Keywords

1 Introduction

Discovery of X-ray, a milestone in the history of science, has largely been attributed to images from organs to molecules scale in both treatment and diagnosis [1]. Medical application of X-ray is divided into two main categories: structural and functional imaging. Structural imaging reveals anatomical structures, while functional imaging measuring changes in biological functions including metabolism, blood flow, regional chemical composition and biochemical processes [2]. Skipping the various applications of X-ray in non-medical fields such as material science to determine sample structure and physical properties, this chapter is devoted to introducing the benefits of contrast agents and the magnificent role of nanotechnology in developing agents of interest for imaging scope.

Upon X-ray beam incidence and followed interaction with sample, due to the weak X-ray absorption (low attenuation coefficient) with light atoms such as carbon, hydrogen and nitrogen as organic phosphors elements, fluorescence is generated from singlet excitons. Upon X-ray beam incidence and interaction with sample, electrons either attenuation or secondary X-ray/luminescence optical excitation occurs [3, 4]. X-ray may be scattered or absorbed by cells and as a result, X-ray radiation intensity attenuates until it reaches the scintillator. The difference or contrast between different X-ray absorption ability of tissues make it possible to distinguish them [5, 6], whereas the transmitted X-ray generates the background noise [7].

Absorption of X-ray energy by electrons provides a situation for every element to emit a unique X-ray fluorescence spectrum at a specific angle which can be detected by a detector [8]. This spectrum behaves like a fingerprint for identifying the element.

Despite the limitation of imaging techniques, they can be used to complete image information. Contrast agents can also be applied in vivo molecular imaging such as tracking kinetics of drugs, cancer diagnosis and beta-amyloid plaques [9, 10].

X-ray attenuation is detected by projection and computed tomography imaging techniques. The contrast between tissues is determined by relative attenuation of objects which itself depends on atomic number and tissue density [11]. Thus, soft tissues (e.g., tumors, muscle, and fat) and materials containing hydrocarbon backbone with low density and atomic number produce less contrast in comparison to tissues like bone [12].

Computed tomography provides 3D images through mathematical back projection algorithms by converting sinograms to tomograms [13]. This highly efficient imaging technique apart from many other applications has been used for diagnosis as high resolution data can be analyzed easily. Exposure period must neither be long nor short but long enough to prevent poor resolution, mainly in soft tissue. To increase the quality of the image, several imaging techniques or contrast agents can be employed. Contrast agent material should have a high atomic number to attenuate X-ray intensity to the desired level and consequently, create the appropriate contrast. Commercial contrast agents for examining soft tissue such as iohexol, iodixanol and barium sulfate contain iodine and barium with atomic numbers of 53 and 56 respectively, which enables them to reduce X-ray density more than the soft tissue does.

Disadvantages of conventional X-ray:

  • Soft tissues are not distinguished from each other. Although contrast agents are able to enhance the contrast but it has been shown that it is not sufficient for detection of tumors in early stages and in deep anatomical regions.

  • Low sensitivity is observed when X-ray intensity decreases, because noise which is caused by transmitted X-ray, lowers the contrast.

Contrast agents can be used for highlighting a specific part of tissue or functionalized to target the expressed proteins on the surface of specific cells such as tumor cells. Not all contrast agents attenuate X-ray intensity at the same level, but proportional to their mass attenuation coefficient. This is the reason why different doses of X-ray are employed for different contrast agents. Toxicity and circulation time of the contrast agent should be taken into consideration for specific applications.

The significance of CT as one of the principal radiology techniques which is frequently used in biomedical imaging, has experienced a significant growth with the development of various nanoparticles (NPs) as next generation CT contrast agents. Because of their numerous benefits over traditional contrast agents, such as longer blood circulation period, regulated biological clearance pathways, and precise molecular targeting capabilities, NPs are going to play a substantial role in the future of medical diagnostics. This section outlines different NPs, which play a significant role in X-ray techniques and imaging applications as contrast agents. Traditionally, iodine-based components were employed as contrast agents in CT scans, however after development of NPs with high atomic number, K-absorption edge (K-edge), and appropriate coefficient absorption, iodine started to lose its importance in the literature. The iodine-based elements were compared to modern NPs such as gold, silver, bismuth, thorium, tantalum, and lanthanides. These series of NPs have usually better performance in comparison to the iodine or iohexol in terms of toxicity, and coefficient parameters. However, thorium-based NPs had a malignancy effect as a long-term side effect. Following sections will provide in-depth information regarding these benefits and deficiencies.

