Modalities in modern radiology: A synopsisD Sanghvi1, MG Harisinghani2
1 Department of Radiology, Kokilaben Dhirubhai Ambani Hospital, Mumbai, India
2 Harvard Medical School, Director Abdominal MRI, Massachusetts General Hospital, Boston, MA 02114, USA
While the radiological armamentarium encompasses multiple modalities that can be used to image the patient, the choice of using one or a combination of multiple modalities is dependent on the information that is required. It is important for the practicing physician to be familiar with the advantages and limitations of each modality to request their use in the clinical context. The ensuing articles will highlight the use of these modalities in different primary pathologies.
This brief overview is meant to familiarise the practising clinician with various radiological techniques.
In November 1895, X-rays were inadvertently discovered by the German physicist Wilhelm Conrad Roentgen while he was assessing the properties of a cathode ray tube when an electrical discharge is passed through it. Roentgen saw the very first radiograph, his own flickering ghost-like skeleton on the barium platinocyanide screen. He subsequently reported that it was at this time that he decided to carry on his experiments in secrecy, being concerned for his professional credibility in the event that his observations were incorrect. Roentgen provisionally labelled the new rays as x-rays using the mathematical label for something unidentified. Although these new rays would eventually be named after him in several languages; where they are known as Rφntgen Rays, he in fact preferred the name X-rays. About two weeks following his discovery, he obtained the very first radiographic image using X-rays of his wife's hand, Anna Bertha. When she perceived her skeleton she exclaimed that she had witnessed her death ! Roentgen's initial manuscript, "On A New Kind Of Rays" (άber eine neue Art von Strahlen ), was published two months later on 28 December 1895. , Today, Wilhelm Conrad Roentgen is honoured as the father of diagnostic radiology.
Conventional radiography has developed greatly since the inadvertent discovery of X-rays. Most conventional radiography systems have been replaced by digital radiography equipment which convert X-ray images to electronic data that can be studied using a monitor and archived on a computer disk. Digital techniques permit the radiographs to be viewed instantaneously, additionally allow specific areas of the image to be enlarged, and the contrast of the images can be manipiulated to provide greater visibility of the abnormality. In spite of rapid strides in more advanced imaging modalities, radiographs continue to be customarily obtained for certain clinical indications. The most common and possibly useful indication for obtaining X-rays is to detect pathologies of the skeletal system; mostly the bones but sometimes also the adjacent soft tissues. It remains the first modality for evaluating fractures and for the initial demonstration and subsequent characterisation of bone tumors. The ubiquitous chest X-ray is the bastion of imaging for pulmonary diseases including pneumonia and lung cancers. In certain settings, the use of X-rays is debatable, such as assessing bowel obstruction and detection of renal calculi, which may not always be visible. Also, traditional radiographs provide little or no information in the radiological demonstration of soft tissues such as the brain, spinal cord or muscle. Radiological alternatives for soft tissues are Ultrasound, magnetic resonance imaging (MRI) and computed tomography (CT).
It is imperative for the practicing clinician to be conversant with the issues of radiation safety while ordering for X-rays. It is vital to use X-rays judiciously in view of their carcinogenic potential. Radiologists and X-ray technologists as well as clinicians should be educated and be aware to use the least amount of radiation necessary to acquire the needed results.The use of X-rays is contraindicated in pregnancy considering its teratogenic effects and use in the pediatric population must also be regulated. Radiotherapy in Oncology utilises higher energies of radiation.
Sir Godfrey Hounsfield invented the computed tomographic scanner, and thereby made an exceptional contribution to medicine. Employing his engineering background, he conceived the idea of CT during a weekend ramble in 1967. At first it was unrelated to medicine when he had the simple realisation that one could deduct the contents of a box by obtaining readings at various angles traversing through it.  CT was at first labelled as the EMI scan and was developed by Hounsfield in a research branch of EMI, a company well known for its music recordings. It has been suggested that owing to the commercial successes of the Beatles, EMI was able to subsidize research and create rudimentary models for medical use. As a direct consequence of the Beatles' sensational popularity and success, the scanner's inventor, Geoffrey Hounsfield, could dedicate almost four years expanding the scanner from the 1968 prototype, to equipment that could be utilised in a clinical setting. The recording firm of the Beatles had sold almost 200 million copies of the Fab Four's singles, and was thus able to fund Hounsfield's research and the scanner was completed to be installed in hospitals in the 1970s.
