MRI Chapter 9 Review Questions

Why does an MRI scan make such a loud noise?

• The noise is caused by the gradient coils

• The noise is a safety feature that the magnetic field is on

• The noise is the RF energy being transmitted into the patient

• The noise correlates to the patient's heartbeat

Answer explanation

This relates to the gradient coil. A conductor placed in an external magnetic field will experience a (Lorentz) force. When the current in the gradient coils rapidly changes polarity, it results in a significant vibration of the coil. This is transmitted through the superstructure of the scanner and is transferred to the surrounding air as sound waves. The scanner behaves like a huge loudspeaker.

Why are superconducting magnets preferred in MRI systems?

• They can carry high current with no resistivity losses, allowing for a powerful magnetic field.

• They are less expensive than other types of magnets.

• They are easier to install than permanent magnets.

• They do not require any cooling systems.

Answer explanation

A superconducting magnet (typically spun from niobium/titanium wire) is capable of carrying a high current with no resistivity losses (heating). This permits the generation of a powerful magnetic field over a large imaging volume that will persist even when detached from a power supply.

Tags

NGSS.HS-PS2-5

Why is the body area to be scanned positioned at the center of the magnet bore (isocenter)?

• Isocenter is where the magnetic field is most homogeneous.

• Isocenter is where the magnetic field is the weakest.

• Positioning depends on patient tolerance. Claustrophobic patients don't need to be positioned at isocenter.

• Isocenter is where the gradient noise is the lowest.

Answer explanation

An MRI scanner uses a segmented solenoid magnet to create the main magnetic field. The design of this solenoid creates a homogenous imaging volume over an approximately spherical 40 cm diameter. The isocenter of this imaging volume corresponds to the very center of the magnet bore. The anatomy of interest must be positioned at the isocenter because image distortion increases with distance from this point.

What is meant by shimming?

• Shimming refers to the process required to optimize the homogeneity of an MRI magnet once installed

• Shimming eliminates the need for tuning for each patient

• Shimming transmits RF into the patient during scanning

• Shimming keeps the magnet wire windings cool during scanning

Answer explanation

Shimming refers to the process required to optimize the homogeneity of an MRI magnet once installed. Passive shimming uses strategically placed metal plates to offset any undesirable harmonics in the static field that may be caused by nearby metal structures (such as the steel frame of a building). Dynamic shimming can be adjusted for individual patients and typically uses the windings of the gradient subsystem.

What are the advantages of an open-magnet design vs closed bore?

• Claustrophobic patients and children are less enclosed

• Patients with metal implants can be scanned without screening

• Patients who require high-resolution imaging can be images more quickly

• Can scan patients who need rapid scans due to pain

Answer explanation

Open magnets offer the following advantages: large or broad patients can be accommodated, improved access for biopsy, claustrophobic patients (and nervous children) are less enclosed, sideways table movement allows lateral structures to be positioned close to the isocenter, and flexion/extension views are more easily achievable.

What are the primary functions of spatially varying gradients in an MRI acquisition?

• To create slopes along the main magnetic field

• To cool the MRI machine

• To generate the main magnetic field

• To shield the scanner from RF interference

Answer explanation

The gradient subsystem creates slopes along the main magnetic field. These are used to select slice position, alter slice thickness, locate returning signal along one axis by imposing incremental changes in phase, and locate signal in the other axis by imposing a range of frequencies. They are also used to create an echo in gradient echo sequences and spoil unwanted signal.

Why is a magnet room typically lined with copper or aluminum sheeting?

• To provide radiofrequency shielding

• To improve image resolution

• To reduce patient discomfort

• To enhance magnetic field strength

Answer explanation

This is to provide radiofrequency shielding. The frequencies of RF used in MRI share the same part of the electromagnetic spectrum as other external sources of RF such as those used in VHF wireless communication. These include cordless phones and walkie-talkies. RF interference from these devices and other sources would otherwise degrade the image with zipper artefacts.

What is meant by the “filling factor” of an RF receive coil?

• This term describes how much of the sensitive volume of a coil is filled with the patient’s body tissue

• This term describes how much of the bore a patient fills during the scan

• The term describes how full the patient's stomach is during scanning

• The term describes how much contrast has been loaded into the power injector

Answer explanation

This term describes how much of the sensitive volume of a coil is filled with the patient’s body tissue. For optimum SNR, the coil size should match the area of anatomy being scanned. The close placement of surface coils offers a high-filling factor. Using a large volume coil to image a small region results in a poor filling factor and usually reduces SNR. This is why manufacturers offer multiple receive coils, each tailored to the anatomical region to be examined.

Tags

NGSS.HS-PS4-5

Which of the following is an advantage of a phased-array coil over a standard surface coil?

• Higher SNR with a large field of view

• Lower SNR with a small field of view

• Slower acquisition time

• Reduced image quality

Answer explanation

A phased-array coil is essentially a number of individual coils (elements) arranged together. In short, a phased-array coil offers a high SNR combined with a large field of view. Each coil element may have its own channel to carry signals, this improves SNR. Multiple channels also permit parallel imaging in which each coil element is responsible for acquiring its own lines of data. This reduces acquisition time.

What differentiates ferromagnetism from paramagnetism or diamagnetism?

• Ferromagnetic objects retain their magnetism when removed from an external magnetic field

• Ferromagnetic objects are heavier than paramagnetic or diamagnetic objects

• Nothing, they're all magnetic

• Ferromagnetic objects do not react to the external magnetic field

Answer explanation

Ferromagnetic objects retain their magnetism when removed from an external magnetic field.