Method and system for providing a maximum intensity projection of a non-planar image

An imaging method and system generates images of non-planar portions of a three dimensional data point array wherein the non-planar portion corresponds to a non-planar object. The method includes selecting at least two different intermediate imaging planes, each selected plane including at least a portion of the object to be imaged, generating cross-sectional views perpendicular to each intermediate plane, selecting a viewing plane, projecting the cross-sectional views onto the viewing plane to generate transition value sets, and combining the transition value sets to generate values for each pixel in a display.

BACKGROUND OF THE INVENTION
 This invention relates to nuclear magnetic resonance (NMR) imaging methods
 and systems and, more particularly, to a method and apparatus for
 generating a maximum intensity projection image of a tortuous and
 non-planar vessel.
 The present invention can be used with imaging techniques (e.g. NMR,
 positron emission tomography or PET, computerized tomography or CT, etc.)
 that generate a three-dimensional data point array which is then used to
 generate an image for viewing on a two-dimensional screen. To simplify the
 explanation, the invention is described in the context of an NMR system.
 Any nucleus which possesses a magnetic moment attempts to align itself with
 the direction of the magnetic field in which it is located. In doing so,
 however, the nucleus precesses around this direction at a characteristic
 angular frequency (Larmor frequency) which is dependent on the strength of
 the magnetic field and on the properties of the specific nuclear species
 (the magnetogyric constant .gamma. of the nucleus). Nuclei which exhibit
 this phenomena are referred to herein as "spins".
 While many different tissue samples and various bodies may be examined
 using NMR imaging, the invention, for simplicity, is described in the
 context of an exemplary transaxial volume through a patient's body wherein
 the volume includes the patient's heart. This volume is herein referred to
 as a region of interest. In addition, it is assumed that an NMR imaging
 system includes a three dimensional imaging area having Cartesian
 coordinate x, y and z axes and that the patient is positioned within the
 imaging area with the patient's height (i.e. from head to feet) defining
 an axis along the z axis.
 When the region of interest is subjected to a uniform magnetic field
 (polarizing field B.sub.0), the individual magnetic moments of the spins
 in the region attempt to align with the polarizing field, but precess
 about the direction of the field in random order at their characteristic
 Larmor frequencies. A net magnetic moment M.sub.z is produced in the
 direction of the polarizing field, but the randomly oriented magnetic
 components in the perpendicular, or transverse, plane (x-y plane) cancel
 one another.
 If, however, the region of interest is subjected to a magnetic field
 (excitation field B.sub.1) which is in the x-y plane and which is near the
 Larmor frequency, the net aligned moment Mz may be "tipped" into the x-y
 plane to produce a net transverse magnetic moment M.sub.t which is
 rotating or spinning in the x-y plane at the Larmor frequency.
 The practical value of this phenomenon resides in the signal emitted by the
 excited spins after the excitation signal B.sub.1 is terminated. The
 emitted signal is a function of at least one and typically several
 physical properties of the spin which generates the signal and therefore,
 by examining the emitted signal, the properties of the spin can be
 determined. The emitted NMR signals are digitized and processed to
 generate an NMR data set.
 To be useful, an NMR data set requires that the point of origin of each NMR
 signal sensed be known. To determine the point of origin of an NMR signal,
 each NMR signal is encoded with special information. An exemplary position
 encoding technique is commonly referred to as "spin-warp" and is discussed
 by W. A. Edelstein et al. in "Spin Warp NMR Imaging and Applications to
 Human Whole-Body Imaging", Physics in Medicine and Biology, Vol. 25, pp.
 751-756 (1980) which is incorporated herein by reference.
 In the spin-warp technique, special encoding is accomplished by employing
 three magnetic gradient fields (G.sub.x, G.sub.y, and G.sub.z) which have
 the same direction as polarizing field B.sub.0 and which have gradients
 along the x, y and z axes, respectively. By controlling the strength of
 these gradients during each NMR cycle, the spatial distribution of spin
 excitation can be controlled and the point of origin of the resulting NMR
 signals can be identified.
 A generally useful acquisition technique is known as a slice or two
 dimensional technique wherein NMR data are acquired for each of several
 single transaxial slices of a region of interest consecutively, and then
 the slices are "stacked" together to form a three dimensional data set.