CT is an imaging modality employing ionising radiation combined with tomography. Digital geometric processing is innovatively employed to create a 3D picture of the inside of an object from several sets of 2D X-ray images acquired using a single axis of rotation. CT generates a volume of data which may be manipulated, utilising a method called windowing, in order to reveal various structures based on their capacity to obstruct the X-ray beam. Present CT scanners permit this data to be reformatted isotropically in all planes and even as volumetric or 3D representations of structures, though historically the images available were only in the transverse or axial plane.
The launch of the helical or spiral CT scanner in 1969 and the more recent innovation of the multislice CT in 1993 has brought greater speed and better resolution. These unique attributes have have enabled CT to become the mainstay of several imaging algorithms.
Two researchers are eminent in the annals of ultrasound and medical imaging. The Austrian Karl Dussik published the preliminary manuscript on medical ultrasonics in 1942, expounding on his research regarding transmission ultrasound investigation of the brain. Subsequently, in the 1950s, the Scottish professor Ian Donald created practical technology and applications for ultrasound. Although ultrasound is used to image soft tissues of the entire body, its greatest applications are in obstetrics, to image the fetus. Since there is no ionising radiation utilised, it is safe to use in pregnancy, children and for repeat studies. It is a widely available and inexpensive investigation with accurate results in experienced hands.
Ultrasound waves are generated in a transducer. A transducer is a device that acquires power from a source, changes the energy into another form, and finally distributes the power to the final target. Here, the transducer changes electric signal into ultrasound waves, and collects the reflected waves converting them once again into electrical signals. The electrical signals obtained from the transducer are utilised to create images on a monitor.
In the mid-1800s, the Austrian physicist Christian Doppler was the foremost to illustrate the frequency shift that happens when sound or light is emitted from a moving source and this effect is now named after him. Ironically, Doppler's discovery had no realistic applicability during his own life but almost a century later had implications on medicine and also metereology and cosmology.  Doppler ultrasound is currently employed for non-invasive velocity measurements of blood flow thereby generating data regarding arterial stenoses. It is also often used to demonstrate venous thrombosis. The subsequent development of real-time 2D images fused with colour Doppler flow imaging to designate blood or tissue velocity allows assessment and recognition of flow patterns and also facilitates recognition of anatomy and various pathologies. The more recent development of power Doppler demonstrates areas of tissue perfusion and is not dependent on flow direction and velocity.
For a Positron Emission Tomography (PET) study, a shortlived radioactive tracer isotope is administered intravenously. The tracer is chemically incorporated into a biologically active molecule, and finally decays thereby emitting a positron. The patient is imaged following a waiting period at which time the active molecule concentrates within the tissues of interest. The molecule most often employed for this study is a sugar called fluorodeoxyglucose (FDG).
When the radioisotope undergoes positive beta decay or positron emission decay, it emits a positron which is a particle that possesses the opposite charge of an electron. Subsequent to moving a few millimeters, the positron comes across and annihilates with an electron, generating a pair of annihilation or gamma photons that travel in opposite directions. These are perceived when they reach a scintillator material within the scanner, generating a burst of light that is detected by photomultiplier tubes better known as silicon avalanche photodiodes (Si APD). This technique is thus centered on the simultaneous or the coincident detection of a photon pair. On the other hand, photons that do not arrive in pairs during a timing window of a few nanoseconds, are ignored.To summarise, PET imaging involves the perception of a pair of gamma rays emitted indirectly by a positron-emitting tracer that is injected with a biologically active molecule. Pictures of tracer concentration within a 3D space within the human body are subsequently reconstructed by computer analysis. In a PET/CT machine, this reconstruction is often accomplished with the help of a CT scan performed on the patient at the same time, in the same equipment
The initial commercial PET scanner was launched in 1975. Later in the 1970s and and in the 1980s, PET was primarily employed in research. In the early 1990s, PET was initiated into clinical practice. The first PET/CT scanner was instituted in 2000. This hybrid technology marries two concepts, simultaneously revealing both molecular function and anatomical information, providing inclusive information regarding the metabolism and anatomical sites of cancer.
