(SE) The most common pulse sequence used in MR imaging is based of the detection of a spin or Hahn echo. It uses 90° radio frequency pulses to excite the magnetization and one or more 180° pulses to refocus the spins to generate signal echoes named spin echoes (SE).
In the pulse sequence timing diagram, the simplest form of a spin echo sequence is illustrated.
The 90° excitation pulse rotates the longitudinal magnetization (Mz) into the xy-plane and the dephasing of the transverse magnetization (Mxy) starts.
The following application of a 180° refocusing pulse (rotates the magnetization in the x-plane) generates signal echoes. The purpose of the 180° pulse is to rephase the spins, causing them to regain coherence and thereby to recover transverse magnetization, producing a spin echo.
The recovery of the z-magnetization occurs with the T1 relaxation time and typically at a much slower rate than the T2-decay, because in general T1 is greater than T2 for living tissues and is in the range of 100-2000 ms.
The SE pulse sequence was devised in the early days of NMR days by Carr and Purcell and exists now in many forms: the multi echo pulse sequence using single or multislice acquisition, the fast spin echo (FSE/TSE) pulse sequence, echo planar imaging (EPI) pulse sequence and the gradient and spin echo (GRASE) pulse sequence;; all are basically spin echo sequences.
In the simplest form of SE imaging, the pulse sequence has to be repeated as many times as the image has lines.
Contrast values:
PD weighted: Short TE (20 ms) and long TR.
T1 weighted: Short TE (10-20 ms) and short TR (300-600 ms)
T2 weighted: Long TE (greater than 60 ms) and long TR (greater than 1600 ms)
With spin echo imaging no T2* occurs, caused by the 180° refocusing pulse. For this reason, spin echo sequences are more robust against e.g., susceptibility artifacts than gradient echo sequences.

See also Pulse Sequence Timing Diagram to find a description of the components.

(IR) The inversion recovery pulse sequence produces signals, which represent the longitudinal magnetization existing after the application of a 180° radio frequency pulse that rotates the magnetization Mz into the negative plane. After an inversion time (TI - time between the starting 180° pulse and the following 90° pulse), a further 90° RF pulse tilts some or all of the z-magnetization into the xy-plane, where the signal is usually rephased with a 180° pulse as in the spin echo sequence. During the initial time period, various tissues relax with their intrinsic T1 relaxation time.
In the pulse sequence timing diagram, the basic inversion recovery sequence is illustrated. The 180° inversion pulse is attached prior to the 90° excitation pulse of a spin echo acquisition. See also the Pulse Sequence Timing Diagram. There you will find a description of the components.
The inversion recovery sequence has the advantage, that it can provide very strong contrast between tissues having different T1 relaxation times or to suppress tissues like fluid or fat. But the disadvantage is, that the additional inversion radio frequency RF pulse makes this sequence less time efficient than the other pulse sequences.

Contrast values:
PD weighted: TE: 10-20 ms, TR: 2000 ms, TI: 1800 ms
T1 weighted: TE: 10-20 ms, TR: 2000 ms, TI: 400-800 ms
T2 weighted: TE: 70 ms, TR: 2000 ms, TI: 400-800 ms

See also Inversion Recovery, Short T1 Inversion Recovery, Fluid Attenuation Inversion Recovery, and Acronyms for 'Inversion Recovery Sequence' from different manufacturers.

Contrast enhanced MRI is a commonly used procedure in magnetic resonance imaging. The need to more accurately characterize different types of lesions and to detect all malignant lesions is the main reason for the use of intravenous contrast agents.
Some methods are available to improve the contrast of different tissues. The focus of dynamic contrast enhanced MRI (DCE-MRI) is on contrast kinetics with demands for spatial resolution dependent on the application. DCE-MR imaging is used for diagnosis of cancer (see also liver imaging, abdominal imaging, breast MRI, dynamic scanning) as well as for diagnosis of cardiac infarction (see perfusion imaging, cardiac MRI). Quantitative DCE-MRI requires special data acquisition techniques and analysis software.
Contrast enhanced magnetic resonance angiography (CE-MRA) allows the visualization of vessels and the temporal resolution provides a separation of arteries and veins. These methods share the need for acquisition methods with high temporal and spatial resolution.
Double contrast administration (combined contrast enhanced (CCE) MRI) uses two contrast agents with complementary mechanisms e.g., superparamagnetic iron oxide to darken the background liver and gadolinium to brighten the vessels. A variety of different categories of contrast agents are currently available for clinical use.
Reasons for the use of contrast agents in MRI scans are:
Common Indications:
Brain MRI : Preoperative/pretreatment evaluation and postoperative evaluation of brain tumor therapy, CNS infections, noninfectious inflammatory disease and meningeal disease.
Spine MRI : Infection/inflammatory disease, primary tumors, drop metastases, initial evaluation of syrinx, postoperative evaluation of the lumbar spine: disk vs. scar.
Breast MRI : Detection of breast cancer in case of dense breasts, implants, malignant lymph nodes, or scarring after treatment for breast cancer, diagnosis of a suspicious breast lesion in order to avoid biopsy.

For Ultrasound Imaging (USI) see Contrast Enhanced Ultrasound at Medical-Ultrasound-Imaging.com. See also Blood Pool Agents, Myocardial Late Enhancement, Cardiovascular Imaging, Contrast Enhanced MR Venography, Contrast Resolution, Dynamic Scanning, Lung Imaging, Hepatobiliary Contrast Agents, Contrast Medium and MRI Guided Biopsy.

Lung imaging is furthermore a challenge in MRI because of the predominance of air within the lungs and associated susceptibility issues as well as low signal to noise of the inflated lung parenchyma. Cardiac and respiratory triggered or breath hold sequences allow diagnostic imaging, however a comparable image quality with computed tomography is still difficult to achieve.
Assumptions for lung MRI:
The current trends in MRI are the use of new imaging technologies and increasingly powerful magnetic fields. Among these technologies are parallel imaging techniques as well as ventilation agents like hyperpolarized helium for the use as an inert inhalational contrast agent to study lung ventilation properties. With hyperpolarized gases clear images of the lungs can be obtained without using a large magnetic field (see also back projection imaging). Single shot sequences (e.g. TSE or Half Fourier Acquisition Single Shot Turbo Spin Echo HASTE) used in lung MR imaging benefits from parallel imaging techniques due to reduced relaxation time effects during the echo train and therefore reduced image blurring as well as reduced motion artifacts.
In the future, more effective contrast agents may provide an alternative solution to the need for high field MRI. Dynamic contrast enhanced MRI perfusion has demonstrated a potential in the diagnosis of pulmonary embolism or to characterize lung cancer and mediastinal tumors. 3D contrast enhanced magnetic resonance angiography of the thoracic vessel.

See also the related poll result: 'MRI will have replaced 50% of x-ray exams by'