2 Nanoparticle Based Contrast Agents

2.1 Gold Nanoparticle Based Contrast Agents

Gold, one of the chemical elements with high atomic number (79), has been identified as a metaphysical and healing power source since ancient times, and it attracted attention of many researchers in traditional medicine and many of recent investigators were also fascinated by its different applications in nanomaterial and biomedical sciences [14]. In 2004, Smilowitz and his colleagues utilized the high atomic number of gold in the field of X-ray imaging, which has since acquired significant attraction across the world [15]. Since 2004, it is clearly seen that most of the research on inorganic nanoparticles based X-ray contrasting media have focused on gold nanoparticles or their hybrids. Aside from the fact that gold particles exhibit great attenuation in X-ray, research has also concentrated on the significant control of their physical, chemical, and biological characteristics, making it one of the most appropriate candidates for X-ray CT and multimodal imaging [16]. AuNPs are not only stable and inert, but they also exhibit a variety of intriguing characteristics, including self-assembly in conjugation with organization motifs and templates, size-related electrical and optical properties, and uses as catalyst and biological processes. The following points highlight the uses of AuNPs for medical diagnostics and X-ray CT imaging: (i) radiopacity characteristics that have been shown to be superior to standard CT contrast agents (e.g., iodinated), (ii) a highly adaptable and simple surface chemistry that allows for a broad flexibility in surface functionalization for in vivo stability or coating, as well as conjugation with specific functional molecules for active targeting to specific organs and cancerous cells. (iii) and biocompatible properties.

In 1847, Faraday described the production of red gold solutions by reduction of gold chloride and phosphorus in carbon disulfide. Similarly, synthesis process of gold NPs is based on the reduction of gold salt by a variety of reduction agents and chemical methods. Citrate reduction is one of the methods which are introduced by J. Turkevich. This is the simplest protocol which allows the synthesis of monodisperse and spherical shape, citrate stabilized AuNPs with a size range of 10–20 nm [17,18,19]. However, size of NPs can be changed from 16 to 147 nm using improved methods and adding amphiphilic items into the synthesis reaction [20,21,22,23]. In the previous decade, AuNPs were used as X-ray contrast agents due to their high stability against oxidation, exceptional absorption coefficient (5.16 cm2g−1) and high Z-number (79) instead of traditional iodine-based alternatives with low absorption coefficient (1.94 cm2g−1) and lower Z-number (53) where Au exhibits 2.7 times more contrast per unit weight in comparison to iodine at 100 keV. Furthermore, gold imaging at 80–100 keV minimizes interferences from bone and soft tissue absorption and finally lowers the overall radiation dosage and exposure [24].

When AuNPs are used as blood pool contrast agents (BPCA), it is worth noting these three factors: (i) stabilization of AuNPs suspensions in bulk media is required which is related to the synthesis steps, (ii) AuNPs should demonstrate proper in vivo pharmacokinetics properties and good in vivo stability which need suitable coating, for example PEG-coatings which show poor interactions with plasma proteins [19, 20], (iii) active targeting should be achieved by surface modifications to enable site-specific targeting between AuNPs and receptors of tumor cells. Active targeting usually links the functionalized NPs and receptor-specific target using three distinct biomarkers that are highly expressed in cancer cells site which include epidermal growth-factor receptor, matrix metalloprotease and oncoproteins that are related to the human papillomavirus infection [25]. Figure 1 depicts the surface functionalization of AuNPs as well as their potential for surface coating and nano encapsulation. Table 1 shows different applications of AuNPs in BPCA and CT applications. In addition, Table 1 shows that citrate reduction is obviously the preferable method for the production of AuNPs core because it is the simplest approach to provide a stable dispersion in aqueous media. Besides, PEGylation of AuNPs can also be beneficiary in BPCA applications and imaging processes, and it can mitigate the effect of reticuloendothelial system (RES) by minimizing the accumulation of AuNPs in liver and spleen. Targeted CT contrast agents using AuNPs have achieved global success ranging from long circulation vascular contrast to targeting tumor cells [19, 20, 22, 26].

