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1


What is the primary advantage of using organic radical contrast agents over traditional gadolinium-based contrast agents (GBCAs) in MRI?

Lower toxicity

Organic radical contrast agents are increasingly studied as alternatives to traditional gadolinium-based contrast agents (GBCAs) in MRI due to their significantly lower toxicity. GBCAs carry risks such as Nephrogenic Systemic Fibrosis (NSF) in patients with kidney issues and long-term gadolinium retention in tissues, including the brain, even in patients with normal kidney function. This has raised safety concerns (Source: FDA Safety Communications, 2017). 7

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2


Which property of dendrimers makes them suitable as scaffolds for organic radical contrast agents?

Monodisperse and well-defined macromolecular structure

Dendrimers are highly branched, symmetrical macromolecules with a well-defined, monodisperse structure. These characteristics make them excellent scaffolds for organic radical contrast agents because they allow precise control over the placement and density of functional groups. Unlike other polymers, dendrimers have uniform size and structure, enabling reproducible behavior in biological environments. This is critical for applications like MRI contrast agents where precision is crucial. 7

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3


What are nitroxides commonly used for in the context of MRI contrast agents?

Enhancing signal intensity by shortening the T1 relaxation time

Nitroxides are commonly used in the context of MRI contrast agents because of their paramagnetic nature. The T1 relaxation time refers to the time it takes for protons in the tissue to return to their equilibrium state after being disturbed by the magnetic field. Shortening this time leads to a faster recovery of the signal, which enhances the brightness or intensity in T1-weighted MRI images. 7

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4


Which type of dendrimer has been fully functionalized with TEMPO radicals and studied for MRI contrast agents?

PAMAM dendrimers

PAMAM dendrimers (Polyamidoamine dendrimers) are a widely studied class of dendrimers that have been fully functionalized with TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) radicals for use as MRI contrast agents. PAMAM dendrimers are especially suitable for such modifications due to their monodisperse structure and the availability of multiple functional groups at the periphery of their structure. 7

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5


What challenge do nitroxides face that limits their widespread use as MRI contrast agents?

Rapid in vivo reduction and low paramagnetic relaxivity

Nitroxides face a significant challenge in their use as MRI contrast agents due to their rapid in vivo reduction. In biological environments, nitroxides can undergo reduction to a non-paramagnetic form (hydroxylamine), which diminishes their paramagnetic relaxivity and reduces their effectiveness as contrast agents. Nitroxides are a class of paramagnetic radicals that induce efficient MRI contrast by shortening the T1 relaxation time of nearby water protons. However, when in vivo, they undergo reduction either by enzymes or antioxidants to their nonparamagnetic form, reducing their efficiency as an MRI contrast agent. 7

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6


A dendrimer-based contrast agent contains 48 TEMPO radicals, each contributing 0.14 mM⁻¹ s⁻¹ to relaxivity. What is the total relaxivity of this dendrimer-based contrast agent?

6.7 mM⁻¹ s⁻¹

The total relaxivity is 6.7 mM⁻¹ s⁻¹ We now the number of radicals (48) and the relaxivity per radical (0.14), so the total relaxivity is 48 * 0.14, which is 6.72. 7

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7


If a fourth-generation dendrimer with 32 PROXYL radicals has a relaxivity of 5 mM⁻¹ s⁻¹, what is the relaxivity per PROXYL radical?

0.15 mM⁻¹ s⁻¹

The relaxivity per radical is 0.5 mM⁻¹ s⁻¹ We know the total relaxivity (5) and number of radicals (32), so the relaxivity per radical would be 5 / 32 = 0.156mM⁻¹ s⁻¹ 7

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8


A dendrimer-based MRI contrast agent releases its radical at a rate of 0.5 mM/day. If the initial concentration of the radical is 10 mM, how many days will it take for the concentration to reduce to 2 mM?

16 days

It would take 16 days From this, we know that the initial concentration is 10mM, final is 2mM, and rate of release is 0.5mM/day. Using the first-order rate equation, we know that 2mM=10mM−(0.5mM/day)×t Therefore, t = 16 days. 7

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9


If the relaxivity of a G1-Tyr-PROXYL dendrimer is 2.9 mM⁻¹ s⁻¹ and that of Gd-DTPA is 3.2 mM⁻¹ s⁻¹, what is the percentage difference in relaxivity between the two agents?

