Introduction to Medical Imaging Systems

1 Chapter 1 Introduction to Medical Imaging SystemsContentsIntroduction .. philosophy .. Imaging Systems overview .. ideal Medical Imaging modality .. to primary modalities .. Imaging .. Imaging .. magnetic resonance Imaging (NMR) or MRI .. Imaging .. comparison .. modalities .. begin with a brief overview of what is meant byimaging, some history and philosophy, and a survey of the modalitiesthat willbe books have been written on Medical Imaging [1 6], Medical physics and instrumentation [7 9], Imaging more generally[10 12], tomography and image reconstruction [13 18], MRI[19 22], and many other related topics.(A few of these are on reserve at Engineering Library for EECS516.) J. Fessler, August 2, 2009, 21:21 (student version) Heritage Dictionary, 3rd edition: A reproduction of the form of a person or an object, especially a sculptured likeness.

2 Physics. An optically formed duplicate, counterpart, or otherrepresentative reproductionof an object, especially an opticalreproduction of an object formed by a lens or mirror. One that closely or exactly resembles another; a double: He is the image of his uncle..History Prior to the development of Medical Imaging , doctors diagnosed and treated patients without being to see their insides, exceptthrough exploratory surgery. Medical Imaging dates to the discovery ofX-raysby William R ontgen on Nov. 8, after their discovery, X-rays were used to produce shadowgrams of the body. 1896 Antoine Henri Becquerel discoversradioactivityaccidentally, while investigating phosphorescence in uranium salts. 1898 Pierre and Marie Curie (PhD student of Becquerel) discover radium and polonium (named after Marie s native Poland)Marie got her PhD 5 years later in 1903. That same year she and Pierre and Becquerel shared the Nobel prize in Physics!

3 She got a 2nd Nobel Prize in Chemistry in 1911 for her radium purification work. After an initial flurry of activity in the early 20th century culminating in the invention of the first practicalX-ray tubebyCoolidge in 1913, Radiography ( , the use of X-rays to image internal organs) progressed slowly until the early 1960 s. Since the 1960s, with the invention ofX-ray computed tomography(which lead to the 1979 Nobel Prize for Physiology andMedicine to Cormack and Hounsfield), radiographic applications have increased enormously. For a fascinating history of X-ray Imaging , see [23]. Starting in the 1970s and continuing to the present, many other modalities have been applied to Medical Imaging :Ultrasound, magnetic resonance Imaging (MRI), andNuclear Medicine:SPECTandPET Since the 1970s, two other modalities have been investigated with limited commercial success: Microwave Imaging [24]Why?

4 ?? Visible Light Imaging (or near visible)Why???(Nevertheless, both remain active research areas andoptical imagingis seeing increased interest.) Two other modalities have been used with some success for a limited set of applications:Resistive Imaging ( , low-frequency EM), andThermal Imaging (surface or near surface features) The general technical development of Medical Imaging sincethe mid 1960s has followed two paths: More sophisticated Systems and applications using conventional modalities. These improvements have been based primarilyon the availability of more powerful computers and electronics, , helical scan CT, fully 3D PET. Identification of new ,photo-acoustic Imaging , or major variants of old modalities, ,Doppler ultrasound. Today, Medical Imaging is a vibrant field where innovative researchers continually discover new ways to improve imagequality and explore novel philosophyMedical Imaging is a very interdisciplinary field, and uses concepts from mathematics, physics, statistics, engineering, biology,and medicine.

5 Obviously a single course cannot cover all aspects of all modalities!The focus in this course will be the Systems aspects as follows. The basic physics governing each Imaging modality will be developed as needed to understand Imaging principles. Sources and detectors will be described phenomenologically as part of the Introduction to each modality. A system model of each Imaging system will be developed. The basic Imaging equations will be derived. Differences in noise characteristics and image artifacts due to physical differences in the basic physics will be identified. Fundamental similarities between the Imaging equations ofdifferent modalities will be stressed. For example, we willsee howFourier reconstruction of NMR and CT data is very similar to lens forming in phased array ultrasound. Fundamental differences between data from different modalities will also be J.