Fig. 1
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Reproduced with permission from Ref. [27]. Copyright 2012, with permission from Wiley

Schematic illustration of surface functionalization of inorganic NPs with different materials.

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Table 1 Main results regarding the use of AuNPs as X-ray contrast agent and imaging
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To increase contrast effect and better surface modification, gold NPs can be used with gadolinium to provide hybrid NPs. Gadolinium is in the middle of the lanthanide family and it contains seven unpaired electrons, producing a strong signal in magnetic resonance imaging (MRI). Coupling gold with gadolinium illustrates a suitable detectable dual nanosystem both by X-Ray CT and MRI. The core of this hybrid nanosystem is made up of AuNPs and the shell layer is gadolinium where disulfide bonds keep them together. The reduction of gold salt in the presence of dithiolated derivatives of diethylenetriaminepentaacetic acid was followed by the addition of Gd3+. Through disulfide connections, the two thiol groups were critical in the creation of the multilayered ligand shell. Favorably, modest concentrations of gold (10 mg/mL) and gadolinium (5 mg/mL) could be easily identified and tracked using MRI and X-ray imaging [31]. Presence of cysteine [32], penicillamine [33] and 4-aminothiophenol [34] on Gd added AuNPs demonstrated high r1 relaxivity [33].

Furthermore, Jon et al. also evaluated the ability of coupled superparamagnetic iron oxide with AuNPs [35, 36]. In 2009, synthesis of PEG coated iron oxide core and gold shell hybrid NPs were also reported. The particle surfaces were coated with PEG to assure biocompatibility and a lengthy circulation duration in the bloodstream even at high concentrations. Amphiphilic poly(DMA-r-mPEGMA-r-MA) prevent aggregation and increasing water solubility [35]. Mulder and colleagues demonstrated another method for developing multifunctional probes for fluorescence imaging, CT, and MRI. They coated a fluorescent and paramagnetic lipid layer containing a Gd conjugation and a special dye Cy5.5 over gold/silica core-shell NPs which exhibited highly sensitive signal enhancements of 24 and 50% for MRI and CT in mice liver, respectively [37].

In another study, Salehi et al. used PEGylated AuNPs with deoxycholic acid (DCA-PEG-GNPs) (Fig. 2c) in comparison to commercially available contrast agent Visipaque. DCA-PEG-GNPs with the size of 17 nm in an oval-like shape as shown in Fig. 2a demonstrated significant stability in a various range of pHs (2.5–11) and temperatures (−78–48 °C). Also, AuNPs had similar cell viability with Visipaque in the concentration value of under 100 μg/ml on A549 lung cells. MTT assay also suggested that in the high concentration (200 μg/ml), AuNPs showed low cell viability than Visipaque as shown in Fig. 2b. Notably, CT performance of DCA-PEG-GNPs were higher than Visipaque. This result is also comparable with the iodine. For example, at kVp = 120 keV, the attenuation value of 0.8 mg ml−1 for iodine was close to the attenuation value of 0.49 mg ml−1 for gold as shown in Fig. 2d [38]. In addition, Wang and his colleagues designed Au nanocage@PEG nanoparticles (AuNC@PEGs) as shown in Fig. 3 in different concentrations (0, 50, 100, 200, 500, and 1000 μg/ml) and compared with iodine and showed that Au nanocages were possible alternative CT contrast agents to iodine [39]. Park et al. synthesized gold@silica NPs, and their cytotoxicity studies displayed that they were not hazardous, and while tiny deformations on the silica shell had no effect on the stability of gold@silica NPs, larger deformations on the silica shell resulted in agglomeration. However, in vivo applications revealed effective contrast enhancement in CT imaging [40]. Real-time in vivo X-ray images showed that gold@silica NPs with folic acid (FA) as a targeting agent can visualize MGC803 gastric cancer after intravenous injection in nude mice as show in Fig. 4 [41, 42].

