety profiles of these agents are all extremely attractive, especially in comparison to io-dinated x-ray contrast agents [39, 54,70,79, 80, 82, 105, 107, 127, 132], possible problems associated with the least stable of these agents (gadodiamide and gadoversetamide) [54] have recently come to light. Specifically, both gadodiamide and gadoverse-

Table 1.

Contrast agent type

Manufacturer

Principal Mechanism

Extracellular Gd agents

Gadopentetate dimeglumine; Gd-DTPA (Magnevist®)

Schering1 / Berlex2

T1 shortening

Gadoteridol;

Gd-HP-DO3A (ProHance*)

Bracco3,4

T1 shortening

Gadodiamide;

Gd-DTPA-BMA (Omniscan*)

GE Healthcare5

T1 shortening

Gadoversetamide; Gd-DTPA-BMEA (OptiMARK®)

Tyco Healthcare6

T1 shortening

Gadoterate meglumine; Gd-DOTA (Dotarem®)

Guerbet7

T1 shortening

Gadobutrol; Gd-BT-DO3A (Gadovist®)

Schering1

T1 shortening

Hepatobiliary agent

Mangafodipir trisodium; Mn-DPDP (Teslascan®)

GE Healthcare5

T1 shortening

Combined extracellular / hepatobiliary agents

Gadobenate dimeglumine; Gd-BOPTA (MultiHance®)

Bracco3,4

T1 shortening

Gadoxetate;

Gd-EOB-DTPA (Primovist®)

Schering1 / Berlex2

T1 shortening

SPIO agent

Ferumoxides (Feridex®; Endorem®)

Berlex8 / Guerbet7

T2 shortening

USPIO agents

SH U 555 A (Resovist®)

Schering1

T1 & T2 shortening

AMI-227 (Combidex®; Sinerem®)

Advanced Magnetics8/ Guerbet7

T1 & T2 shortening

1 = Berlin, Germany; 2 = Wayne NJ, USA; 3 = Milano, Italy; 4 = Princeton NJ, USA; 5 = Chalfont St. Giles, UK; 6 = St. Louis MO, USA; 7 = Aulnay-Sous-Bois, France; 8 = Cambridge MA, USA

1 = Berlin, Germany; 2 = Wayne NJ, USA; 3 = Milano, Italy; 4 = Princeton NJ, USA; 5 = Chalfont St. Giles, UK; 6 = St. Louis MO, USA; 7 = Aulnay-Sous-Bois, France; 8 = Cambridge MA, USA

Table 2. Physicochemical characteristics of commercially-available T1 -shortening MR contrast agents

Characteristic

Extracellular agents

Combined extracellular / hepatobiliary agents

Hepatobiliary agent

Magnevist

ProHance

Omniscan

OptiMARK

Dotarem

Gadovist

MultiHance

Primovist

Teslascan

(0.5 mol/L)

(0.5 mol/L)

(0.5 mol/L)

(0.5 mol/L)

(0.5 mol/L)

(1.0 mol/L)

(0.5 mol/L)

(0.25 mol/L)

(0.05 mol/L)

Molecular structure

Linear,

Cyclic,

Linear,

Linear,

Cyclic,

Cyclic,

Linear,

Linear,

Not applicable

ionic

non-ionic

non-ionic

non-ionic

ionic

non-ionic

ionic

ionic

Thermodynamic stability

22.1

23.8

16.9

16.6

25.8

21.8

22.6

23.4

Not applicable

constant (log Keq)

Conditional stability

18.1

17.1

14.9

15.0

18.8

N/A

18.4

18.7

Not applicable

constant at pH 7.4

Osmolality (Osm/kg)

1.96

0.63

0.65

1.11

1.35

1.6

1.97

0.69

0.30

Viscosity (mPa«s at 37°C)

2.9

1.3

1.4

2.0

2.0

4.96

5.3

1.19

0.8

rl relaxivity 0.2 T

4.7a

NA

NA

NA

NA

5.5a

10.9a

NA

NA

in plasma 0.47 T

4.8", 3.8C, 4.9"

4.9\4.8e

4.4b, 4.4e

5.7e

4.3e

5.6\ 6.1e

9.2e, 9.7"

8.7e, 8.7e

3.6e

(L/mmohs-1) 1.5 T

3.9a, 4.1c

4.1e

4.3e

4.7e

3.6e

4.7a, 5.2e

8.1% 6.3e

6.9e

3.6e

3 T

3.3a, 3.7e

3.7e

4.0e

4.5e

3.5e

3.6a, 5.0e

6.3a, 5.5e

6.2e

2.7e

r2 relaxivity 0.2 T

9.6'