 To determine the z-axis origin of a signal, signal generation during slice
 data acquisition is limited to a specific transaxial slice of the region
 of interest, at one time, using gradient field G.sub.z. To this end, the
 Larmor frequency F of a spin can be expressed as:
EQU F=(B.sub.0 +B.sub.z).gamma. (1)
 where B.sub.z is essentially the strength of gradient G.sub.z within a
 specific transaxial slice of the region of interest. Because the gradient
 field strength varies along the z-axis, each z-axis slice has a different
 Larmor frequency F. When the excitation signal B.sub.0 is provided at a
 specific excitation frequency, only those spins within the "selected"
 z-axis slice which are at the excitation frequency are tipped. Therefore,
 when the excitation signal B.sub.0 is turned off, only spins within the
 selected z-axis slice generate NMR signals.
 A similar technique is used to spatially encode NMR signals along the x
 axis. To this end, instead of providing a single excitation signal B.sub.0
 frequency, excitation signal B.sub.0 is provided at a small range of
 frequencies. The x axis gradient G.sub.x is small enough that all of the
 spins along the x axis have Larmor frequencies within the small range of
 excitation signal frequencies and therefore each of the spins along the x
 axis generates an NMR signal when the excitation signal is turned off,
 each x-axis NMR signal having a unique and identifiable frequency. Hence,
 x-axis position can be determined by identifying the frequency of each NMR
 signal received during an acquisition. Because x axis position is encoded
 using signal frequency, this type of encoding is known as frequency
 encoding.
 To encode y axis position within NMR signals the y axis gradient G.sub.y is
 employed to cause spins along the y axis to have different phases.
 Consequently, NMR signals resulting from spins along the y axis have
 different phases which can be used to determine y axis position. Because y
 axis position is encoded using signal phase, this type of encoding is
 known as phase encoding.
 After data have been acquired for one region of interest slice, the
 acquisition process is repeated for adjacent regions of interest slices
 until data have been acquired for every slice within the region of
 interest. After digitizing and processing, the slice data are combined to
 provide a three dimensional data point (TDDP) array representing one or
 more physical properties at regular grid positions within the interior of
 the region of interest. The TDDP array includes a plurality of sets of
 three dimensional (x,y,z) coordinates distributed at regular positions in
 a lattice within the region of interest, at least one value (Vxyz) of the
 physical property being associated with each respective one of the
 coordinate positions. Each cubically adjacent set of eight such positions
 defines a cubic volume, or "voxel", with a physical property value
 specified for each of the eight voxel vertices.
 After a complete TDDP array has been acquired and stored, the array can be
 used to form an image of the region of interest using one of many well
 known reconstruction techniques. Typical imaging screens used to display
 NMR images are only two dimensional. Thus, while shading and the like can
 give the appearance of a three dimensional image, in reality only two
 dimensions of pixels can be displayed at any given time. This hardware
 constraint requires that certain decisions be made as to what aspects of
 the TDDP array are important for examination purposes.
 For example, assume a TDDP array is observed from a specific perspective
 "viewing angle" wherein array data point columns are perpendicular to, and
 along the line of sight of, the viewing angle. In examining data points
 along one of the columns, if a bright data point is behind a dim data
 point, then, from the perspective view, the bright data point would be
 "hidden" and valuable information in the image might be lost. This is true
 of each of the data point columns. This problem is exacerbated because NMR
 systems generate an appreciable amount of electromagnetic noise which is
 reflected in a TDDP array, and a perspective view including data point
 intensities from only the most proximate array within data point columns
 would be relatively useless as many of the intensities correspond to
 noise. Consequently, in most cases after array data has been collected and
 stored, a subset of data is selected for generating an image. For example,
 one useful visualization technique is known as a maximum intensity
 projection (MIP). To form a MIP, a specific array viewing angle is
 selected wherein data point columns are along the viewing angle line of
 sight. For each column, a processor selects the highest intensity data
 point in the column and provides that data point in an associated two
 dimensional array of data points for display on the imaging screen. This
 MIP technique is valuable in that the MIP image is relatively noise free
 (i.e. is not dominated by noise) and provides an image which is akin to an
 x-ray.
 Another useful visualization technique is to select a transaxial slice
 through an NMR data set which is parallel to one of the x, y and z axes so
 that a cross sectional view of the data, and hence the region of interest,
 is obtained. This cross section technique allows a physician to observe
 the detailed spatial relationship between internal structures within the
 region of interest for diagnosing and prescribing purposes.
 One other useful visualization technique is known as oblique reformatting.
 The industry has generally recognized that in many instances it is
 desirable to select a cross sectional slice through an NMR data set which
 is orthogonal to a structural interface and which may form some oblique
 angle (hence the phrase "oblique imaging") to the orientation of the data
 acquisition slices. For example, it may be advantageous to observe the
 length of a vessel which traverses various x, y and z coordinates within
 the three dimensional data array.