Fig. 2
figure 2

Reproduced with permission from Ref. [38]. Copyright 2020, with permission from Wiley

a TEM image of DCA-PEG-GNPs, b MTT assay on A549 cells in different concentrations of DCA-PEG-GNPs and Visipaque, c Structure of DCA-PEG-GNPs, d In vitro X-ray CT images at different kVp values, the vials contained 1.0 mg ml−1 AuNPs or iodine.

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Fig. 3
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Reproduced from Ref. [39] with the permission of the Creative Commons Attribution 4.0 International License http://creativecommons.org/licenses/by/4.0/). Copyright 2020, Springer

a In vitro CT images of AuNC@PEGs and iodine-based at different concentrations. b Hu values for AuNC@PEGs and iodine-based contrast agents, ***P < 0.001.

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Fig. 4
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Reproduced with permission from Ref. [41]. Copyright 2011, with permission from Elsevier

Real-time in vivo X-ray images. a The tumor tissue, b X-ray image at 0 h, c X-ray image at 0 h (in color), d X-ray image at 12 h, e X-ray image at 12 h (in color). f X-ray image at 24 h (in color).

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Interestingly, Guo et al. designed multifunctional core–shell tecto dendrimers (CSTDs) with AuNPs core and β-cyclodextrin modified generation 5 poly(amidoamine) dendrimers for dual-mode CT and MRI of tumors. Au CSTDs with the size of 11.61 nm displayed excellent stability, strong X-ray attenuation and good r1 relaxivity (9.414 mM–1 s–1), and desirable cytocompatibility as well. They also showed that CSTDs act as suitable CT/MRI dual mode imaging probe (Au/Gd) in cancer diagnosis applications as shown in Fig. 5 [43].

Fig. 5
figure 5

Reproduced with permission from Ref. [43] Copyright 2021, with permission from ACS

a CT images and b T1-weighted MR image of Gd@Au CSTDs in vivo.

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2.2 Silver Nanoparticle Based Contrast Agents

Silver, a metal with atomic number of 47, is recognized as fascinating NPs in different applications such as biomedical sciences, food, textiles, consumer products and industrial purposes, due to their unique physical and chemical properties.

Image processing at two separate energy windows is known as dual-energy imaging (low and high) where images with reduced contrast between background tissues is achieved by weighting variables. Because of good X-ray attenuation properties of Ag as appealing contrast material with k-edgeFootnote 1 of 25.5 keV and average diameter of 2–6 nm by Brust method demonstrate slight cellular toxicity in T6-17 fibroblast cells. AgNPs, stabilized with PEG, can provide similar or better contrast in comparison to the iodine [44].

Silver iodide (AgI) and silver oxide (Ag2O) were introduced as contrast agents in the earlier twentieth century. However, their usage was immediately halted due to the significant toxicity and reported deaths [45,46,47]. Yet, silver was maintained in the liver for a long enough period of time to allow CT scanning with a contrast difference of 4–5 times that of the background [48]. Lui et al. showed the X-ray contrasting properties of generation 5 poly(amidoamine) dendrimer which is appropriate for the stabilization of silver nanoparticles for in vivo CT imaging. Size and concentration of AgNPs are two main parameters for the X-ray absorption coefficient. They were not only able to readily control the size of AgNPs from 8.8 to 23.2 nm by changing the molar ratio of dendrimer/Ag salt, but also kept AgNPs stable in the phosphate buffered saline, fetal bovine serum and water at pH range of 5–8 in room temperature conditions. It is quite interesting to note that X-ray attenuation behavior of particles with a diameter of around 16 nm were identical to those of an iodine-based contrast agent and in vivo studies with mice revealed that the AgNPs provided sustained contrast enhancement while having no severe toxicity. In another study, Cormode and his colleagues explored potential of silver telluride (Ag2Te) NPs in an in vivo setting at a low dose (2 mg Ag per kg) for X-ray imaging. An astonishing contrast agent performance was observed compared to the AgNO3, Na2TeO3 and iodine which were frequently used in the previous studies as shown in Fig. 6 [49].