NA

NA

NA

NA

10.1a

18.9a

NA

NA

in plasma 0.47 T

4.1e, 6.3d

6.1e

4.6e

5.7e

5.5e

7.4e

12.9e, 12.5"

13.0e, 8.7e

4.3e

(L/mmohs-1) 1.5 T

5.3a, 4.6e

5.0e

5.2e

6.6e

4.3e

6.8a, 6.1e

18.7a, 8.7e

8.7e

7.1e

3 T

5.2a, 5.2e

5.7e

5.6e

5.9e

4.9e

6.3a, 7.1e

17.5a, 11.0e

11.0e

9.3e

NA = not available ; a = Pintaske et al [88].; b = Vogler et al [141]; c = Rohrer et al.[100]; d = De Haen et al.[20], e = Schuhmann-Giampieri [119]

n.b. rl and r2 relaxivity values from Rohrer et al. were determined from curves of relaxation rate versus concentration for just two concentrations of contrast agent and hence are subject to variability. Values from Pintaske et al. were determined for a minimum of eight contrast agent concentrations.

ai IQ

Ln Ln tamide, but none of the other approved gadolinium agents, have been shown to cause spurious hypocal-cemia as a result of interference with laboratory tests for serum calcium [25,54,71,90,91]. The issue of gadolinium chelate stability in the case of gado-diamide has also been raised in a study to determine the extent to which gadolinium is deposited in bone following intravenous injection of this agent [33].

As paramagnetic compounds, gadolinium chelates shorten T1 tissue relaxation times when injected intravenously. At recommended doses of 0.1-0.3 mmol/kg their principal effect is to shorten the T1 relaxation time resulting in an increase in tissue signal intensity. This effect is best captured on heavily T1-weighted images [64,69,124]. Due to rapid redistribution of gadolinium chelates from the intravascular compartment to the extracellular space, the contrast agents must be administered as a rapid bolus, typically at 2-3 ml/sec. Thereafter, imaging of the entire liver is performed in a single breath-hold during the dynamic phase of contrast enhancement. This is most commonly undertaken with a 2D or 3D T1-weighted spoiledGRE sequence with serial imaging in the arterial dominant phase (25-30 sec post-injection), the portal-venous phase (60-80 sec post-injection), and the equilibrium phase (3-5 min post-injection).

A schematic representation of the enhancement behavior seen after the bolus injection of non-specific extracellular gadolinium chelates is shown in Fig. 1. In the hepatic arterial dominant phase, enhancementoccurs principally in the arterial tree and mainly in arterially-perfused tissues and tumors [22,72,81,148,150]. This is important since most focal lesions, especially primary liver tumors like hepatocellular carcinoma (HCC) and metastases are supplied primarily via the hepatic arteries [41, 72, 148]. Enhancement during the arterial dominant phase is important in order to detect perfusion abnormalities. For example, transient increased segmental enhancement in liver segments may indicate that portal-venous flow is compromised due to compression or thrombosis [114, 152]. This can be of value in patients where findings on unenhanced images are equivocal.

Typically, maximal enhancement of the hepatic parenchyma is seen in the portal-venous phase. In this phase, hypovascular lesions such as cysts, hy-povascular metastases and scar tissue are most clearly revealed as regions of absent or diminished enhancement [41]. Patency or thrombosis of hepatic vessels is also bestshown during this phase.

Enhancement of tissues with enlarged extracellular spaces, such as focal liver lesions and scars of focal nodular hyperplasia (FNH), is usually best seen in the equilibrium phase. Likewise, typical signs of malignancy, such as peripheral wash-out in colorectal metastases, are best seen during the equilibrium phase. Such features frequently contribute to accurate lesion characterization [73, 74].

Imaging with gadolinium during the arterial phase has been shown to improve the rate of detection of suspected HCC in cirrhotic patients compared to unenhanced imaging [81, 150, 152]. For lesion characterization, characteristic enhancement patterns have been identified for a variety of benign and malignant masses (Figs. 2,3) of both hepatocellular and non-hepatocellular origin [26,41,42,72-74,148].