The initial PET/CT scanners incorporated a single-slice spiral CT integrated with a PET camera which employed BGO detectors. In the present time, the configurations have changed radically. It is presently possible to choose between a dual-slice CT scanner incorporated with an advanced PET camera having the novel and much faster LSO crystals or alternatively a clinically advanced 16-row CT scanner with the very same sophisticated LSO PET equipment.
The most frequently used PET radiopharmaceutical - 2-Deoxy-2-[ 18 F] fluoro-D-Glucose or FDG is a radioactive form of sugar.Though the use of this particular tracer is the most popular type of PET scan, other tracer molecules are employed in PET to study the tissue concentration of several other kinds of molecules of interest.
Emphasising the fundamental applicability and importance of Magnetic Resonance Imaging (MRI) in medicine, Paul Lauterbur  from the University of Illinois at Urbana-Champaign and Peter Mansfield of the University of Nottingham were honoured with the Nobel Prize in Physiology or Medicine in 2003. Over the last two decades, Fourier transform imaging techniques have greatly hastened the development of MRI. 
The basic principle of MRI being nuclear resonance, it was at first termed NMR which stands for nuclear magnetic resonance. However, due to anxiety regarding the use of the term "nuclear", it is now popularly referred to as magetic resonance imaging or MRI. When a person is in a MRI scanner, the hydrogen nuclei or protons found freely in the human body in water molecules align with the powerful main magnetic field. Another electromagnetic field, that oscillates at radiofrequencies and is perpendicular to the main field, is then pulsed to modify some of the protons out of alignment with reference to the main field. These protons subsequently drift back into alignment with the main field, emanating a detectable radiofrequency signal in the process. Protons in diverse tissues of the human body (e.g., fat versus bone) realign at differing speeds and thus the various structures of the body can be demonstrated.
As MRI is dependent on inherent proton composition, it is able to offer spatial and soft tissue contrast resolution which is superior to other imaging modalities. This trait together with the ability to image in many planes has made MRI the problem-solving modality for assessment of pathologies in diverse anatomical sites. Using the intravenous contrast agent gadolinium with MRI permits evaluation of enhancement characertistics. The limiting factor for modern MRI machines is the time taken to assess a particular anatomic area. So, for example, while CT can demonstrate the abdomen and pelvis in a matter of a few seconds, modern MRI equipment takes around 30-45 min to completely evaluate the same anatomic area. This restricts the widespread use of MRI as a screening modality. The scanners are nevertheless getting faster and may prevail over the time constraint in the near future.
MRI signal intensity is related to many parameters, which includes proton density, T1 and T2 relaxation times. Different pathologies can be assessed by the appropriate choice of pulse sequence parameters. Repetition time (TR) is the time flanked by two successive RF pulses measured in milliseconds. For a certain nucleus in a given setting, TR determines the amount of T1 relaxation. Echo time or TE is the time from the initiation of an RF pulse to the generation of the MR signal. TE decides how much decay of the transverse magnetisation is permitted to occur before the signal is read. It thereby controls the extent of T2 relaxation. The application of RF pulses at varying TRs and the receiving of signals at different TEs generates differences in contrast in MR images.
More recently, with further sophistication in MRI technology, there has been a shift in imaging, from merely providing anatomical information to providing information regarding physiology. These recent developments include diffusion and perfusion imaging, MR spectroscopy, tractography or diffusion Tensor Imaging (DTI) and Functional MRI.
The last few decades have witnessed dramatic innovations and improvisations in imaging technology. Historically, from the discovery of X-rays by Roentgen in 1895, to the introduction of MRI by Damadian in 1969, radiological advances have revolutionised the practice of modern medicine.
Imaging now uses a wide gamut of modalities that vary in their mode of image acquisition. In order to request the correct imaging technique and thereby improve patient management, it is useful for the practising clinician to be conversant with all imaging techniques available, their advantages as well as limitations, indications and contraindications of each modality.