Fig. 6
figure 6

Reproduced with permission from Ref. [49]. Copyright 2020, with permission from RSC

In vitro imaging with CT. a CT phantom images, b CT attenuation rate of the different samples at various energies (80,100, 120, and 140 kV).

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Luo et al. produced heterostructure samples with bismuth and silver NPs conjugated with poly(vinylpyrrolidone) (PVP) to design Bi-Ag@PVP NPs for quadra applications including CT, photoacoustic imaging, photodynamic therapy and photothermal therapy (PDT/PTT) into one nanotheranostic platform. They also compared Bi-Ag@PVP with the commercial contrast agent, iohexol, in different range concentrations from 0 to 20 mg ml−1 as shown in Fig. 7 [50].

Fig. 7
figure 7

Reproduced from Ref. [50] with the permission of the Creative Commons Attribution 4.0 International License http://creativecommons.org/licenses/by/4.0/. Copyright 2021, ACS

a CT images of Bi-Ag@PVP and iohexol, b CT signals-concentration fitting curves, c CT imaging capability of Bi-Ag@PVP NPs in tumor site (red), and d CT signal intensity before and after injection.

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2.3 Bi Nanoparticle Based Contrast Agents

Bismuth is a heavy metal with a high atomic number of 83. Bismuth was one of the first X-ray contrast agents used on human gastrointestinal tract before the 1990s [51,52,53]. The surface of Bi2S3 NPs was modified with poly(vinylpyrrolidone) (PVP), a biocompatible polymer, and a rectangular flat plate morphology, size range of 10–50 nm and 4 nm in thickness was obtained. These NPs were highly soluble in water, inert and had a longer blood half-life. Noticeably, the X-ray opacity of Bi2S3–PVP NPs was about fivefold higher than the commercial iodine. In vivo experiment revealed the temporal evolution of the blood after injection where NPs accumulated in liver and spleen after 24 h [53].

RadiopaqueFootnote 2 objects block radiation rather than allow it to pass through. This concept was cleverly employed in a study where Bi2S3 NPs with the inner core layer covered with alginate-poly-L-lysine-alginate were used to avoid toxic effects in the X-ray process. Results displayed microcapsules were not only visualized individually in the rabbit and mice, but also kept their contrast properties for two weeks due to low water solubility [54].

Nosrati et al. evaluated the ability of Bi2S3-Au semiconductor heterojunction NPs which can improve the contrast of CT images and free radical generation via the Schottky barrier in addition to their intrinsic radiosensitizing ability [10]. As it is declared earlier, under X-ray irradiation, NPs with high-Z number (here bismuth) components produce secondary and Auger electrons via the photoelectric and Compton processes, resulting in the generation of large quantities of reactive oxygen species (ROS) within the cells [10, 55, 56]. Figure 8 shows successful targeting and effective tumor accumulation of drug loaded heterojunction NPs (Bi2S3@BSA-Au-BSA-MTX-CUR). It increase the Hounsfield units (HU)Footnote 3 in the tumor site from 15 to 81 (white arrow). They also showed that CT contrast intensity directly depended on the concentration of the final sample (Bi2S3@BSA-Au-BSA-MTX-CUR) as shown in Fig. 8c and e.

Fig. 8
figure 8

Reproduced from Ref. [10] with the permission of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/). Copyright 2022, Elsevier

a Schematics of designed nano drug delivery system of Bi2S3@BSA-Au-BSA-MTX-CUR, b TEM image of Bi2S3@BSA-Au-BSA-MTX-CUR, c CT contrast value curve of Bi2S3@BSA-Au-BSA-MTX-CUR at different concentrations, d CT images of mice with a 4T1 tumor recorded at (i) pre and (ii) post intravenous injection of Bi2S3@BSA-Au-BSA-MTX-CUR, and e First row: Typical photograph of each concentration; Second and Third rows: CT images of each concentration.

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In a study, Pan et al. demonstrated CT contrast effect for detection of clot with thrombus-targeted NanoK with a diameter of 180 and 250 nm (Fig. 9b) [57].