Pharmacokinetic Model Spleen Contrast
Fig 1. Enhancement scheme of the liver after injection of extracellular gadolinium chelates. Extracellular Gadolinium chelates initially distribute to the intravascular space and then rapidly filter into the extracellular space of normal tissue

Fig. 2a-f. Hemangioma. A large hypoechoic mass noted in the liver on ultrasound (a) is seen as hyperintense on the T2-weighted MR image (b). With dynamic Tl-weighted imaging following the bolus administration of Gd-DTPA, the lesion demonstrates intense peripheral nodular enhancement with progressive filling-in on the arterial (c), early and late portal-venous (d and e, respectively) and equilibrium (f) phase images. The enhancement pattern is characteristic of a benign hemangioma

Fig. 2a-f. Hemangioma. A large hypoechoic mass noted in the liver on ultrasound (a) is seen as hyperintense on the T2-weighted MR image (b). With dynamic Tl-weighted imaging following the bolus administration of Gd-DTPA, the lesion demonstrates intense peripheral nodular enhancement with progressive filling-in on the arterial (c), early and late portal-venous (d and e, respectively) and equilibrium (f) phase images. The enhancement pattern is characteristic of a benign hemangioma

Fig. 3a-d. Focal nodular hyperplasia. A large well-defined mass in the liver is seen as hyperintense in comparison to the normal background parenchyma on the T2-weighted fast spin-echo image (a). A bright central scar (arrow) is also evident. Dynamic Tl-weighted imaging following the administration of Gd-DTPA reveals that the lesion shows intense enhancement during the arterial phase (b) followed by rapid wash-out during the portal-venous (c) phase. The central scar is seen as hypointense on the arterial phase image and as hyperintense on the equilibrium phase image (d) (arrow)

¡v

c

kp y 1 *

Fig. 3a-d. Focal nodular hyperplasia. A large well-defined mass in the liver is seen as hyperintense in comparison to the normal background parenchyma on the T2-weighted fast spin-echo image (a). A bright central scar (arrow) is also evident. Dynamic Tl-weighted imaging following the administration of Gd-DTPA reveals that the lesion shows intense enhancement during the arterial phase (b) followed by rapid wash-out during the portal-venous (c) phase. The central scar is seen as hypointense on the arterial phase image and as hyperintense on the equilibrium phase image (d) (arrow)

Hepatocyte-Targeted Contrast Agents

Hepatocyte-selective contrast agents are taken up by hepatocytes and are eliminated, at least in part, through the biliary system. A prototypical, dedicated hepatocyte-selective contrast agent is man-gafodipir trisodium (Teslascan®, Mn-DPDP; GE Healthcare) which was approved for clinical use in 1997 [23,42,70,98,105,122,143].As with gadolinium chelates, mangafodipir trisodium is considered to have an acceptable safety profile although injection-related minor adverse events such as flushing, nausea and dizziness are relatively common [5, 27, 67, 98]. Moreover, Mn-DPDP dissociates rapidly following administration to yield free Mn++ ions [31]) which, in patients with hepatic impairment, may be associated with increased neurological risk [44,77].

Like the gadolinium agents, mangafodipir trisodium is a paramagnetic contrast agent and primarily affects T1 relaxation times [135]. The increased signal intensity generated in functioning hepatocytes improves the contrast against non-enhancing tissues on T1-weighted images [5, 9, 27, 78,108,137].A schematic representation of the enhancement behavior seen after administration of hepatobiliary agents such as mangafodipir trisodi-um is shown in Fig. 4.

This agent is administered as a slow intravenous infusion over 1-2 min, which unfortunately precludes dynamic phase imaging in the manner performed with gadolinium-based agents [5, 47]. Moreover, because the 5-10 mmol/kg dose of man-gafodipir is 10% or less than that of the gadolinium agents, imaging with mangafodipir during its distribution phase in the extracellular fluid compartment does not contribute to diagnosis. Doses above 10 mmol/kg also do not contribute any additional enhancement [5,143]. Liver enhancement is maximal within 10 min of mangafodipir trisodi-um infusion and persists for several hours. Since dynamic images are not acquired with this agent, any T1-weighted sequences can be used. Fat saturation has been shown to improve contrast [99, 108,137]. More importantly, higher spatial resolu-

drip infusion

Mn-DPDP

Delayed hepatobiliary evaluation

Fig. 4. Hepatobiliary agents such as mangafodipir trisodium (Mn-DPDP, Teslascan®) are taken up by hepatocytes and imaging is performed during a delayed hepatobiliary phase tion imaging can be used effectively even if the entire liver cannot be covered in one data acquisition. On state of the art scanners, a useful sequence would be a 2D or 3D spoiled GRE sequence with a matrix size of 512/256 x 512 (Fig. 5).

Because liver enhancement in patients with cirrhosis is limited with mangafodipir trisodium [69], liver lesion detection on mangafodipir-en-hanced MR imaging is primarily effective in patients with normal liver parenchyma. In these patients, non-hepatocellular focal lesions generally appear hypointense compared to the normal liver on post-contrast T1-weighted images [6,144].