 Cline et al. U.S. Pat. No. 4,984,157, "System and Method for Displaying
 Oblique Planar Cross Sections of a Solid Body Using Tri-Linear
 Interpolation To Determine Pixel Position Data", issued Jan. 8, 1991 and
 assigned to the instant assignee (hereinafter "the '157 patent"), is
 incorporated herein by reference. The '157 patent teaches one method and
 apparatus for selecting oblique reformatting planes and thereafter
 converting data point intensities to pixel intensities for display in the
 oblique image plane.
 As an alternative to generating a TDDP array and oblique reformatting
 thereafter to generate oblique images, oblique image data can be acquired
 initially via an oblique slice through a patient's body and the acquired
 data can then be used, without reformatting, to generate a desired oblique
 image. Methods to acquire oblique image data are well known in the art.
 Unfortunately, even conventional oblique imaging techniques have several
 shortcomings. One shortcoming of oblique imaging is that many vessels are
 tortuous, so that the vessel is not neatly contained within a single
 imaging plane. For example, the coronary arteries which are formed on an
 external surface of the heart are tortuous and multi-planar. In this
 instance, while a first portion of a vessel may be imageble via selection
 of a proper oblique imaging plane, other portions of the vessel which lie
 in different planes cannot be imaged along with the first portion. In
 addition, where a selected oblique plane passes through one or more heart
 chambers which include blood pools, the blood obfuscates the resulting
 image.
 SUMMARY OF THE INVENTION
 An exemplary embodiment of the invention includes a method for selecting a
 plurality of imaging planes which are tangent to different sections of a
 tortuous vessel and which are often non-planar, combining the data point
 intensities from each of the images into a single image, and then
 generating a MIP of the combined images to provide a relatively complete
 image of the tortuous vessel.
 In the exemplary embodiment of the invention, different intermediate planes
 through a region of interest are selected consecutively and separate
 intermediate sets of data corresponding to each plane are generated. Each
 intermediate set is a cross sectional view of the region of interest along
 an associated intermediate plane. A viewing plane is also selected which
 indicates the perspective of an image to be displayed. After the viewing
 angle is selected and the intermediate data sets are generated, the
 intermediate data sets are combined to generate values for display element
 positions associated with the viewing plane. Thereafter, the viewing plane
 element values are used to drive a display for displaying an image of the
 tortuous vessel from the viewing plane perspective.

DESCRIPTION OF A PREFERRED EMBODIMENT
 I. SYSTEM HARDWARE
 FIG. 1 illustrates the major components of a preferred NMR system which
 incorporates the present invention and is sold by the General Electric
 Company under the trademark "SIGNA". The system generally includes an
 operator control console 100, a computer system 107, a system control 122,
 a set of gradient amplifiers 127, a physiological acquisition controller
 129, a scan room interface 133, a patient positioning system 134, a magnet
 assembly 141, a preamplifier 153, an RF (radio frequency) power amplifier
 151, a transmit/receive switch 154, a power supply 157, data storage
 devices 111 and 112 and various data lines and busses which link the
 aforementioned components.
 The system is controlled from console 100 which includes a console
 processor 101 that scans a keyboard 102 and is controlled by a human
 operator through a control panel 103 and a plasma display/touch screen
 104. Console processor 101 communicates through a communications link 116
 with an application interface module 117 in computer system 107. Through
 keyboard 102 and controls 103, an operator controls the production and
 display of images by an image processor 106 in computer system 107, which
 is coupled to a video display 118 on console 100 through a video cable
 105.
 Display 118 includes a two dimensional array of pixels and is driven by a
 display driver (not shown in FIG. 1) to generate medical images for
 observation by a system user.
 Computer system 107 includes a number of modules which communicate with
 each other through a backplane 162. In addition to application interface
 117 and image processor 106, other system 107 modules include a CPU module
 108 for controlling the backplane, and an SCSI interface module 109 which
 couples computer system 107 through a bus 110 to a set of peripheral
 devices, including disk storage 111 and tape drive 112, for storing data
 during NMR signal acquisition and subsequent processing. Computer system
 107 also includes a memory module 113, known in the art as a frame buffer,
 for storing image data arrays, and a serial interface module 114 which
 links computer system 107 through a high speed serial link 115 to a system
 interface module 120 of a system control cabinet 122.