Fig. 9
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Reproduced with permission from Ref. [57]. Copyright 2020, with permission from ACS

a Structure of Bi-based NanoK, b Cross-sectional slices of fibrin clot targeted with NanoK presenting calcium.

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Zhang et al. explored ultrasmall FA and bovine serum albumin-modified Bi–Bi2S3 heterostructure NPs (Bi–Bi2S3/BSA&FA NPs) as CT contrast agents. Bi–Bi2S3/BSA&FA NPs with the size of 10 nm showed not only an appropriate stability due to formation of Bi2S3 NPs on the surface of Bi without any agglomeration even after the one month, also displayed photothermal conversion efficiency (35%). As demonstrated in Fig. 10, the X-ray absorption coefficient of Bi–Bi2S3/BSA&FA NPs (50.4 HU L g–1) is higher than commercial contrast agent iohexol (24.1 HU L g–1), therefore, Bi–Bi2S3/BSA&FA NPs can be a candidate CT contrast agent [58].

Fig. 10
figure 10

Reproduced with permission from Ref. [58]. Copyright 2019, with permission from ACS

a CT images of Bi and iohexol (I) for different concentrations from 0 to10 g.L-1, b Quantification of CT value for Bi–Bi2S3/BSA&FA NPs and iohexol samples, c In vivo CT images of tumor (green circles) and bladder (blue circles).

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Yang et al. explored bismuth functionalized S-nitrosothiol (Bi–SNO NPs) with a size of 36 nm for the combination therapy (X-ray radiotherapy and 808 nm PTT). X-rays can break down the S–N bond and commence a large amount of NO-release which is a cancer killing agent at concentrations above 60 µM [59]. CT value increased from 47.52 HU to 232.21 HU at a NP concentration of 2 mg.mL−1 after the injection [59]. Shan et al. designed Bi/Se NPs loaded with Lenvatinib (Len) in a simple reduction reaction for in vivo CT imaging of mice. They showed that Bi/Se-Len NPs can increase CT value from 27 to 638 HU after the injection in mice [60]. Luo and his colleagues also designed hybrid NPs containing bismuth and lanthanide conjugated PVP as BiF3Ln@PVP as a CT contrasting agent. CT value increased from 48.9 HU to 194.58 HU post-injection, as shown in Fig. 11 [61].

Fig. 11
figure 11

Reproduced from Ref. [61] with the permission of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/). Copyright 2021, Springer

In vivo CT imaging of BiF3Ln@PVP NPs, a CT images before and after the injection of BiF3Ln@PVP NPs (tumor site is shown in red circle), b Corresponding CT values (HU).

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2.4 Thorium Oxide Nanoparticle Based Contrast Agents

Thorium, one of the actinide group element (group 4, period 7) with a high atomic number of 90 is represented with the symbol Th. In 1828, Jöns Jacob Berzelius, a Swedish scientist, discovered Th in the mineral thorite (ThSiO4). Heyden chemical company produced colloidal thorium oxide (ThO2) as a radiological contrast agent in 1920s [62, 63]. Favorably, ThO2 or Thorotrast (trade label) does not aggregate after intravenous administration with a size of 3–10 nm and showed appropriate contrast agent performance due to the high atomic weight. ThO2 was also used as oral administration in gastric mucosa and upper part of gut for imaging applications. However, it is found that ThO2 not only deposits lifelong in the RES of many organs such as spleen, liver, lymph nodes and bone marrow, but also causes formation of malignant tumors, blood dyscrasias, liver fibrosis and leukemia [64,65,66,67,68]. Table 2 shows some of these problems which cause malignant effects in human and animal models [65]. Later, the usage of colloidal ThO2 was eventually abandoned due to the severe toxicity.