Several studies have shown a benefit for liver lesion detection with mangafodipir-enhanced hepatic MR imaging compared with unenhanced MRI [5,6,27,137,143,144]. Moreover, since hepa tocellular lesions such as FNH, hepatic adenoma and HCC generally enhance with mangafodipir, it is frequently possible to differentiate lesions of hepatocellular origin from lesions of non-hepatocel-lular origin [78, 83, 102]. Unfortunately, because mangafodipir often causes the enhancement of both benign and malignant lesions of hepatocellu-lar origin, it is not always possible to differentiate between benign and malignant lesions [16]. In a study of 77 patients with histologically-confirmed diagnoses, the sensitivity and specificity of man-gafodipir-enhanced MRI for the differentiation of histologically-confirmed malignant versus benign lesions was 91% and 67%, respectively, while that for the differentiation of hepatocellular versus non-hepatocellular lesions was 91% and 85%, respectively [83]. Enhancement of both benign and

Fig. 5a, b. Detection of liver metastasis with mangafodipir trisodium in a young male patient with primary colorectal cancer. Post-man-gafodipir-enhanced images obtained with standard (128 x 256) resolution (a) and high (256 x 512) resolution (b) are shown. Compared to the routine T1-weighted gradient-echo image more lesions (arrows, circle) are seen with the high resolution technique

Fig. 5a, b. Detection of liver metastasis with mangafodipir trisodium in a young male patient with primary colorectal cancer. Post-man-gafodipir-enhanced images obtained with standard (128 x 256) resolution (a) and high (256 x 512) resolution (b) are shown. Compared to the routine T1-weighted gradient-echo image more lesions (arrows, circle) are seen with the high resolution technique

Fig. 6a, b. Mangafodipir-enhanced MRI of the pancreas in a middle-aged man with a history of sarcoma. Compared to the pre-contrast T1-weighted image (a) strong enhancement of the pancreas is seen on delayed images after the administration of mangafodipir (b). Multiple metastatic deposits in the pancreas (whitearrows) are much better appreciated on the T1-weighted fat-suppressed post-contrast image. Additionally, the conspicuity of the lesions in the liver is improved (blackarrow)

Fig. 6a, b. Mangafodipir-enhanced MRI of the pancreas in a middle-aged man with a history of sarcoma. Compared to the pre-contrast T1-weighted image (a) strong enhancement of the pancreas is seen on delayed images after the administration of mangafodipir (b). Multiple metastatic deposits in the pancreas (whitearrows) are much better appreciated on the T1-weighted fat-suppressed post-contrast image. Additionally, the conspicuity of the lesions in the liver is improved (blackarrow)

malignant hepatocellular neoplasms limits the usefulness of this agent for the accurate differentiation of hepatocellular lesions and this, combined with the frequent need for delayed imaging at 4-24 hrs post-contrast [102], represents the principal shortcoming of this agent [9,16,78,83,99] .

Apart from the inability to adequately differentiate benign from malignant lesions of hepatocel-lular origin, a further potential limitation of man-gafodipir-enhanced liver MRI appears to be inadequate characterization of non-hepatocellular lesions. Common benign tumors such as heman-giomas and cysts, as well as non-neoplastic masses such as focal fatty infiltration and focal fat sparing may mimic malignancy in patients with known or suspected cancer. In these settings Gd-chelate-en-hanced dynamic multiphase MRI is invaluable for satisfactory lesion characterization.

Although mangafodipir trisodium is primarily considered an agent for MRI of the liver, a number of early studies demonstrated a potential usefulness for imaging of the pancreas (Fig. 6) as well [32,68,76].

Moreover, since the Mn++ ion is excreted in part through the biliary system, mangafodipir trisodium may prove effective for biliary tract imaging [65].

Agents with Combined Extracellular and Hepatocyte-Specific Distribution

In 1998, gadobenate dimeglumine (MultiHance®, Gd-BOPTA; Bracco Imaging SpA) became available in Europe for MRI of the liver. Today, gadobenate dimeglumine is approved in Europe and other parts of the world for MRI of the liver, and in the United States, Europe and other parts of the world for MRI of the central nervous system and related tissues [2, 17,18,19,58,103,115]. It is also under development for other indications including MR angiography and MRI of the breast and heart [3,14,55,57,59,62, 86,92,111,112,115,142,149].

Gadobenate dimeglumine differs from the purely extracellular gadolinium agents as it combines the properties of a conventional non-specific gadolinium agent with those of an agent targeted specifically to hepatocytes [2, 52, 98]. With this agent it is possible to perform both dynamic phase imaging as performed with conventional gadolinium-based agents, and delayed phase imaging as performed with mangafodipir trisodium [37, 49, 50, 51, 87, 89]. Thus, arterial, portal-venous and equilibrium phase images are readily attainable using identical sequences to those employed with the conventional non-specific gadolinium agents [ 116]. Unlike the conventional agents, however, approximately 3-5% of the injected dose of gadobenate dimeglu-mine is taken up by functioning hepatocytes and ultimately excreted via the biliary system [129]. As with mangafodipir, a result of the hepatocytic uptake is that the normal liver parenchyma shows strong enhancement on delayed T1-weighted images that is maximal approximately 1 hr after administration [11,129,130].

