A Review Quiz On Magnetic Resonance Imaging!

Active magnetic shielding refers to the use of additional magnets and their magnetic fields to contain or reduce the magnetic field of the main magnet. This technique involves actively manipulating the magnetic field by generating counteracting magnetic fields to cancel out or confine the field of the main magnet. This is in contrast to passive magnetic shielding, which relies on the use of high-permeability materials to redirect and absorb the magnetic field. Therefore, the given correct answer of "active" aligns with the explanation provided.

Paramagnetic substances are those that show a slight increase in their magnetic field when exposed to an external magnetic field. This is due to the alignment of their atomic or molecular magnetic moments with the applied field. They have low and positive susceptibility, meaning that their magnetic response is weak and in the same direction as the applied field.

Superparamagnetic substances exhibit positive susceptibility, meaning they are attracted to a magnetic field. They are stronger than paramagnetic substances but weaker than ferromagnetic substances. Superparamagnetic substances are commonly used as T2 contrast agents, which means they can enhance the visibility of certain tissues or structures in magnetic resonance imaging (MRI).

A phase array allows for increased area of coverage without the reduction in signal-to-noise ratio (SNR) by having multiple coils and receivers. This means that the phase array system can receive signals from multiple directions simultaneously, improving the overall coverage area. It achieves this by using constructive and destructive interference of the signals received by the individual coils to steer and focus the beam in the desired direction. This makes phase arrays particularly useful in applications such as radar systems and wireless communication networks.

The gradient subsystem in MRI is responsible for multiple tasks. It is used to select the slice plane, which determines the specific area of the body to be imaged. It also helps select the imaging plane, which determines the orientation of the image. Additionally, the gradient subsystem spatially encodes the MR signal, allowing for the creation of detailed and accurate images. Therefore, the correct answer is "all of the above."

The given correct answer is "Superconducting." This is because superconducting magnets use direct current applied to a coil of wire that is cooled down by being submerged in liquid helium or cryogen. This cooling process helps to lower the temperature of the wires, allowing them to conduct electricity with zero resistance, which is a characteristic of superconductivity.

A narrow bandwidth allows for thinner slices because bandwidth refers to the range of frequencies that can be transmitted or processed. In the context of slicing, a narrow bandwidth means that only a small range of frequencies is being used. This limited range allows for more precise and detailed slicing, resulting in thinner slices.

The receiver bandwidth is determined by the number of frequency samples in the matrix. This means that the more frequency samples there are in the matrix, the wider the receiver bandwidth will be. This is because each frequency sample represents a specific frequency range that the receiver can detect. Therefore, having more frequency samples allows the receiver to detect a wider range of frequencies, resulting in a larger bandwidth. Conversely, having fewer frequency samples would result in a narrower bandwidth.

The correct answer is x because the gradient coil is responsible for creating a varying magnetic field in different directions. In this case, it specifically varies the intensity of the magnetic field in the right to left direction. The x-axis is commonly associated with the right to left direction in many coordinate systems, making it the correct answer.

The given statement indicates that the magnet can be used in both horizontal and vertical field systems, has strengths up to 0.3 T, and can be turned off. The term "resistive" suggests that this type of magnet operates using resistive materials, which can control the magnetic field strength and be turned off when needed. Therefore, the answer "Resistive" is appropriate based on the information provided.

Passive magnetic shielding refers to the use of metal, typically steel, in the walls of a scan room to confine the fringe field generated by a magnetic source. This type of shielding does not require any external power or active control mechanisms to contain the magnetic field. Instead, it relies on the inherent properties of the metal to redirect and absorb the magnetic field lines, preventing them from escaping the scan room and interfering with nearby equipment or individuals.

The z gradient coil is responsible for varying the intensity of the magnetic field in the head to foot direction. This coil is positioned along the z-axis, which runs from the head to the foot of the subject. By varying the current in this coil, the magnetic field strength can be changed in a controlled manner along this direction. This is important in magnetic resonance imaging (MRI) as it allows for the creation of detailed images by manipulating the magnetic field strength in different regions of the body.

The difference in precessional frequency of the hydrogen in fat and water is known as the chemical shift. Chemical shift is a phenomenon in nuclear magnetic resonance (NMR) spectroscopy, where the resonance frequency of a nucleus is shifted due to its chemical environment. In the case of hydrogen, the chemical shift is caused by the surrounding electron density and molecular structure. The chemical shift is an important parameter in NMR spectroscopy as it provides information about the chemical composition and structure of molecules.

Diamagnetic substances are those that exhibit a very slight negative or repelling effect when placed in an externally applied magnetic field. They have low and negative susceptibility, meaning they are weakly repelled by magnetic fields. This is in contrast to paramagnetic substances, which are weakly attracted to magnetic fields, and ferromagnetic substances, which are strongly attracted to magnetic fields. Superparamagnetic substances, on the other hand, exhibit a unique behavior where they switch between being paramagnetic and diamagnetic depending on the size and strength of the applied magnetic field.

