Dual Energy CT (2)

One Basic Reason for Use of Dual Energy CT: Material Differentiation

• By scanning a patient at two different energy spectra (on an example above, at 56 kV and 76 kV), the attenuation difference of the same material is different.
• Iodine has higher attenuation difference, compared to bone
• With this nature, scanning allows the computer to process bone and iodine content on images differently.

Routine Use of Dual-energy CT for Material Differentiation

• Creation of 3D vascular images ("Direct Angio") by easy removal of bony structures
• Plaque analysis (calcified vs. soft plaques)
• Lung perfusion
• Virtual unenhanced scan (creation of unenhanced scan from enhanced images by deleting iodine content from the images)
• Calculi characterization (uric acid vs. others)

Reference: Fletcher JG, Takahashi N, Hartman R, et al. Dual-energy and dual-source CT: is there a role in the abdomen and pelvis? Radiol Clin N Am 2009; 47:41-57.

Dual Energy CT (1)

A Growing Need for a New Kind of CT

• We need a better delineation of tissue components than just simple Hounsfield Units
• We need to scan faster to image the beating heart (decrease temporal resolution), esp. in cases with high heart rate, irregular heart rate

What Is Dual Energy CT?

• CT that uses the data of two different energy spectra to display anatomy and physiology (regular CT scans utilize one energy spectrum to create images)
• Dual-source CT describes CT that has two x-ray sources and two detectors mounted on a single gantry. The x-ray sources can produce two different energy spectra, or one spectrum. Dual-source CT can run in either dual- or single-energy mode

What Are Possible Hardware Configurations for Dual Energy CT?

• Two x-ray sources (dual-source CT)
• Rapid kV switching (single-source CT)
• Sandwich detector CT (two layers of detectors detecting x-rays at different energy spectra)

Image credit: medical.siemens.com

Reference:

Fletcher JG, Takahashi N, Hartman R, et al. Dual-energy and dual-source CT: is there a role in the abdomen and pelvis? Radiol Clin N Am 2009; 47:41-57.

CT Slice Thickness

Single Detector Array Scanners

• Determined by physical collimation of the x-ray beam (green structure in the picture)
• As this gap widens, the slice thickness increases
• Upper limit of slice thickness depends on the width of the detector

Multiple Detector Array Scanners

• Determined by width of the detectors (between blue lines in the picture)
• Width of the detectors can be changed by 'binning' different numbers of individual detector elements together
• Can be used in conventional axial scanning (no table movement) and in helical scanning protocols

Tradeoffs Respect to Slice Thickness

• Number of detected x-ray photons increases linearly with slice thickness, for scans performed at the same kV and mAs
• As number of detected x-ray photons increases, the signal-to-noise ratio (SNR) increases.
• Higher SNR means better contrast resolution
• Example: going from a 1-mm to 3-mm slice thickness triples the number of detected photons, and the SNR increases by square root of 3 = 73%
• Increased slice thickness will reduce spatial resolution and increase volume averaging in the thickness dimension

References:

Bushberg, et al. Essential Physics of Medical Imaging, 2nd ed, 2001

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Acoustic Noise in MRI

Acoustic (Sound) Noise in MRI

• One of the most disturbing obstacles for patients receiving an MRI particularly children and psychiatric patients
• Interferes with communication between MRI technologists and patients
• Interferes with functional MRI studies by producing unwanted stimulus. Noise may introduce changes in oxygenation in cortex and blood capillaries that may in turn affect signal receiving during fMRI.
• They were anecdotal reports of temporary hearing loss related to MRI, but these have never been proven.
• Depends on pulse sequence, types of scanner (coil structures and coil supports)
• Average relative noise = 94-107 dB (in MRI bore) and 87-98 dB (in MR scanning room)
• Noise from gradient recalled echo (GRE) sequences typically is louder than spin echo (SE) and echo planar imaging (EPI).

Etiology

• Main source: Lorentz forces acting on gradient coils and pulsing particularly two gradients along x- (frequency encode) and y- (phase encode) axes
• Other sources: vibrations of conducting cryostat inner bore of MR machine due to eddy current, vibrations of radiofrequency body coil, etc (including several "unknown" pathways)

Image from www.noise-busters.com

References:

1. Cho ZH, Park SH, Kim JH, et al. Analysis of acoustic noise in MRI. Magn Reson Imaging 1997;15:815-822.

2. Edelstein WA, Hedeen RA, Mallozzi RP, et al. Making MRI quieter. Magn Reson Imaging 2002; 20:155-163.

Hounsfield Unit

What is Hounsfield Unit?

• CT number
• Unit used to measure attenuation of tissue on CT
• A representative of relative attenuation coefficient of tissues, using water as a reference
  
• Water is defined as zero Hounsfield unit
• Matters with higher physical density (g/cm3), higher electron density (e/cm3) than water will have higher HU; and vice versa
• HU depends on kV peak (used in CT image acquisition) and filtration technique (used in CT image reconstruction)
• HU values on an image are only approximate (because it will change if kV changes)
  

Who is Hounsfield?

• Godfrey Hounsfield is a British electrical engineer
• He shared a 1979 Nobel Prize in physiology or medicine with Allan Cormack for the development of computed tomography (CT)

Reference:
Huda W, et al. Review of radiology physics. 2003
www.wikipedia.com