Table 2 Risk of malignancy after thorotrast administration
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2.5 Tantalum Nanoparticle Based Contrast Agents

Tantalum is a chemical element with the atomic number of 73 and the symbol Ta in the periodic table. Tantalum has demonstrated certain benefits over traditional iodinated contrast agents as a X-ray contrast agent due to the high radiopacity with a density of 16.6 g/mL [78,79,80]. Tantalum oxide (Ta2O5) is a biocompatible material and used in gastrointestinal imaging and angiography [81,82,83]. Bonitatibus et al. designed a tantalum-based NP in the form of a core–shell structure with the total size of less than 6 nm. Silica was used as a coating layer to make NPs stable and water soluble. In vivo experiment showed that tantalum-based NPs displayed appropriate contrast compared to iodine [84]. Krivoshapkin and his colleagues also showed Ta2O5 NPs, at a concentration of 20 mg.ml−1, can increase CT value from 47.1 ± 5.7 HU to 426.1 ± 2.8 HU at tumor site in 10 min Fig. 12 [85].

Fig. 12
figure 12

Reproduced with permission from Ref. [84]. Copyright 2020, with permission from RSC

a CT image of Ta2O5 NPs from the coronal views in GI tract, b CT images and values of Ta2O5.

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2.6 Rare Earth Nanoparticles Based Contrast Agents

Rare earth elements display very complex luminescence activity which arises from f-f transitions of the 4f shell and f-d transitions in the 4f-5d shell. Upconversion nanoparticles (UCNPs) and downconversion nanoparticles (DCNPs) are the two primary types of rare-earth based NPs. UCNPs can convert low energy with long-wavelength light to high energy with short wavelength while DCNPs convert high energy to low energy. Rare-earth NPs have been widely employed in vitro and in vivo imaging of biomolecules due to the outstanding properties of rare-earth ions, such as their low energy losses, different absorption and emission wavelengths, and low photobleaching [86]. Below, two rare-earth NPs are introduced as contrast agents in imaging applications.

2.6.1 Gadolinium Based Nanoparticles

Gadolinium (Gd) is an appropriate contrast agent for MRI [87] and CT imaging due to its high atomic number (64), large number of unpaired electrons and high K-edge (52 keV) which is greater than iodine. Havoron et al. evaluated Gd oxide as a contrast agent in 1970s. Their animal test resulted that there were no observable toxic effects when poly(vinylpyrrolidone) was used to stabilize the Gd2O3 microparticles [88, 89]. In a study, Prosser et al. used Gd NPs in both CT and MRI applications. Their data indicated that NaGdF4 (with a low polydispersity and 20–22 nm size) and 50:50 mixture of GdF3 and CeF3 (with the 10–12 nm size) not only displayed high relaxivities at 1.5 and 3 T (35–40 mL.s−1 mg−1) in MRI which was better than Gd3+-DTPA (gadopentetate dimeglumine, Magnevist®) complex but also could be functionalized with folic acid for targeting purpose in human cancer cells [90].

In another study, Watkin et al. described Gd2O3-albumin NPs with a diameter of 20–40 nm and embedded them within protein microspheres as CT contrast agent, which had 40 to 100 times higher contrast effect than iopamidolFootnote 11 at the same concentration [91, 92]. In a similar approach, Ru(bpy):Gd(III)@SiO2 NPs were developed as a contrast agent for CT imaging, MRI and diffuse optical tomography. Ru(bpy) can act as a dye to prohibit photo bleaching and Gd has paramagnetic properties which is useful in enhancing MR contrast for both longitudinal (T1) and transverse (T2) proton relaxation rate measurements. NPs displayed higher contrast enhancement than the commercial contrast agent, Gadoteridol, however, the contrast effect was less than the trade mark Omnipaque® at the same concentration [27, 93].

Qui et al. explored fabrication of neodymium (Nd)-doped and gadolinium tungstate NPs with a hydrophilic layer (NaGd(WO4)2:Nd@PDA–HA NPs) for CT, MRI and fluorescence imaging. Their result demonstrated that NPs displayed a higher CT contrast (11.67 ± 0.46 HU.mM−1) compared to the commercial contrast agent, iohexol (4.31 ± 0.09 HU.mM−1). NPs were proposed as an appropriate CT contrast agent in breast tumor at a concentration of 2.5 mg.mL–1 [94].

