Non radiation imaging Flashcards
What is the physical phenomenon at the base of MRI imaging?
Explain how this works.
- NMR: magnetic resonance is a phenomenon that any particle having spin can undergo. Spin is an intrinsic property of the particle, often defined as a magnetic angular momentum, whose direction can be manipulated through the application of a magnetic field.
1) application of strong magnetic field –> spins align parallel/antiparallel according to the Boltzmann statistics and thus producing a net magnetization vector
2) The direction of the spins can be temporarily tiled by the application of a variable magnetic field in the radio-frequency range (RF pulse). If the frequency of the RF pulse equals the resonance frequency of the area of interest, we have a tilt of 90°
3) The realignment process is gradual and consists in a precession motion of the magnetization vector around the direction of the main magnetic field. The signal we measure is that of the precessing spins, which is done by coils.
4) There are two characteristic times of this precession motion and by manipulating the time-to-repetition (TR) and the time-to-echo (TE) we can obtain images with different contrast, useful to differentiate tissues.
Give the definitions of T1, T2 and T2*
T1: spin-lattice relaxation time. Describes the recovery of the longitudinal magnetization after the application of the RF pulse (time taken to recover the 63% of the transverse magnetization).
T2: spin-spin relaxation time. Describes the decay of the transverse magnetization after the application of the RF pulse (time taken to decay by 37%).
T2*: effective transverse relaxation time. It takes the inhomogeneities of the magnetic field into account (for the value of the magnetization vector).
List the components of the MRI machine
1) Main magnet coils: generate the strong constant magnetic field.
2) Gradient coils + shim coils: The first generate a gradient to the magnetic field in each of the three spatial directions and that will be needed in the imaging process. The shim coils generate a magnetic field that compensates the inhomogeneities of B0.
3) Integral RF coil: generates and transmits the RF pulses that excite the spins and produce the signal.
4) RF head coil: collects the resonance signal and is put as close as possible to the body to maximize it.
Describe how the images are obtained
Assume patient axis along the z-direction
1) Spatial encoding: The gradient is applied to the magnetic field along the direction of the axis of the body. This way each slice has a specific range of resonance frequencies that excite it to produce the MRI signal. By choosing this as the RF pulse frequency we can select the slice of interest.
–> All the protons in the slice are spinning together
2) Frequency encoding: This gradient is applied along the x-axis and allows us to implement the spatial dependence of the Larmor frequency s.t. the signal can be localized through Fourier transform of the cumulative measured signal.
–> we measure a complex combined signal which is the superposition of the signals from each voxel. Those in the same column will experience the same magnetic field and the same Larmor frequency. The amplitudes will be different but we can only measure the combined value.
3) Phase encoding: Phase gradient applied along the Y-axis but not in a continuous way (due to periodicity of the phase and the directions of the gradients); this is switched on and off. Moreover, the process is repeated multiple times with different gradient strengths in order to recover the information about the amplitude.
What are the applications of MRI imaging in radiotherapy?
1) Contour definition, treatment planning, and plan adaptation
2) Functional MRI
3) Onboard monitoring (IGRT)
4) Dose calculation for MR protons
5) 4D MRI
What are the advantages and disadvantages of MRI scans
A) high soft tissue contrast, high spatial resolution, arbitrary slice selection, selectable contrast, functional information, no dose is delivered to the patient
D) Acquisition is slow, the costs are high, It is sensitive to metals and doesn’t show compact bone structure.
Explain the working principle of ultrasound imaging
US imaging is a non-radiation technique that produces an image of the internal structure of the body by sending sound waves to the target (produced in a transducer by inverse piezoelectric effect) and measuring the reflected signal (excitation happens through piezoelectric effect). The reflection coefficient, and with that the amplitude of the reflected wave, depends on the acoustic impedance difference of tissues (reason why we need gels).
The visualization is made possible by the designation of a color scale (BW), each color associated to the amplitude of the measured signal.
What are some important parameters for the ultrasound imaging?
W) Wavelength: correlated to the frequency
I) Intensity: Related to the amplitude of the particle oscillation
F) Frequency: set by the transducer. Between 2-18 MHz
V) Velocity: Faster in bone than air; 1540 m/s in soft tissue
What are the possible modes one can use to acquire ultrasound images?
A) Amplitude: Measures the distance between tissues or the thickness of one tissue
B) Brightness: To the amplitude peaks detected in A mode are assigned dots with different brightness. The detector sends multiple pulses in all directions to form all the image lines
M) Motion: displays the temporal evolution of a single image line
D) Doppler: used for cardiovascular studies, is able to track the movement of blood or other fluids by measuring the shift in frequency due to movement.
What are advantages and disadvantages of US imaging?
A) real time imaging, good spatial resolution, no dose delivered to the patient, cost-effective
D) Operator dependent, low SNR