A schematic representation of the enhancement behavior seen after administration of dual agents such as gadobenate dimeglumine is shown in Fig. 7.

-

pr

A

Gd-BOPTA

Fig. 7. Combined extracellular/hepatobiliary agents such as Gd-BOPTA and Gd-EOB-DTPA distribute initially to the intravascular and ex-travascular spaces and then, like Mn-DPDP, are taken up by hepatocytes via a specific transporter

Fnh Like Lesion

Fig. 8a-e. Characterization of FNH with Gd-BOPTA. On the unenhanced T2-weighted image the lesion (arrows) appears slightly hyperintense against the surrounding normal parenchyma. A focus of slight hyperintensity (arrowhead) is indicative of a central scar. The corresponding unenhanced T1-weighted image (b) reveals a slightly hypointense lesion with a markedly hypointense central area (arrowhead corresponding to the scar. The enhancement pattern observed following the bolus injection of Gd-BOPTA is typical of that observed for FNH after the administration of conventional gadolinium agents, i.e. rapid homogenous enhancement during the arterial phase (c) followed by rapid wash-out during the portal-venous phase (d). The T1-weighted image acquired during the delayed hepatobiliary phase (e) reveals a lesion that is isointense against the surrounding normal parenchyma. This indicates that the lesion contains functioning hepatocytes that are able to take up Gd-BOPTA in the same way as normal hepatocytes. The central scar is seen as hypointense on the delayed image

Fig. 8a-e. Characterization of FNH with Gd-BOPTA. On the unenhanced T2-weighted image the lesion (arrows) appears slightly hyperintense against the surrounding normal parenchyma. A focus of slight hyperintensity (arrowhead) is indicative of a central scar. The corresponding unenhanced T1-weighted image (b) reveals a slightly hypointense lesion with a markedly hypointense central area (arrowhead corresponding to the scar. The enhancement pattern observed following the bolus injection of Gd-BOPTA is typical of that observed for FNH after the administration of conventional gadolinium agents, i.e. rapid homogenous enhancement during the arterial phase (c) followed by rapid wash-out during the portal-venous phase (d). The T1-weighted image acquired during the delayed hepatobiliary phase (e) reveals a lesion that is isointense against the surrounding normal parenchyma. This indicates that the lesion contains functioning hepatocytes that are able to take up Gd-BOPTA in the same way as normal hepatocytes. The central scar is seen as hypointense on the delayed image

A second feature unique to gadobenate dimeg-lumine is that the contrast-effective moiety of this agent interacts weakly and transiently with serum albumin [12,20]. This interaction slows the tumbling rate of the Gd-BOPTA chelate and results in a longer rotational correlation time with inner shell water protons for Gd-BOPTA compared to gadolinium agents that do not interact with serum albumin. This in turn results in a T1 relaxivity in human plasma that is approximately twice that of the conventional gadolinium agents [20, 88] (Table 2). Not only does this increased relaxivity permit lower overall doses to be used to acquire the same information in the dynamic phase as available with conventional agents at a standard dose of 0.1 mmol/kg [116], it also facilitates the improved performance of gadobenate dimeglu-mine for both intra- and extrahepatic vascular imaging [57,92].

A principal advantage of the selective uptake by functioning hepatocytes is that the normal liver enhances, while tumors of non-hepatocytic origin, such as metastases (Fig. 9) and cholangiocellular carcinoma (Fig. 10), as well as non-functioning he-patocytic tumors that are unable to take up Gd-BOPTA (Fig. 11), remain unenhanced, thereby increasing the liver-lesion contrast-to-noise ratio (CNR) and hence the ability to detect lesions [11, 89,104,129].

Imaging is typically performed with 2D or 3D T1-weighted gRe sequences while the use of fat saturation has been shown to improve CNR on delayed, hepatobiliary phase images. In the delayed hepatobiliary phase, high-resolution imaging is recommended.

Clinical studies and routine clinical practice have shown that dynamic phase imaging is particularly important for lesion characterization (Fig. 12), while delayed phase imaging in the hepatobiliary phase increases the sensitivity of MRI for liver lesion detection [11, 87, 89]. However, delayed phase imaging can also contribute to the improved characterization of lesions, particularly when the results of unenhanced and dynamic imaging are equivocal or when atypical enhancement patterns are noted on dynamic imaging [35,36]. A particularly interesting and clinically important finding concerning the characterization of lesions on delayed hepatobiliary phase imaging after gadobe-nate dimeglumine administration is that focal nodular hyperplasia can be accurately differentiated from hepatic adenoma, thereby eliminating the need for lesion biopsy [37].