Gadilinium is an example of a paramagnetic substance because it possesses unpaired electrons in its outermost energy level. Paramagnetic substances are weakly attracted to a magnetic field and their magnetic properties are due to the presence of these unpaired electrons.

Permanent magnets are made of materials that are naturally ferromagnetic, meaning they have a high magnetic permeability and can retain their magnetization over a long period of time. These magnets are created by exposing ferromagnetic materials to a strong magnetic field, aligning the magnetic domains within the material. Once magnetized, permanent magnets can generate their own magnetic field without the need for an external power source. They are commonly used in various applications such as motors, generators, and speakers.

Ferromagnetic substances have the property of positive susceptibility, meaning they are attracted to a magnetic field. Additionally, they can retain their magnetization even after the external magnetic field is removed. This behavior is due to the alignment of magnetic moments within the material, which creates a strong magnetic field. Therefore, the given statement describes ferromagnetic materials accurately.

Superconductive magnets are limited to 4 Tesla in clinical MRI by the FDA. This means that the maximum magnetic field strength allowed for clinical MRI machines is 4 Tesla. Higher magnetic field strengths can pose safety risks and may not have sufficient evidence of efficacy for clinical use. Therefore, the FDA sets this limit to ensure patient safety and the reliability of MRI results.

The frequency encoding gradient is responsible for determining the spatial location of the signal within the image. It is applied during the data acquisition process and helps in encoding the frequency information of the MR signal. The receiver bandwidth refers to the range of frequencies that are sampled during this process. By adjusting the receiver bandwidth, different frequency components can be captured, allowing for the acquisition of specific frequency information and improving the image quality.

The phase encoding gradient represents the line in k-space on which the data will be plotted during the readout period. K-space is a mathematical space used in MRI to represent the spatial frequencies of the image. The phase encoding gradient is applied along one direction during the data acquisition process, causing the signal to be spatially encoded along that direction in k-space. By varying the strength and duration of the phase encoding gradient, different lines in k-space can be sampled, allowing for the reconstruction of different spatial frequencies and ultimately forming the final image.

Permanent magnets have field strengths in the range of 0.06 - 0.35T. This means that the magnetic field produced by permanent magnets typically falls within this range. Field strength refers to the intensity or magnitude of the magnetic field, and it is measured in Tesla (T). The given answer suggests that the magnetic field strength of permanent magnets is relatively low, ranging from 0.06T to 0.35T.

The correct answer is 1 T. This is because 1 Tesla is equal to 10,000 gauss. Gauss and Tesla are both units of magnetic field strength, with Tesla being the SI unit and gauss being the CGS unit. Therefore, 10,000 gauss is equivalent to 1 Tesla.

The term "slew rate" refers to the rate at which the gradient magnetic field increases to its maximum amplitude, measured in units of T/m/s. It describes how quickly the field strength changes, specifically in relation to acceleration. The rise time refers to the time it takes for the gradient magnetic field to reach its maximum amplitude, while coil configuration refers to the arrangement of the coils used to generate the magnetic field. Therefore, the most appropriate explanation for the given answer is "slew rate."

The signal induced in a receiver coil immediately following an RF excitation pulse is known as the free induction decay. This term refers to the decay of the induced signal that occurs after the excitation pulse has ended. It is a fundamental concept in magnetic resonance imaging (MRI) and is used to generate the signal that is processed to create an image. The free induction decay signal contains information about the properties of the sample being imaged and is crucial for obtaining high-quality images in MRI.

Reducing the voxel volume is the only way to increase the spatial resolution. Spatial resolution refers to the ability to distinguish between two separate points in an image. By reducing the voxel volume, the size of each voxel decreases, allowing for more voxels to be present in the same field of view. This increase in the number of voxels leads to a higher level of detail and improved spatial resolution in the resulting image. Increasing the field of view (FOV) would have the opposite effect, as it would spread the same amount of information over a larger area, resulting in a decrease in spatial resolution. Decreasing the phase encodings or increasing the voxel volume would not directly impact spatial resolution.

Quadrature coils are designed with additional loops and circuitry to enhance the efficiency of the magnetic resonance (MR) signal induction in the coil. This design feature helps increase the signal-to-noise ratio (SNR) by approximately 40% compared to coils of the same size.

A solenoid magnet is created by aligning wires side by side. A solenoid is a coil of wire that carries an electric current, and when current flows through the coil, it creates a magnetic field. The alignment of the wires side by side allows for a stronger and more uniform magnetic field to be produced. This is why a solenoid magnet is the correct answer in this case.

To obtain a thicker slice, a lower amplitude of the Z gradient is selected. The Z gradient is responsible for the slice thickness in magnetic resonance imaging (MRI). By reducing the amplitude of the Z gradient, the magnetic field variation in the Z direction is decreased, resulting in a thicker slice being excited and detected during the imaging process.

The slice selection gradient is applied during both the 90 and 180 degree RF pulses. This gradient is used to spatially encode the signal and select a specific slice for imaging. By applying the gradient during both RF pulses, the slice selection process is optimized and ensures accurate and precise slice selection.