6 Fessler, August 2, 2009, 21:21 (student version) Imaging Systems overview Medical Imaging Systems are based on the physical interaction between some energy source and the human body. (Exceptions,such asphonocardiographyandthermography, that useinternalenergy sources within the body are rare and represent veryfew applications). The following figure gives a generic block diagram of a typical modern electronic Medical Imaging J. Fessler, August 2, 2009, 21:21 (student version) this course our view of a Medical Imaging system will look more like one of the FunctionOutput ImageInput ImageDesign ParametersSf(x, y)g(x, y)f(x, y, z)g(x, y, z)g=S[f]Design ParametersOutput ImageImageFormationDesign ParametersInput ImageRaw DataSystem FunctionInstrumentSf(x, y)g(x, y)f(x, y, z)g(x, y, z)g=S[f]The input image is also called the object being output image is often routed to image processing to enhance visualization or to quantify object each modality, our primary goals will be to Understand whatf(x, y)represents physically (with minimal discussion of the Medical relevance off(x, y)).

7 Understand howg(x, y)is formed from the acquired raw data. DetermineSor equivalently the associatedpoint spread function(PSF). Examine howSchanges with the various instrumentation design parameters and image formation design system is to be useful, the outputgshould be a representative reproduction of the ideal Medical Imaging modalityBefore introducing the specifics of the common Medical Imaging modalities, it is useful to consider the following are are the characteristics of an ideal Medical imagingsystem? Unfortunately, no single modality provides all of these, hence modern radiology departments are equipped with severaldifferenttypes of Systems , each with their own strengths and J. Fessler, August 2, 2009, 21:21 (student version) to primary modalitiesThis course emphasizes the following modalities. Basic Radiography and Fluoroscopy(Transmission of X-rays through the body) X-ray Computed Tomography (CT) Nuclear Medicine (SPECT, PET) (Emission of -rays from decaying radioisotopes deposited ( , injected) into the body) magnetic resonance Imaging (MRI)(Concentration and decayparameters of resonant1H nuclei) Ultrasound(Reflection of ultrasonic pulses transmitted into the body)The general principles and methods of analysis that we use also apply to the many other Medical Imaging modalities, and tonon- Medical Imaging as will discuss image post-processing methods little (if atall)

8 , because they generally are less effective than makingimprovementsto the Imaging method imaging10 410 210010 1010 810 610 410 2100 Wavelength [Angstroms]TransmissionTransmission of EM Waves Through 25cm of Soft TissueDiagnosticX rays10 210010210 1010 810 610 410 2100 Wavelength [meters] T MRI H2O63 MHz = TV 3 The figure above, adapted from [1, Fig. ], graphs the transmission coefficient through 25 cm of tissue (roughly the diameter ofa head) as a function of the EM free space constants for interpreting that figure. 1 A=10 8cm =10 10m = 100 pm c= 103(speed of light in m/s) h= 10 27(Planck s constant in erg sec) 10 12erg = 1 eV (electron volt)To convert wavelength in meters to photon energyEin eV, useE=h(c/ )/( 10 12)So A= 1 10 12m = 1 pm is associated with a photon energy of about MeV, A= 5 10 11m = 50 pm is associated with a photon energy of about 25 keV.

9 At long wavelengths the attenuation is manageable, but the resolution is very poor. At a free space wavelength of 1 cm, the wavelength in the body is about 1 , the transmission coefficient through 25 cm of tissue at this wavelength is only about10 , direct absorption of EM waves up to microwavefrequencies is not very useful for Medical Imaging . At wavelengths in the X-ray region (1-50 pm, corresponding to photon energies of 25 keV - 1 MeV), attenuation is , in this region wavelengths are much shorter than typical image resolutions of mm - 10 mm, which ensures thatdiffraction will not distort the Imaging system and that allrays travel in straight lines. (Making a lens to focus X-raysis hard.) At -ray wavelengths (<1 pm) the body is essentially transparent, providing no contrast and, therefore, no possibility of highc J. Fessler, August 2, 2009, 21:21 (student version) transmission Imaging .

10 Radiographic Imaging , including conventional shadowgrams, fluoroscopy, and CT scans, is fundamentally transmission imag-ing. Contrast in these images is provided by the differential absorption of X-rays among different tissues. These images displayanatomy, often to exquisite imagingIn nuclear Imaging , biologically important chemicals are labeled with radioactive materials, where the specific labeled compoundis chosen depending on the disease or physiological processof interest. Through nuclear decays, high energy -rays are emittedand detected. Depending on the photon energy, a great many ofthe photons are absorbed or scattered in the tissue, a processcalledattenuation. Because of this attenuation, and because of the high (relative to X-ray) energies, a relatively smaller numberof photons are available for detection at the same patient dose. Consequently, the counting statistics are poor, resulting in noisyimages with unspectacular resolution.