 System control 122 includes a series of modules interconnected by a common
 backplane 118. Backplane 118 is comprised of a number of bus structures,
 including a bus structure controlled by a CPU module 119. A serial
 interface module 120 couples backplane 118 to high speed serial link 115,
 and a pulse generator module 121 couples backplane 118 to operator console
 100 through a serial link 125. Through serial link 125, system control 122
 receives commands from the operator which call for performance of a scan
 sequence.
 Pulse generator module 121 operates the system components to carry out the
 desired scan sequence, producing data which indicate the timing, strength
 and shape of RF pulses that are to be produced, and the timing of, and
 length of, a data acquisition window. Pulse generator module 121 is also
 coupled through serial link 126 to the set of gradient amplifiers 127, and
 conveys data thereto which indicate the timing and shape of the gradient
 pulses to be produced during the scan. Pulse generator module 121 also
 receives patient data through a serial link 128 from physiological
 acquisition controller 129. Physiological acquisition controller 129 can
 receive a signal from a number of different sensors connected to the
 patient; for example, it may receive electrocardiogram (ECG) signals from
 electrodes or respiratory signals from a bellows, and produce pulses for
 pulse generator module 121 that synchronizes the scan with the patient's
 cardiac cycle or respiratory cycle. Pulse generator module 121 is also
 coupled through a serial link 132 to scan room interface circuit 133 which
 receives signals at inputs 135 from various sensors associated with the
 position and condition of the patient and the magnet system. It is also
 through scan room interface circuit 133 that patient positioning system
 134 receives commands which move the patient cradle and transport the
 patient to the desired position for the scan.
 The gradient waveforms produced by pulse generator module 121 are applied
 to gradient amplifier system 127 which is comprised of G.sub.x, G.sub.y
 and G.sub.z amplifiers 136, 137 and 138, respectively. Each amplifier 136,
 137 and 138 is utilized to excite a corresponding gradient coil in an
 assembly 139. Gradient coil assembly 139 forms part of magnet assembly 141
 which includes a main or polarizing magnet 140, typically superconductive,
 for producing a polarizing field such as a 0.5 or a 1.5 Tesla polarizing
 field which extends horizontally through a bore 142 in the magnet
 assembly. Gradient coils 139 encircle bore 142 and, when energized, coils
 139 generate magnetic fields in the same direction as the main polarizing
 magnetic field, but with gradients G.sub.x, G.sub.y and G.sub.z directed
 in the orthogonal x-, y- and z-axis directions of a Cartesian coordinate
 system. That is, if the magnetic field B.sub.0 generated by main magnet
 140 is directed in the z direction, and the total magnetic field in the z
 direction is B.sub.z, then G.sub.x =.differential.B.sub.z
 /.differential.x, G.sub.y =.differential.B.sub.z /.differential.y and
 G.sub.z =.differential.B.sub.z /.differential.z, and the magnetic field at
 any point (x,y,z) in the bore 142 of magnet assembly 141 is given by
 B(x,y,z)=B.sub.0 +G.sub.x x+G.sub.y y+G.sub.z z. The gradient magnetic
 fields are utilized to encode spatial information into the NMR signals
 emanating from the patient being scanned.
 Located within bore 142 is a circular cylindrical whole-body RF coil 152
 which produces a circularly polarized RF field in response to RF pulses
 provided by a transceiver module 150 in system control cabinet 122. These
 pulses are amplified by RF amplifier 151 and coupled to an RF coil 152 by
 transmit/receive switch 154. Waveforms and control signals are provided by
 pulse generator module 121 and utilized by transceiver module 150 for RF
 carrier modulation and mode control. The resulting NMR signals radiated by
 the excited nuclei in the patient may be sensed by the same RF coil 152
 and coupled through transmit/receive switch 154 to preamplifier 153. The
 amplified NMR signals are demodulated, filtered, and digitized in the
 receiver section of transceiver 150. Transmit/receive switch 154 is
 controlled by a signal from pulse generator module 121 to couple RF
 amplifier 151 to coil 152 during the transmit mode and to couple coil 152
 to preamplifier 153 during the receive mode. Transmit/receive switch 154
 also enables a separate RF coil (for example, a head coil or surface coil)
 to be used in either the transmit or receive mode.
 In addition to supporting polarizing magnet 140, gradient coils 139 and RF
 coil 152, the main magnet assembly 141 also supports a set of shim coils
 156 associated with main magnet 140 and used to correct inhomogeneities in
 the polarizing magnet field. The main power supply 157 is utilized to
 bring the polarizing field produced by superconductive main magnet 140 to
 the proper operating strength and is then disconnected from the magnet.