In addition to the hepatic imaging capability of this agent, its partial biliary excretion also facilitates its use for biliary tract imaging (Fig. 13), while the increased relaxivity deriving from weak protein interaction may prove beneficial for hepatic MR angiography (Fig. 14). Both of these features have proven advantageous for the pre-operative evaluation of potential liver donors in transplant surgery [34, 66]. Finally, preliminary studies have already indicated its potential for MR colonography [56].

A second agent with combined extracellular and hepatobiliary properties is gadolinium ethoxybenzyldiethylenetriaminepentaacetic acid (Primovist®, Gd-EOB-DTPA; Schering AG, Germany) which has recently been approved for use in Europe, albeit at a formulation of only 0.25 mol/L and at an approved dose of only 0.025 mmol/kg bodyweight [40, 97, 140] (Table 2). Like Gd-BOP-TA, this agent has a higher T1 relaxivity compared to the conventional extracellular agents [97] and distributes initially to the vascular and interstitial compartment after bolus injection. However, whereas only 3-5% of the injected dose of Gd-BOPTA is taken up by hepatocytes and eliminated in the bile, in the case of Gd-EOB-DTPA, 50% of the injected dose is taken up and eliminated via the hepatobiliary pathway after approximately 60 min [40, 118]. The maximum increase of liver parenchyma signal intensity is observed approximately 20 min after injection and lasts for approximately 2 hrs [40,106,118,119,120,140].

As with Gd-BOPTA, the dynamic enhancement patterns seen during the perfusion phase after injection of Gd-EOB-DTPA are similar to those seen with Gd-DTPA. During the hepatobiliary phase, Gd-EOB-DTPA-enhanced images have been shown to significantly improve the detection rate of metastases, HCC, and hemangiomas (Figs. 15, 16), compared with unenhanced and Gd-DTPA-enhanced images [45, 46, 97, 140]. Moreover, Gd-EOB-DTPA may also be a suitable agent for biliary imaging [10].

Like Gd-BOPTA [53], Gd-EOB-DTPA has a safety profile that is not dissimilar from those of the conventional extracellular gadolinium agents [8,40,97].

RES-Specific Contrast Agents

Iron oxide particulate agents are selectively taken up by Kupffercells of the reticulo-endothelial system (RES), primarily in the liver [39,121],but also in the spleen and the bone marrow. Iron oxide particles of different sizes have been developed which are referred to as superparamagnetic iron oxides (SPIO, mean size > 50 nm) and ultrasmall superparamagnetic iron oxides (USPIO, mean particle size < 50 nm). Of the various formulations, two have so far been developed clinically for MR imaging: ferumoxides (Feridex®, Berlex Laboratories and Endorem®, Laboratoire Guerbet) which has particles ranging between 50 and 180 nm and SH U 555 A (Resovist®, Schering AG) which has

And Spio Mri

Fig. 9a-f. Characterization of hypovascular metastasis with Gd-BOPTA. Unenhanced T2-weighted and T1-weighted images (a and b, respectively) both reveal a lesion that is hypointense against the normal liver parenchyma (arrowin a). The lesion remains hypointense with a hyperintense peripheral rim on arterial (arrowin c), portal-venous (d) and equilibrium (e) phase images acquired after the administration of Gd-BOPTA. The hyperintense appearance of the rim is due to the presence of peripheral edema. On the T1-weighted image acquired during the delayed hepatobiliary phase (f) the lesion is still hypointense against a strongly enhanced surrounding normal parenchyma. This indicates that the lesion does not take up Gd-BOPTA and is therefore malignant in nature

Fig. 9a-f. Characterization of hypovascular metastasis with Gd-BOPTA. Unenhanced T2-weighted and T1-weighted images (a and b, respectively) both reveal a lesion that is hypointense against the normal liver parenchyma (arrowin a). The lesion remains hypointense with a hyperintense peripheral rim on arterial (arrowin c), portal-venous (d) and equilibrium (e) phase images acquired after the administration of Gd-BOPTA. The hyperintense appearance of the rim is due to the presence of peripheral edema. On the T1-weighted image acquired during the delayed hepatobiliary phase (f) the lesion is still hypointense against a strongly enhanced surrounding normal parenchyma. This indicates that the lesion does not take up Gd-BOPTA and is therefore malignant in nature

