(J) The SI unit of work or energy.
Definition: The work done by a force of 1 newton acting to move an object through a distance of 1 meter in the direction in which the force is applied.
Since kinetic energy is one half the mass times the square of the velocity, 1 joule is the kinetic energy of a mass of two kilograms moving at a velocity of 1 m/sec.
The joule is named for the British physicist James P. Joule.

Spread in frequency of a resonance line in a MR spectrum. A common measure of the line width is full width at half maximum (FWHM).

Liver imaging can be performed with sonography, computed tomography (CT) and magnetic resonance imaging (MRI). Ultrasound is, caused by the easy access, still the first-line imaging method of choice; CT and MRI are applied whenever ultrasound imaging yields vague results. Indications are the characterization of metastases and primary liver tumors e.g., benign lesions such as focal nodular hyperplasia (FNH), adenoma, hemangioma and malignant lesions (cancer) such as hepatocellular carcinomas (HCC). The decision, which medical imaging modality is more suitable, MRI or CT, is dependent on the different factors. CT is less costly and more widely available; modern multislice scanners provide high spatial resolution and short scan times but has the disadvantage of radiation exposure.
With the introduction of high performance MR systems and advanced sequences the image quality of MRI for the liver has gained substantially. Fast spin echo or single shot techniques, often combined with fat suppression, are the most common T2 weighted sequences used in liver MRI procedures. Spoiled gradient echo sequences are used as ideal T1 weighted sequences for evaluating of the liver. The repetition time (TR) can be sufficiently long to acquire enough sections covering the entire liver in one pass, and to provide good signal to noise. The TE should be the shortest in phase echo time (TE), which provides strong T1 weighting, minimizes magnetic susceptibility effects, and permits acquisition within one breath hold to cover the whole liver. A flip angle of 80° provides good T1 weighting and less of power deposition and tissue saturation than a larger flip angle that would provide comparable T1 weighting.
Liver MRI is very dependent on the administration of contrast agents, especially when detection and characterization of focal lesions are the issues. Liver MRI combined with MRCP is useful to evaluate patients with hepatic and biliary disease.
Gadolinium chelates are typical non-specific extracellular agents diffusing rapidly to the extravascular space of tissues being cleared by glomerular filtration at the kidney. These characteristics are somewhat problematic when a large organ with a huge interstitial space like the liver is imaged. These agents provide a small temporal imaging window (seconds), after which they begin to diffuse to the interstitial space not only of healthy liver cells but also of lesions, reducing the contrast gradient necessary for easy lesion detection. Dynamic MRI with multiple phases after i.v. contrast media (Gd chelates), with arterial, portal and late phase images (similar to CT) provides additional information.
An additional advantage of MRI is the availability of liver-specific contrast agents (see also Hepatobiliary Contrast Agents). Gd-EOB-DTPA (gadoxetate disodium, Gadolinium ethoxybenzyl dimeglumine, EOVIST Injection, brand name in other countries is Primovist) is a gadolinium-based MRI contrast agent approved by the FDA for the detection and characterization of known or suspected focal liver lesions.
Gd-EOB-DTPA provides dynamic phases after intravenous injection, similarly to non-specific gadolinium chelates, and distributes into the hepatocytes and bile ducts during the hepatobiliary phase. It has up to 50% hepatobiliary excretion in the normal liver.
Since ferumoxides are not eliminated by the kidney, they possess long plasmatic half-lives, allowing circulation for several minutes in the vascular space. The uptake process is dependent on the total size of the particle being quicker for larger particles with a size of the range of 150 nm (called superparamagnetic iron oxide). The smaller ones, possessing a total particle size in the order of 30 nm, are called ultrasmall superparamagnetic iron oxide particles and they suffer a slower uptake by RES cells. Intracellular contrast agents used in liver MRI are primarily targeted to the normal liver parenchyma and not to pathological cells. Currently, iron oxide based MRI contrast agents are not marketed.
Beyond contrast enhanced MRI, the detection of fatty liver disease and iron overload has clinical significance due to the potential for evolution into cirrhosis and hepatocellular carcinoma. Imaging-based liver fat quantification (see also Dixon) provides noninvasively information about fat metabolism; chemical shift imaging or T2*-weighted imaging allow the quantification of hepatic iron concentration.

See also Abdominal Imaging, Primovistâ„¢, Liver Acquisition with Volume Acquisition (LAVA), T1W High Resolution Isotropic Volume Examination (THRIVE) and Bolus Injection.

For Ultrasound Imaging (USI) see Liver Sonography at Medical-Ultrasound-Imaging.com.

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'

When a multi shot technique is applied, each shot will have its own effect on the prepulse, with a scan time increase. Multiple shots allow a shorter IR delay but at the cost of increased scan time.
In multi shot technique (also called mosaic imaging), a group of samples, which are contiguous in k space are acquired in the same sequence repetition. The phase encoding steps or profiles are split into 'shots' (sub-acquisitions). The shot interval is the time between the shots. Usually kept as short as possible. Because the acquisitions are divided into different shots, each shot will have less T1 variation, thereby increasing T1 contrast. Two excitations, each requiring the data for one half of k-space, are the simplest variation of multi shot techniques (e.g. positive versus negative phase encoding). The alternative to this mosaic strategy for multi shot EPI is interleaving. In interleaved sequences, each repetition acquires every nth (n is the number of shots) line in k-space and for the complete raw data set the various repetition data are interlaced.

See also Single Shot Technique.