This is a broad question that would require a detailed introduction to MRI for a full answer.

In short, different tissues are characterized by different proton densities (PD) and different relaxation time constants (T1 for the longitudinal relaxation, T2 for the transverse relaxation).

Depending on the MRI acquisition technique (pulse sequence) and the sequence parameters (such as repetition time TR, echo time TE, or inversion time TI), the resulting image can reflect one of the tissue parameters much stronger than the others. E.g., a T2-weighted image (with very long TR and TE in the range of T2) emphasizes differences in T2 (showing tissues with long T2 such as fluids much brighter than those with shorter T2). In a T1-weighted image (with very short TE and TR in the range of T1), differences in T2 are hardly visible, but tissues with long T1 appear darker than those with shorter T1.

For a conventional spin-echo sequence, this is described by the signal equation:

S(TE, TR; T1, T2) = S0 (1 – exp(–TR/T1)) exp(–TE/T2)

See also my answer to the following related question:

https://www.researchgate.net/post/What_is_the_difference_between_MRI_scans_T1_T1c_T2_Flair_Is_there_another_type_of_scan

(and e.g. http://mriquestions.com/image-contrast-trte.html )

This is a broad question that would require a detailed introduction to MRI for a full answer.

In short, different tissues are characterized by different proton densities (PD) and different relaxation time constants (T1 for the longitudinal relaxation, T2 for the transverse relaxation).

Depending on the MRI acquisition technique (pulse sequence) and the sequence parameters (such as repetition time TR, echo time TE, or inversion time TI), the resulting image can reflect one of the tissue parameters much stronger than the others. E.g., a T2-weighted image (with very long TR and TE in the range of T2) emphasizes differences in T2 (showing tissues with long T2 such as fluids much brighter than those with shorter T2). In a T1-weighted image (with very short TE and TR in the range of T1), differences in T2 are hardly visible, but tissues with long T1 appear darker than those with shorter T1.

For a conventional spin-echo sequence, this is described by the signal equation:

S(TE, TR; T1, T2) = S0 (1 – exp(–TR/T1)) exp(–TE/T2)

See also my answer to the following related question:

https://www.researchgate.net/post/What_is_the_difference_between_MRI_scans_T1_T1c_T2_Flair_Is_there_another_type_of_scan

(and e.g. http://mriquestions.com/image-contrast-trte.html )

Dear Mahdi,

T1 (longitudinal relaxation time) is the time constant which determines the rate at which excited protons return to equilibrium. It is a measure of the time taken for spinning protons to realign with the external magnetic field

T2 (transverse relaxation time) is the time constant which determines the rate at which excited protons reach equilibrium or go out of phase with each other. It is a measure of the time taken for spinning protons to lose phase coherence among the nuclei spinning perpendicular to the main field.

PD-weighted image, is the tissues with the higher concentration or density of protons (hydrogen atoms) which produce the strongest signals and appear the brightest on the image

Below is the Tissue differentiation link:

https://mrimaster.com/characterise%20image%20pd.html

Refer the figure (Fig.(a)(b)(c)) and table (Table.2.5) in the attachment file - RG ans TI T2 PD.pdf

1) The strength of the MRI signal and the contrast between brain tissues depend primarily on three parameters, PD, T1 and T2 (Atlas, 2002). The greater the density of protons, the larger the signal will be. For most "soft" tissues in the body, the proton density is very homogeneous and therefore does not contribute in a major way to signal differences (see Fig.(c)). However, T1 and T2 can be dramatically different for different soft tissues, and these parameters are responsible for the major contrast between soft tissues as shown in Fig.(a) and (b). The time parameters T1 and T2 are strongly influenced by the viscosity or rigidity of a tissue. The greater the viscosity and rigidity, the smaller the value for T1 and T2. The relaxation time and image contrast of tissues of brain structures are summarized in Table 2.5 (Woodward, 2001).

2) The relaxation times mentioned as long, intermediate and short in Table 2.5 are approximately equivalent to 2200-2400 ms, 900 ms, 780 ms respectively for T1 images and 500-1400 ms, 100 ms, 90 ms respectively for T2 images in a steady magnetic field of 1.5 Tesla (Dhawan, 2003).

3) WM appears a light gray in T1 and a dark gray in T2 images. GM appears gray in both images. The CSF appears black in T1 and white in T2 images. The background of the image (air), dense calcification, fibrous tissue and flowing blood in spin echo (SE) sequence typically provide little to no signal on MR images and thus appear dark on both T1 and T2 sequences. T1 images are typically used for anatomic information, as they are also highly sensitive for paramagnetic contrast media, fat, fluids with high protein content and subacute haemorrhage. T2 images offer high sensitivity to most pathologic processes. A prolongation of T2, which provides high signal intensities on long TR and long TE images, is seen with edema, infarction, demyelination, infection, neoplasm and most fluid collections (Edelman et al., 1996).

Reference:

“Brain Portion Extraction and Brain Abnormality Detection from Magnetic Resonance Imaging of Human Head Scans”, Pallavi Publications South India Pvt. Ltd., 2011 (ISBN: 978-93-80406-76-3).

Thank you all for your detailed answers.

https://www.radiologycafe.com/radiology-trainees/frcr-physics-notes/t1-t2-and-pd-weighted-imaging