The outer lines of K space hold edge detail. This means that the information about the edges of objects in an image is encoded in the outer lines of the K space. K space is a mathematical representation of the raw data acquired during magnetic resonance imaging (MRI). By analyzing the outer lines of K space, one can extract information about the edges of objects in the image, which is important for detecting and characterizing boundaries between different tissues or structures.

T2* refers to the decay time of the transverse magnetization in a magnetic resonance imaging (MRI) system. The difference in precessional frequency, due to the chemical/molecular environment, affects the T2* value. This means that different chemical or molecular environments will cause the transverse magnetization to decay at different rates, resulting in different T2* values. Therefore, T2* can be used to study and analyze the chemical or molecular composition of a sample in an MRI system.

The middle lines of K space contain over 90% of the image detail and image contrast. This means that the majority of the important information and distinctions between different structures in the image are captured in these middle lines. The outer lines of K space contribute less to the overall detail and contrast of the image.

The slice location in magnetic resonance imaging (MRI) is determined by the transmit frequency of the RF pulse. The RF pulse is used to excite the protons in the body, and the transmit frequency determines the energy level at which the protons resonate. By adjusting the transmit frequency, different slices of the body can be selected for imaging. The other options mentioned, such as the phase gradient and receiver frequency, are not directly related to determining the slice location.

MRI primarily relies on the properties of hydrogen nuclei. This is because hydrogen atoms are abundant in the human body, particularly in water and fat. The hydrogen nucleus, which is a single proton, has a magnetic moment that allows it to align with or against a magnetic field. When placed in a strong magnetic field, these hydrogen nuclei can be manipulated using radiofrequency pulses, causing them to emit signals that can be detected and used to create detailed images of the body's internal structures. The abundance and favorable properties of hydrogen make it the ideal target for MRI.

The Fourier Transform is a mathematical tool used to analyze signals and functions in the frequency domain. It converts a signal from its original time domain representation to a representation in the frequency domain, revealing the frequency components present in the signal. By altering the Fourier Transform, one can manipulate the frequency components of a signal, which in turn can affect the FID (Free Induction Decay). The FID is a signal obtained in nuclear magnetic resonance (NMR) experiments and is used to extract information about the sample being studied. Therefore, altering the Fourier Transform can impact the FID signal.

The given explanation suggests that the correct answer is "vertical" because it is more efficient than linear coil and can be combined with other coils electronically to improve signal uniformity. It also mentions that solenoid coils are required due to the orientation of the B0, which implies that the vertical coil is the suitable option in this scenario.

Magnetic Resonance Imaging (MRI) is a non-invasive imaging technology that produces three-dimensional detailed anatomical images. It is primarily used for imaging the internal organs, including the brain, spinal cord, heart, and other soft tissues, providing a clearer picture than other imaging methods like X-rays or CT scans for these areas.

The receiver bandwidth is determined by the number of frequency samples collected and the sampling period. In this case, since 256 frequency samples are collected and the sampling period is 8 ms, we can calculate the receiver bandwidth by dividing the number of samples by the sampling period. Therefore, 256 samples divided by 8 ms equals 32 kHz. However, since the Nyquist-Shannon sampling theorem states that the maximum frequency that can be accurately represented is half the sampling rate, the receiver bandwidth would be half of 32 kHz, which is 16 kHz.

A solenoid is a type of superconducting magnet that is commonly used in various applications. It is designed in a cylindrical shape with wire coils wrapped around a central axis. The magnetic field produced by a solenoid is typically uniform and parallel to the axis of the cylinder. In the given answer, "solenoid horizontal" suggests that the solenoid is oriented in a horizontal direction, meaning the axis of the cylinder is aligned horizontally. Therefore, the solenoid will produce a horizontal magnetic field.

Phase encoding takes place during the FID (Free Induction Decay), after the slice selection and before the readout. This means that after the specific slice of tissue is selected, the phase encoding gradients are applied to encode the spatial information of the image in the phase dimension. This allows for the reconstruction of the image in the correct spatial orientation during the readout phase.

Gradient echo imaging is a technique used to evaluate images with T2* contrast. This technique involves using gradient magnetic fields to manipulate the magnetization of tissues, which allows for the generation of images with T2* contrast. Therefore, in order to evaluate images with T2* contrast, gradient echo images are required.

A Faraday cage is a type of enclosure made of conductive material that blocks electromagnetic fields. It is used to protect sensitive electronic equipment from interference and to prevent electromagnetic radiation from escaping. RF shielding refers to the practice of using materials or structures to block or reduce radio frequency signals. Therefore, RF shielding is known as a Faraday cage because it effectively blocks RF signals and provides electromagnetic shielding.

The slice thickness in imaging is determined by the amplitude of the gradient and the bandwidth of the RF pulse. The gradient is used to spatially encode the signal, and the amplitude of the gradient determines the range of spatial encoding. A higher amplitude gradient will result in a thicker slice. The RF pulse is used to excite the spins in the desired slice, and the bandwidth of the RF pulse determines the range of frequencies excited. A wider bandwidth will result in a thicker slice. Therefore, both the amplitude of the gradient and the bandwidth of the RF pulse play a role in determining the slice thickness.