 The NMR signals picked up by RF coil 152 are digitized by transceiver
 module 150 and transferred to a memory module 160 which is part of system
 control 122. When the scan is completed and an entire array of data has
 been acquired in memory module 160, an array processor 161 operates to
 Fourier transform the data into an array of image data. This array of
 image data is conveyed through serial link 115 to computer system 107 and
 stored in disk memory 111. In response to commands received from operator
 console 100, this array of image data may be archived on tape drive 112,
 or it may be further processed by image processor 106 and conveyed to the
 operator console and presented on video display 118.
 As shown in FIGS. 1 and 2, transceiver 150 includes components which
 produce an RF excitation field B.sub.1 through power amplifier 151 at a
 coil 152 and components which receive the resulting NMR signal induced in
 a coil. As indicated above, the coils 152 may be a single wholebody coil
 as shown in FIG. 1. The base, or carrier, frequency of the RF excitation
 field is produced under control of a frequency synthesizer 200 which
 receives a set of digital signals (CF) through backplane 118 from CPU
 module 119 and pulse generator module 121. These digital signals indicate
 the frequency and phase of the RF carrier signal which is produced at an
 output 201 of the synthesizer. The commanded RF carrier is applied to a
 modulator and up converter 202 where its amplitude is modulated in
 response to a signal R(t) also received through backplane 118 from pulse
 generator module 121. The signal R(t) defines the envelope, and therefore
 the bandwidth, of the RF excitation pulse to be produced. The RF
 excitation pulse is produced in module 121 by sequentially reading out a
 series of stored digital values that represent the desired envelope. These
 stored digital values may, in turn, be changed from the operator console
 to enable any desired RF pulse envelope to be produced. Modulator and up
 converter 202 produces an RF pulse at the desired Larmor frequency at an
 output 205.
 The magnitude of the RF excitation pulse output is attenuated by an exciter
 attenuator circuit 206 which receives a digital command TA from backplane
 118. The attenuated RF excitation pulses are applied to RF power amplifier
 151 driving RF coil 165. For a more detailed description of this portion
 of transceiver 122, reference is made to Stormont et al. U.S. Pat. No.
 4,952,877 issued Aug. 28, 1990, which is assigned to the instant assignee
 and incorporated herein by reference.
 The NMR signal produced by the patient is picked up by receiver coil 166
 and applied through preamplifier 153 to the input of a receiver attenuator
 207 which further amplifies the NMR signal and attenuates the signal by an
 amount determined by a digital attenuation signal (RA) received from
 backplane 118. The receive attenuator 207 is also turned on and off by a
 signal from pulse generator module 121 such that it is not overloaded
 during RF excitation.
 The received NMR signal is at or around the Larmor frequency, which in the
 preferred embodiment is around 63.86 MHz for 1.5 Tesla and 21.28 MHz for
 0.5 Tesla. This high frequency signal is down converted in a two step
 process by a down converter 208 which first mixes the NMR signal with the
 carrier signal from frequency synthesizer 200 and then mixes the resulting
 difference signal with a reference signal, from a reference frequency
 generator 203 on line 204, of 2.5 MHz in a preferred embodiment. The
 resulting down converted NMR signal from down converter 208 has a maximum
 bandwidth of 125 kHz and is centered at a frequency of 187.5 kHz. The down
 converted NMR signal is applied to the input of an analog-to-digital (A/D)
 converter 209 which samples and digitizes the analog signal at a rate of
 250 kHz in a preferred embodiment. The output signal of A/D converter 209
 is applied to a digital detector and signal processor 210 which produces
 16-bit in-phase (I) values and 16-bit quadrature (Q) values corresponding
 to the received digital signal. The resulting stream of digitized I and Q
 values of the received NMR signal is furnished through backplane 118 to
 memory module 160 where it is employed to reconstruct an image.
 To preserve the phase information contained in the received NMR signal,
 both the modulator and up converter 202 and down converter 208 are
 operated with common signals. More particularly, the carrier signal at the
 output 201 of frequency synthesizer 200 and the 2.5 MHz reference signal
 at the output 204 of reference frequency generator 203 are employed in
 both frequency conversion processes. Phase consistency is thus maintained
 and phase changes in the detected NMR signal accurately indicate phase
 changes produced by the excited spins. The 2.5 MHz reference signal, as
 well as 5, 10 and 60 MHz reference signals, are produced by reference
 frequency generator 203 from a common 20 MHz master clock signal. The
 latter three reference signals are employed by frequency synthesizer 200
 to produce the carrier signal on output line 201. For a more detailed
 description of the receiver, reference is made to Stormont et al. U.S.
