Magnetic resonance imaging
MRI is a noninvasive and nonionizing imaging modality that has been extensively applied for the study of cartilage degradation and regeneration both in vitro and in vivo in animals and in clinical studies. MRI has the best soft tissue contrast among all imaging modalities. The principle of MRI is briefly discussed here (see Figure 9.3), but the readers are referred to excellent texts with detailed description elsewhere . MRI detects nuclei that have either an odd number of protons or an odd number of neutrons. Such nuclei possess magnetic moment, м and angular momentum, J, and have a quantum mechanical property called nuclear spin (Figure 9.3 (a)). Many biologically relevant nuclei such as protons (:H), sodium (23Na), phosphorous (31P), and carbon (13C) possess nuclear spin and can therefore be observed using MRI if there is suitable hardware available. MRI involves the detailed manipulation of nuclear spins using a magnetic field, orthogonal gradient fields, and radiofrequency pulses.
The nuclear magnetic moment, м and angular momentum, J, are collinear and are related through a constant called gyromagnetic ratio y. This is an important relationship and one that is the basis of image creation in MRI. When these nuclei are placed in a magnetic field (shown as B0 in Figure 9.3 (b)), they are said to be in precession around the magnetic field with the Larmor frequency ш0 that is related to the magnetic field with (ш0 = yB0). Once placed in a magnetic field, nuclear state with spin I is degenerated (2I + 1) fold. For example, :H with spin Уг will have two energy states (-1/2 and +1/2) and 23Na with spin 3/2 will have four energy levels (-3/2, -1/2, +1/2, and +3/2), etc. In classical terms, we can say that these states are parallel and antiparallel states of nuclei with respect to the magnetic field. Using suitable radiofrequency pulses matching the Larmor frequency ю0, resonance condition can be created, or in other words, nuclei can absorb energy and flip from antiparallel to parallel states. Since a large number of nuclei are involved, it is easy to deal with another terminology involving nuclear magnetization M. Those nuclei oriented toward the main magnetic field effectively
Figure 9.3 Schematic diagram depicting the basic principles of MRI acquisition. (a) a nuclear spin, (b) precessing nuclei in a magnetic field, (c) an example MRI pulse sequence for image acquisition, (d) an example MRI image of tissue engineered cartilage samples.
create what is called nuclear magnetization, denoted by M (Figure 9.3 (b)). The size of magnetization depends on the strength of the main magnetic field, B0, therefore the magnetic field strength plays a key role in MRI signal strength. The M can be manipulated using radiofrequency (RF) pulses.
To create an image out of this excited nuclear state, the main magnetic field B0 in all directions is varied with the help of gradient fields. This makes magnetization vector M position-dependent. Using radiofrequency pulses to manipulate the magnetization, M, as well as the magnetic field in all directions (as shown in Fig. 9.3(c)), one can create an image in k-space. When the fast Fourier transform is applied, the image is obtained in x-y space. Based on the pulse sequence, MRI images can have different contrasts such as water relaxation times (T1, T2, and T1rho), apparent diffusion coefficient (ADC), magnetization transfer (MT), etc. By choosing suitable acquisition parameters, either a parameter-weighted image can be created or a parameter map can be obtained for tissue. The beauty of MRI is that the MR parameters often depend on the local environment within the tissue and are therefore representative of the tissue’s physiologic state. MRI of tissue-engineered cartilage
Figure 9.4 Different MRI systems are needed for the assessment of tissue-engineering cartilage in vitro and in vivo. An example of each type is given. (a) A vertical-bore NMR spectrometer suitable for in vitro MRI analysis. (b) A small animal-dedicated MRI scanner suitable for mice, rats, and rabbits. (c) A human clinical MRI scanner suitable for large animals or humans.
assessment and visualization typically uses water protons because of high water content in tissue-engineered cartilage . Commonly used contrasts in MRI assessment of tissue-engineered cartilage in vitro are Tb T2, T1rho, ADC, and MT. However, in vivo T2 is the most favorable MR parameter for MRI assessment. Sodium MRI has also been used to assess proteoglycans in cartilage tissue engineering and regeneration [53,59].
Depending upon the choice of tissue-engineering strategy and sample size, a number of imagers can be used for the assessment of tissue-engineered cartilage, as shown in Fig. 9.4. For in vitro or ex vivo samples that are typically small in size (~few mm to few cm), wide vertical-bore NMR spectrometers (similar to one shown in Fig. 9.4(a)) can be converted to micro-MRI systems by installing additional hardware. These spectrometers can be operated at a very higher magnetic field (up to 21 Tesla) compared to clinical MRI scanners (1.5-7 Tesla). The advantages of high field micro- MRI systems are their cost effectiveness and very high resolution (~ 10-20 qm) due to small and dedicated RF resonators and high magnetic field. For small animal MRI, commercially available dedicated scanners similar to one shown in Fig. 9.4(b) can be used. Such scanners are typically equipped with animal monitoring systems to carry out in vivo studies and also have better resolution than clinical MRI. Finally, clinical scanners can be used for large animals and clinical studies, as shown in Fig. 9.4(c). These scanners are used with a lower resolution of the order of 2-5 mm in order to be time efficient.