2.6.2 Holmium Based Nanoparticles

Holmium with electron configuration of [Xe] 4f11 6s2 is soft and stable in the room temperate but it oxidizes rapidly in moisture. Similar to gadolinium, it exhibits an effective contrasting agent for both MRI and CT imaging due to its paramagnetic nature, high atomic number (67) and high attenuation coefficient [95,96,97,98]. Bult et al. synthesized HoAcAc NPs (Fig. 13a) by dissolving holmium acetylacetonate in chloroform, followed by emulsifying in an aqueous surfactant solution, and stabilizing with polyvinyl alcohol and didodecyldimethylammonium bromide. Spherical HoAcAc NPs had a diameter of 78 ± 10 nm (Fig. 13b) and had higher CT contrast effect (15.6 HU·mg-1) in comparison to the iodine. Figure 13c shows agarose MRI phantom images for different NP concentration, where increase in the amount of NPs resulted in an increase in the degree of darkness [99].

Fig. 13
figure 13

Reproduced from Ref. [99] with the permission of the Creative Commons Attribution Noncommercial License (https://creativecommons.org/licenses/by-nc/2.0). Copyright 2010, Springer

a Structure of HoAcAc NPs, b STEM image of HoAcAc NPs with the size of 78 ± 10 nm, c Phantom image (left) and photograph of  HoAcAc NPs at different concentration.

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Ocana et al. explored the role of dysprosium vanadate (DyVO4) and holmium vanadate (HoVO4) NPs functionalized with poly(acrylic acid) (PAA) with a diameter of 60 nm for contrast agent application. Such NPs displayed (Fig. 14) 2.4 fold more contrast than the commercial agent iohexol [98].

Fig. 14
figure 14

Reproduced with permission from Ref. [98]. Copyright 2020, with permission from ACS

a Quantification of HU for HVO4 and DyVO4 NPs functionalized with PAA in comparison to the iohexol, b CT phantom images for different NP concentrations.

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Cormode et al. used cerium oxide NPs as a CT contrast agent with k-edge of 40.4 keV for imaging the gastrointestinal tract (GIT) and inflammatory bowel disease (IBD). Dextran was used to increase the accumulation in the IBD inflammation sites. Dextran-coated cerium oxide NPs (Dex-CeNP) was compared to the FDA-approved commercial contrast agent iopamidel (Fig. 15). More than 97% of oral dosage of Dex-CeNP was eliminated from the body within 24 h. Therefore, Dex-CeNP has the potential to be used as a CT contrast agent for GIT imaging in IBD [100].

Fig. 15
figure 15

Reproduced with permission from Ref. [100]. Copyright 2020, with permission from ACS

a Photograph of a solution of Dex-CeNP, and schematic illustration of Dex-CeNF, b TEM image of Dex-CeNP, and c Micro-CT images of colitis mice, before and after oral administration of Dex-CeNP and iopamidol where “S”, “SI” and “LI” are the stomach, small intestine, and large intestine, respectively. The yellow arrow shows Dex-CeNP accumulation in a colitis site.

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3 Conclusion and Future Perspective

Advances in CT equipment, along with better CT contrast formulations are propelling the field of CT imaging forward in both laboratory and clinic   throughout the world. Contrast enhancement and high-resolution CT images, as well as the low cost and widespread availability of clinical CT scanners facilitate the diagnosis process. High atomic number based NP based contrast agents can be a prosperous milestone in the CT imaging to decrease the radiation doses while maintaining sensitivity and specificity, simultaneously. The reviewed studies displayed novel metal NPs can be appropriate candidates as contrast agents in CT imaging and MRI applications in compared to the traditional iodine-based contrast agent as long as long-term side effects are evaluated precisely in the clinical phase to prevent possible health concerns. Furthermore, the ability to integrate imaging and treatment as a theranostic approach in a single nanoplatform is another promising approach in biomedical applications. Appropriate design and functionalization of nanostructures can target specific tissue/cell for imaging and therapy aims. Future research and development of novel CT contrast agents is expected to result in new chemical structures, modes of different functionality, and improved image resolution, and nanoparticle based commercial CT contrast agents could be possible as long as long term in vivo concerns are well documented.