Horizontal comparison: the difference between MRI T1 and T2

1. T1 is better for observing anatomical structures.

2. T2 shows better tissue lesions.

3. Water has long T1 and long T2, while fat has short T1 and short T2.

4. Long T1 is black and short T1 is white.

5. Long T2 is white and short T2 is black.

6. Water T1 is black, T2 is white.

7. Fat T1 is white and T2 is grayish white.

8. T2 is sensitive to bleeding because water T2 is white.

T1-weighted imaging, T2-weighted imaging

The so-called weighting means "highlighting"

T1-weighted imaging (T1WI) ---- highlight differences in tissue T1 relaxation (longitudinal relaxation)

T2-weighted imaging (T2WI)----highlights differences in tissue T2 relaxation (transverse relaxation).

In any sequence of images, the larger the transverse magnetization vector at the time of signal acquisition, the stronger the MR signal.

T1-weighted image short TR, short TE - T1-weighted image, T1 image characteristics: the shorter the T1 of the tissue, the faster the recovery and the stronger the signal; the longer the T1 of the tissue, the slower the recovery and the weaker the signal.

T2-weighted image: long TR, long TE - T2-weighted image, characteristics of T2 image: the longer the T2 of the tissue, the slower the recovery and the stronger the signal; the shorter the T2 of the tissue, the faster the recovery and the weaker the signal.

Proton density weighted image: long TR, short TE - proton density weighted image, image characteristics: the greater the rH of the tissue, the stronger the signal; the smaller the rH, the weaker the signal. Brain white matter: 65% Brain gray matter: 75% CSF: 97%

Characteristics of conventional SE sequences

The most basic and most commonly used pulse sequence.

Obtain standard T1 WI and T2 WI images.

T1 WI is good for observing anatomy.

T2 WI is good for observing lesions and is more sensitive to bleeding. It has relatively few artifacts (but due to the long imaging time, the patient is prone to movement). The imaging speed is slow.

FSE pulse sequence

Principle: FSE pulse sequence applies multiple 1800 complex phase pulses after a 900 pulse, obtains multiple echoes and performs multiple phase encodings, that is, completes the data acquisition of multiple K-space lines within one TR interval, greatly shortening the scanning time.

Images with different weighted properties at the same level are obtained in one imaging.

T1WI - short TE, 20ms short TR, 300~600ms ETL - 2~6

T2WI——Long TE, 100 Long TR, 4000 ETL—8~12

Advantages: short time, can show lesions. Disadvantages: insensitive to bleeding, many artifacts, etc.

IR sequence characteristics

The IR sequence has strong T1 contrast characteristics;

TI can be set to saturate specific tissues to produce characteristic contrast images (STIR, FLAIR);

Short TI contrast is often used for neonatal brain imaging;

The collection time is long and the number of levels is relatively small.

STIR sequence (Short TI Inversion Recovery

During the IR recovery process, the MZ of the tissues must pass through the 0 point, but at different times. This feature can be used to suppress a certain tissue. For example, fat, because its T1 time is shorter than other tissues, TI=0.69T1 (T1 is the fat relaxation time), the signal of fat is better than the 0 point, and its signal cannot be received. Other tissues are highlighted.

FLAIR sequence

When T1 is very long, the MZ of almost all tissues has been restored, and only the MZ of tissues with very long T1 is close to 0, such as water. The liquid signal is suppressed, thus standing out from other tissues. FLAIR (Fluid Attenuation IR) is often used to suppress CSF.

Application of IR sequence

T1 weighting of brain IR can make the contrast of gray and white matter greater. STIR of the orbit can suppress fat signal, increase T2 contrast, and better display the posterior eyeball and optic nerve. The use of FLAIR technology in the spinal cord can suppress the artifacts produced by cerebrospinal fluid pulsation, so as to facilitate the display of cervical and thoracic spinal cord lesions. Microlesions in the liver can be better displayed using IR. Using IR on joints can increase the sensitivity of water and cartilage at the same time.

The residual transverse magnetization vector is "destroyed". After data acquisition, a "destroyed" gradient is applied along the selected gradient direction of the slice, and the residual transverse magnetization vector is used to accelerate the dephasing, thereby eliminating the residual transverse magnetization of the previous cycle.

Clinical Application of MRA

Intracranial vascular MRA

3D-TOF

3D-PC is used for displaying arteries, veins and complex blood flow, and it takes a long time

2D-TOF sagittal sinus and other slow flow display

2D-PC can also be used for sagittal sinus imaging and flow velocity prediction

Neck vascular MRA

Multi-layer 2D-TOF, 2D, 3D-PC for arterial and venous display

Chest vascular MRA

CE-MRA of the aorta and its branches, pulmonary artery and vein

2D and 3D-TOF for aorta display

2D-PC plus ECG synchronization technology is often used for aortic flow analysis

Abdominal vascular MRA

CE-MRA is preferred

3D-TOF and PC can be used for renal artery

Limb Vascular MRA

3D-CE-MRA is good at showing the arterial and venous phases of the blood vessels in the limbs.

2D-TOF can also be used to display blood vessels in the limbs

The commonly used contrast agent is gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA), which is much safer than iodine-containing contrast agents.

The disease is differentially diagnosed based on the presence or absence of enhancement, the degree of enhancement, and the type of enhancement.