7

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10


A dendrimer scaffold improves water solubility by attaching PEG chains. If the original dendrimer has a solubility of 5 g/L and attaching PEG increases the solubility by 60%, what is the new solubility of the dendrimer?

8 g/L

The new solubility of the dendrimer is 8g/L Using the following formula: New Solubility = Original Solubility * (Original Solubility * Increase Percentage) We can find the new solubility as the original solubility (5g/L) and increase percentage (60%) is already given. 7

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11


What is the primary reason for using dynamic amplification factor (DAF) in the analysis of steel truss bridges?

To account for the effects of sudden member failure

7

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12


Which method is traditionally used to calculate the DAF for steel truss bridges, and why is it considered conservative?

Single-degree of freedom model because it assumes a constant DAF

The single-degree of freedom (SDOF) model is traditionally used to calculate the Dynamic Amplification Factor (DAF) for steel truss bridges. This method assumes that the bridge behaves like a simple system with one primary mode of vibration, which simplifies the analysis. 7

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13


What is the damping ratio commonly used in the conventional DAF calculation for steel truss bridges?

5%

7

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14


In the context of this study, what does the empirical equation for DAF primarily depend on?

Maximum bending moment

In the context of steel truss bridge analysis, the empirical equation for the Dynamic Amplification Factor (DAF) typically depends on the maximum bending moment. Maximum axial force: While axial forces (tension or compression) are important in bridge design, they are not typically the primary factor in the empirical equation for DAF, as bending moments tend to have a more significant influence on the dynamic response. 7

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15


Which types of loading are considered in the analysis of progressive collapse in steel truss bridges?

Static loading and dynamic loading

In the analysis of progressive collapse in steel truss bridges, the two main types of loading that are considered are static loading and dynamic loading. When considering progressive collapse, it's important to understand how both static and dynamic loads interact with the structure. Static loads may cause the first failure, while dynamic loads can propagate that failure through the bridge. 7

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16


A member in a steel truss bridge fractures and causes a maximum dynamic stress of 450 MPa. If the yield stress of the member is 315 MPa, what is the dynamic amplification factor (DAF) based on stress?

1.42

The DAF based on stress is approximately 1.42. DAF can be calculated by maximum dynamic stress/yield stress. Therefore, DAF is equal to 450/315 which is 1.429. 7

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17


If the maximum static stress in a bridge member after a fracture is 280 MPa and the corresponding dynamic stress is 392 MPa, what is the dynamic amplification factor (DAF)?

1.40

The DAF is 1.40. DAF can be calculated by maximum dynamic stress/maximum static stress. Therefore, DAF is equal to 392/280 which is 1.4. 7

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18


A bridge member has a yield stress of 250 MPa. During a dynamic event, the maximum stress reaches 375 MPa. What is the stress ratio (𝜎 𝑑𝑦𝑛𝑎𝑚𝑖𝑐 / 𝜎 𝑦𝑖𝑒𝑙𝑑)

1.5

The stress ratio is 1.5. Stress ratio can be found by using the formula maximum dynamic stress/yield stress. Therefore, stress ratio is equal to 375/250 = 1.5. 7

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19


What is the mechanical property that ensures a stent remains flexible and stable in blood vessels?

Flexibility

The flexibility of a stent is the key mechanical property that ensures it remains stable and functional within blood vessels. Stents are designed to expand and support blood vessels, but they must also be flexible enough to conform to the natural shape and movement of the vessel. 7

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20


If the Young's modulus of a stent material is 200 GPa and the applied stress is 50 MPa, what is the strain experienced by the stent?

0.00025

The strain experienced by the stent is 0.00025. Using Hooke's Law for linear elasticity, we can find that strain is equal to stress/Young's Modulus. Since we know both Young's modulus (200e9 Pa) and Stress (50e6 Pa), we can find the strain as 50e6 / 200e6 = 0.00025. 7

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ผลคะแนน 80.25 เต็ม 140

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