Fig. 10a-f. Characterization of cholangiocellular carcinoma with Gd-BOPTA. The unenhanced T2-weighted image (a) reveals a liver of low signal intensity in which a faint area of high signal intensity can be seen in the right lobe (arrows). On the unenhanced Tl-weighted image (b) a marked area of hypointensity is apparent. In addition, capsular retraction is evident (arrow). The lesion retains an initial hypointense appearance on the Tl-weighted arterial phase image acquired after the bolus administration of Gd-BOPTA (c), but thereafter demonstrates progressive delayed heterogeneous enhancement during the portal-venous and equilibrium phase images (d and e, respectively). On the Tl-weighted image acquired during the delayed hepatobiliary phase (f) the lesion is again hypointense compared to surrounding normal parenchyma indicating that the lesion is malignant in nature

Fig. 10a-f. Characterization of cholangiocellular carcinoma with Gd-BOPTA. The unenhanced T2-weighted image (a) reveals a liver of low signal intensity in which a faint area of high signal intensity can be seen in the right lobe (arrows). On the unenhanced Tl-weighted image (b) a marked area of hypointensity is apparent. In addition, capsular retraction is evident (arrow). The lesion retains an initial hypointense appearance on the Tl-weighted arterial phase image acquired after the bolus administration of Gd-BOPTA (c), but thereafter demonstrates progressive delayed heterogeneous enhancement during the portal-venous and equilibrium phase images (d and e, respectively). On the Tl-weighted image acquired during the delayed hepatobiliary phase (f) the lesion is again hypointense compared to surrounding normal parenchyma indicating that the lesion is malignant in nature

Fig. 11a-g. Characterization of hepatocellular carcinoma with Gd-BOPTA. The unenhanced T2-weighted and Tl-weighted in-phase images (a and b, respectively) reveal a lesion (arrows in a) that is essentially isointense with the normal liver parenchyma. The Tl-weighted opposed phase image (c) reveals an isointense lesion with faintly hyperintense margins. The lesion demonstrates marked hyperintensity during the arterial phase after the bolus administration of Gd-BOPTA (d), but thereafter is seen as homogeneously hypointense on portal-venous (e) and equilibrium (f) phase images due to the rapid wash-out of contrast agent. The lesion retains its hypointense appearance on the Tl-weighted image acquired during the delayed hepatobiliary phase (g) indicating that the lesion is malignant in nature

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Fig. 12a-f. Characterization of capillary hemangioma with Gd-BOPTA. A markedly hyperintense lesion (arrowhead) on the unenhanced T2-weighted image (a) is seen as hypointense on the unenhanced Tl-weighted in-phase and opposed phase images (b and c, respectively, arrowheads). The lesion demonstrates marked hyperintensity during the arterial phase after the bolus administration of Gd-BOPTA (d) and retains this hyperintense appearance on the subsequent portal-venous (e) and equilibrium (f) phase images. The persistent hyperintense appearance on the equilibrium phase image is an indication of the benign nature of the lesion

Fig. 12a-f. Characterization of capillary hemangioma with Gd-BOPTA. A markedly hyperintense lesion (arrowhead) on the unenhanced T2-weighted image (a) is seen as hypointense on the unenhanced Tl-weighted in-phase and opposed phase images (b and c, respectively, arrowheads). The lesion demonstrates marked hyperintensity during the arterial phase after the bolus administration of Gd-BOPTA (d) and retains this hyperintense appearance on the subsequent portal-venous (e) and equilibrium (f) phase images. The persistent hyperintense appearance on the equilibrium phase image is an indication of the benign nature of the lesion

Fig. 13a, b. The Tl-weighted fat-suppressed image (a) in the hepatobiliary phase after injection of Gd-BOPTA depicts multiple hypointense liver metastases (arrows). In addition, the bile ducts (arrowheads) are shown with a high SI due to the hepatobiliary excretion of the contrast agent. This high SI of the bile ducts allows for Tl-weighted 3D-GRE imaging with calculation of MIP images (b) from the 3D raw data. Continuity and normal functionality of the bile duct structures as far as the papillary region is demonstrated due to excretion of the contrast agent with the bile. For bile duct imaging it is advisable to inject a dose of 0.1 mmol/kg bodyweight Gd-BOPTA and to perform imaging approximately 1.5-2 hours post-contrast agent injection to reduce the signal from the background

Fig. 15a-c. Hemangioma after Gd-EOB-DTPA. A hypointense lesion on the unenhanced Tl-weighted image (a) demonstrates the enhancement pattern (peripheral nodular enhancement with progressive filling-in) typical of hemangioma on Tl-weighted images acquired during the arterial (b) and portal-venous (c) phases after the administration of Gd-EOB-DTPA