 Pat. No. 4,992,736, issued Feb. 12, 1991, which is assigned to the instant
 assignee and incorporated herein by reference.
 While the invention finds application in imaging any of several different
 types of structures (e.g. vessels, chambers, etc.), it is here described
 in the context of imaging a coronary artery on the surface of a human
 heart. To this end, it is assumed that a full set of NMR imaging data of a
 region of interest, which includes a patient's heart and specifically
 includes the artery to be imaged, has been acquired and processed to
 generate a TDDP array indicating at least one property of the region of
 interest. For example, the physical properties of the TDDP array may be
 spin-spin or lattice-spin relaxation times as are well known in the NMR
 field.
 A TDDP array includes adjacent cubic voxel elements, each element having
 eight vertices. Associated with each vertex is one data value which
 represents the physical property at the corresponding spatial position
 within the region of interest. The spatial positions are located in
 regular patterns defining regularly spaced grid locations within the body.
 The grid locations in turn define a plurality of adjacent voxels within
 the region. For purposes of this explanation it will be assumed that the
 grid positions are aligned with the x, y and z axes of bore 142 where the
 z axis is along the bore length, the x axis is horizontal and the y axis
 is vertical.
 Referring again to FIG. 1 and also to FIG. 5, in one embodiment of the
 invention panel 103 includes an orientation generator 256, a depth
 generator 258, a viewing angle selector 260 and a pixel density selector
 261. In this embodiment image processor 106 includes an intermediate plane
 selector 262, an intermediate element value determiner 264, a transition
 element value determiner 266, a combiner or processor 268 and a video or
 display driver 270.
 Orientation generator 256 allows a user to specify the angular orientation
 of a cut plane through the region of interest along which a cross
 sectional image is to be generated. This orientation can be specified by
 two angles, one from the x plane and one from the z plane. Such angular
 input data can be specified by joystick, rheostat, keyboard, mouse or any
 other suitable input device. In the preferred embodiment a track ball
 having two degrees of angular freedom is used to specify cut plane
 orientation. The x and z axis angle signals are provided to intermediate
 plane selector 262.
 Depth generator 258 selects the depth of the cut plane from the coordinate
 origin at the outer surface of the region of interest. The depth
 generator, like the orientation generator, may be any type of suitable
 input device. The depth signal is provided to intermediate plane selector
 262.
 Pixel density selector 261 allows a user (operator) to select a pixel
 density for an intermediate plane. In the alternative, selector 261 may
 not be utilized and the pixel density may automatically be set to the
 display screen 118 pixel density or, for intermediate planes, to some
 other suitable density value. The density signal is provided to
 intermediate plane selector 262.
 Intermediate plane selector 262 receives each of the selected angle,
 selected depth and pixel density signals from generators 256, 258 and
 pixel density selector 261, respectively, and identifies an intermediate
 plane specified by the angles and depth signal and having an element or
 pixel density specified by the pixel density signal.
 The intermediate plane data are provided to intermediate element value
 determiner 264 which determines the value of each element in the
 intermediate plane as a function of the data point values of the eight
 vertices which enclose the element. Thereafter, the intermediate element
 values for the specific intermediate plane are stored in memory 113 as a
 first intermediate element value set.
 Next, the operator uses depth generator 258, orientation generator 256 and
 pixel density selector 261 to select other intermediate planes which cut
 through the region of interest and to select the density of each
 intermediate plane causing intermediate plane selector 262 and
 intermediate element value determiner 264 to generate other intermediate
 element value sets which are all stored in memory 113. In effect, each
 intermediate element value set includes data corresponding to a separate
 cross sectional view through the region of interest along the
 corresponding intermediate plane.
 The '157 patent, referred to previously, teaches the preferred system for
 selecting oblique imaging planes and using three dimensional data points
 to generate intermediate element position values corresponding to oblique
 cross sectional views, and should be referred to for more detail in this
 regard.
 Viewing angle selector 260 allows an operator to select an angle from which
 to view the cross sectional views corresponding to all, or a subset of,
 the intermediate element value sets. To this end, selector 260, like
 orientation generator 256, includes any of several different suitable
 interface devices (i.e. trackball, keyboard, joystick, etc.). When a
 viewing angle is selected, the two dimensions perpendicular to the
 selected angle are identified as axes x' and y'. In addition, a viewing
 plane is identified which includes a two dimensional array of pixel
 positions corresponding to the pixel density of display 118 (see FIG. 1)
 arranged within the x'-y' plane. The viewing angle, including pixel
 positions in the viewing plane, is provided to transition element value
 determiner 266.
 Transition element value determiner 266, upon receiving the viewing plane,
 retrieves the intermediate element value sets from memory 113 and
 generates a three dimensional data point construct including a data point
 for each intermediate element value in the retrieved intermediate element
 value sets. Geometrically speaking, the three dimensional data point
 construct simply includes a series of interconnected intermediate planes.
 Determiner 266 next identifies the dimensions of the data point construct
 in the x'-y' plane, compares the data point construct dimensions to the
 dimensions of the screen of display 118, and scales the entire data point
 construct either up or down, depending on the difference between the
 screen dimensions and the array x' and y' dimensions.
 After scaling the three dimensional data point construct, for each scaled
 intermediate value set, transition element value determiner 266 projects
 the value set onto the viewing plane pixel positions in the x'-y' plane,
 thereby generating viewing plane element position values for most pixels
 in the viewing plane.
 The term "projecting" is used figuratively to describe the mathematical
 process of determining the values of the viewing plane element. In
 reality, observing a scaled intermediate element value set along a ray
 perpendicular to the x'-y' plane and through a viewing plane pixel or
 element position, at least one, and often several, data points in the
 scaled value set are observable. The projecting process involves
 determining the percentage of each viewing plane pixel or element position
 which is subtended by each observable intermediate element value and
 causing the viewing plane element position value to be proportional to the
 values of the observable intermediate element values and their respective
 percentages. For example, if one half of a viewing plane element position
 is subtended by a first intermediate element value having a relative
 intensity of 10 and the second half of the viewing plane element position
 is subtended by a second intermediate element value having a relative
 intensity of 5, the resulting viewing plane element position value would
 be 7.5 (i.e. (10+5)/ 2=7.5).
 The transition elements corresponding to each intermediate element value
 set are stored as a transition element value set in memory 113. Thus, for
 each intermediate value set, transition element value determiner 266
 generates a separate transition element value set, each transition set
 being in the x'-y' plane and having a density equal to the density of
 display 118. The dimensions of transition sets may overlap and typically
 correspond to a fraction of the display 118 dimensions.
 After all of the transition sets have been generated, combiner 268
 retrieves the transition sets and combines the transition sets to generate
 a separate viewing plane element position value for each display pixel.
 Preferably, combiner 268 combines by performing a maximum intensity
 projection (MIP) on the transition sets, thereby providing data for
 generating a two dimensional image on display 118 (see FIG. 1). The MIP
 data (i.e. viewing element position values) are stored in memory 113.
 Driver 270 retrieves the MIP data and uses that data to drive display 118
 in any of several different manners which are well known in the industry.
 II. OPERATION
 In the interest of simplicity, operation of the system is described in
 conjunction with FIG. 6 in the context of a relatively simple coronary
 artery portion 250 (hereinafter artery 250) as shown in FIG. 3. Artery 250
 includes three adjacent sections 250a, 250b and 250c which generally are
 aligned in different planes. In addition, it is assumed that a desired
 view of artery 250 is along the direction of a viewing angle indicated by
 arrow 252 (hereinafter "angle 252").
 In FIG. 4, artery 250 is illustrated as it would appear from viewing angle
 252 in FIG. 3. It can be seen that artery 250 is tortuous, generally
 having a bent shape into angle 252 (FIG. 3) and an inverted "S" shape from
 the perspective of angle 252 (FIG. 4). Clearly artery 250 is multiplanar
 and therefore an image of artery 250 cannot be generated using data from a
 single plane.
 Referring also to FIGS. 1 and 5, an operator uses console 100 to display a
 cross sectional view through the region of interest by selecting depth and
 orientation of a cut plane through the region of interest using depth
 generator 258 and orientation generator 256.
 The operator modifies the depth and orientation selections until a cross
 section of the heart, including at least a portion of coronary artery 250
 to be imaged, is displayed. The portion of artery 250 selected for viewing
 may simply be a cross section and therefore appear similar to the end of a
 tube, or a dot. Once artery 250 is identified, the operator again modifies
 the depth and orientation selections until a lengthwise section of artery
 250 is displayed. For example, artery 250 may appear as in FIG. 3, after
 this manipulation.
 The remainder of the method of operation is illustrated in FIG. 7. With the
 lengthwise piece of artery 250 displayed, at step 400 the operator uses
 orientation generator 256 and depth generator 258 to select a first
 intermediate plane 280 which, as shown in FIG. 3, includes first artery
 section 250a. This selecting step 400 may be as easy as using an input
 device to move an intermediate plane line across the displayed image until
 the line is aligned with section 250a. In this event, generators 256 and
 258 could be provided by a single input device.