Fig. 15a-c. Hemangioma after Gd-EOB-DTPA. A hypointense lesion on the unenhanced Tl-weighted image (a) demonstrates the enhancement pattern (peripheral nodular enhancement with progressive filling-in) typical of hemangioma on Tl-weighted images acquired during the arterial (b) and portal-venous (c) phases after the administration of Gd-EOB-DTPA

Fig. 16a-c. Lesion detection and characterization with Gd-EOB-DTPA-enhanced MRI in a middle-aged man with carcinoid tumor. Compared to the pre-contrast image (a), the early arterial phase image acquired after Gd-EOB-DTPA injection (b) shows uniform arterial phase enhancement of a large liver lesion. On the delayed phase image acquired 20 min after Gd-EOB-DTPA administration (c), the normal liver parenchyma is strongly enhanced due to uptake of the contrast agent by functioning hepatocytes. The lesion-to-liver contrast (conspicuity) is greatly improved due to the inability of the lesion to take up Gd-EOB-DTPA

particles ranging between 45 and 60 nm. The safety profiles of these agents are less attractive than those of the paramagnetic contrast agents: although serious adverse events are rare, with En-dorem® approximately 3% of patients experience severe back pain while the contrast agent is being administered [4,101].

The principal superparamagnetic effect of the larger SPIO particles is on T2 relaxation and thus MRI is usually performed using T2-weighted sequences in which the tissue signal loss is due to the susceptibility effectsof iron [28,29,94,96] (Fig. 17). Enhancement on Tl-weighted images can also be seen [84] although this tends to be greater for the smaller SPIO and especially for the USPIO formulations [95]. Since there is an overall decrease in liver signal intensity, T2-weighted imaging with SPIO agents requires excellent imaging techniques that are free of motion artifacts. Typically, moderate T2-weighting (TE of approximately 60-80 msec) is adequate for optimizing lesion-liver contrast. Since the larger SPIO agents need to be administered by slow infusion to reduce side effects, for these agents imaging is generally performed some 20-30 min after administration [21,101,125]. Thus, scanning speed is not important and both fast breath-hold and conventional SE imaging can be employed. Pulse sequences that are sensitive to magnetic field heterogeneity tend to be sensitive to the presence of iron oxide. T2*-weighted gradient echo images are very sensitive to SPIO agents [21, 28, 29, 84, 101]. T2-weighted spin-echo sequences are more sensitive than T2-weighted fast (turbo) spin-echo sequences, because the multiple rephasing pulses used in the latter tend to obscure signal losses arising from local variations in the magnetic environment [21]. Administration protocols vary but typically precon-trast T1- and T2-weighted imaging is followed by post-contrast T2-weighted imaging. Schematic representations of the enhancement behavior seen after administration of SPIO and USPIO agents are shown in Figs. 18 and 19.

Since SPIO particles are removed by the RES, the application of these agents is similar to the use of Tc-sulfur colloid in nuclear scintigraphy. Lesions that contain negligible or no Kupffer cells remain largely unchanged, while the signal intensity of the normal liver is reduced on T2-weighted images. As a result the CNR between the normal liver parenchyma and focal liver lesion is increased [21, 29,84,96,101].

Many well-controlled studies using surgical pathology or intraoperative ultrasound (IOUS) as gold standards have supported the efficacy of SPIO-enhanced MRI [21, 29, 84, 96, 101]. For example, an early multi-center Phase III study showed more lesions in 27% of cases than unen-hanced MR and in 40% of cases compared to CT [101]. On the other hand, other early studies were not able to demonstrate a significant benefit over

Fig. 17a, b. On the unenhanced T2*-weighted image (a) a liver metastasis (arrow) from breast cancer is shown with a slightly increased signal intensity. A drop of liver signal intensity is noted on the corresponding T2*-weighted image acquired after the administration of SPIO (b). This is due to the uptake of iron oxide particles by the Kupffer cells of the RES in normal liver parenchyma. The liver lesion does not show significant uptake of SPIO particles, hence the contrast between the metastasis and surrounding normal liver parenchyma is significantly increased after the injection of SPIO

Fig. 17a, b. On the unenhanced T2*-weighted image (a) a liver metastasis (arrow) from breast cancer is shown with a slightly increased signal intensity. A drop of liver signal intensity is noted on the corresponding T2*-weighted image acquired after the administration of SPIO (b). This is due to the uptake of iron oxide particles by the Kupffer cells of the RES in normal liver parenchyma. The liver lesion does not show significant uptake of SPIO particles, hence the contrast between the metastasis and surrounding normal liver parenchyma is significantly increased after the injection of SPIO

drip Delayed hepatic infusion evaluation

AMI 25

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