 When plane 250a is selected, at step 402 intermediate plane selector 262
 generates the plane element positions which correspond to the selected
 plane and intermediate element value determiner 264 determines the values
 of each element position in the selected intermediate plane as described
 above. These values are stored at step 404 as a first intermediate element
 position value set in memory 113. The first intermediate value set is
 schematically and graphically illustrated in FIG. 7 as a plane of data
 points 280'.
 Because there are two additional intermediate planes 282 and 284 to be
 imaged, at decision step 406, the process loops back to step 400 where the
 operator uses generators 256 and 258 to now select a second intermediate
 plane 282 which includes second artery section 250b. When plane 282 is
 selected, at step 402 intermediate plane selector 262 generates the plane
 element positions which correspond to the selected plane and intermediate
 element value determiner 264 determines the values of each element
 position in the selected plane, the values being stored at step 404 as a
 second intermediate element position value set in memory 113. The second
 intermediate element position value set is illustrated in FIG. 7 as a
 plane of data points 282'.
 Once again, at step 400 the operator uses generators 256 and 258 to select
 a third intermediate plane 284 which includes third artery section 250c.
 When plane 284 is selected, at step 402 intermediate plane selector 262
 generates the plane element positions which correspond to the selected
 plane and intermediate element value determiner 264 determines the values
 of each element position in the selected plane, the values being stored at
 step 404 as a third intermediate element position value set in memory 113.
 The third intermediate element position value set is illustrated in FIG. 7
 as a plane of data points 284'. Data sets 280', 282' and 284' can be seen
 to overlap. Now, at decision step 406, because there are no other
 intermediate planes to be imaged in the present example, control passes to
 step 408.
 In addition to selecting the intermediate planes, at step 408 the operator
 uses viewing angle selector 260 to select a viewing angle from which to
 examine artery 250. In this example, with reference to FIGS. 3 and 6; the
 operator selects angle 252. Thereafter, at step 410, transition element
 value determiner 266 retrieves the intermediate element value sets from
 memory 113 and generates a three dimensional data point construct 350
 (FIG. 7) including a data point for each intermediate element value in the
 retrieved value sets. To this end, intermediate arrays 280', 282' and 284'
 are retrieved and, as shown in FIG. 7, three dimensional construct 350 is
 formed. In addition, at step 410, transition element value determiner 266
 identifies the dimensions of data point construct 350 in the x'-y' plane
 (i.e. the plane perpendicular to viewing angle 252), compares array 350 x'
 and y' dimensions to the dimensions of the screen of display 118 and
 scales the entire data point construct 350 either up or down, depending on
 the difference between the screen dimensions and the array 350 x' and y'
 dimensions. For example, if construct 350 x' and y' dimensions are each
 half the dimensions of display 118, size of construct 350 is increased by
 a factor of 2.
 After scaling the data point construct for each scaled intermediate value
 set, transition element value determiner 266, at step 412, projects each
 value set onto the viewing angle x'-y' plane thereby generating three
 transition element value sets 280", 282" and 284", respectively, in the
 viewing angle plane, as shown in FIG. 7. Many methods for performing such
 projection are known in the art and are therefore are not explained here
 in detail.
 After all of the transition element value sets have been generated, combine
 268, at step 414, retrieves the transition sets and combines the sets to
 generate a separate viewing element position value for each display pixel.
 Preferably, combiner 268 performs step 414 by performing a maximum
 intensity projection (MIP) on the transition element value sets, thereby
 providing data for generating a two dimensional image on display 118 (FIG.
 1). The MIP data (i.e. viewing element position values) are stored in
 memory 113.
 At step 416, driver 270 retrieves the MIP data and uses that data to drive
 display 118 in any of several different manners which are well known in
 the industry, thereby providing a multi-oblique plane MIP image for artery
 study.
 If desired, after step 406 and storage of all intermediate value sets, the
 operator can select one viewing angle at step 408 and, if the resulting
 MIP is unsatisfactory, the operator can go back to step 408 and select a
 different, perhaps more suitable viewing angle, using the same
 intermediate value sets to generate a second MIP corresponding to the
 newly chosen angle.
 While only certain preferred features of the invention have been
 illustrated and described, many modifications and changes will occur to
 those skilled in the art. It is, therefore, to be understood that the
 appended claims are intended to cover all such modifications and changes
 as fall within the true spirit of the invention.