JP4087547B2 - Computed tomography equipment - Google Patents

Description

ここで、ｎijとｍijkは一般に整数とならないので、補間計算を行なう。
【００６８】
次に、ステップS8で、次のｋのためにｍijkを更新する。
【００６９】
ｍijk＝ｍijk+△ｍij ・・・（14）
ステップS9,S10,S11でｋ,ｉ,ｊについて繰り返すことで体積全体についてたし込み、ステップS12でφを繰り返し一つの体積に逆投影をたし込んで逆投影処理が終わり、体積の３次元像ができる。
【００７０】
以上の逆投影処理の説明では、説明をわかり易くするため最速な計算式になっておらず、式はループ外に出せる部分を多く含んでいる。
【００７１】
上述したように、本実施の形態によるＣＴスキャナでは、回転軸12方向のボクセルのループ、すなわちｋループを最内としているので、対応チャンネルｎij,ｍijk（あるいは△ｍij）の計算をｋループ内で共通にできるため、前述した従来のように、ｋループをi,jループの外側にする場合よりも計算に無駄が無くなる。
【００７２】
また、データを収集した順に他のデータを待つことなく再構成できるため、再構成速度を上げることが可能となる。
【００７３】
さらに、撮影面11からの傾斜に従って逆投影しているため、回転軸12方向の分解能を上げることが可能となる。
【００７４】
（第１の実施の形態の変形例）
ＢＰウエイトは、１／Ｌ²以外にさらに図４で示すように、回転角によるウエイトｗ(φ)を掛けるようにしてもよい。
【００７５】
この場合、360°＋２αの回転の間、データ収集を行なう。このウェイトを掛けると、360°の回転の前後が平均されるため、撮影中に被検体4が回転テーブル5上で微動しても偽像が生じ難くなる。
【００７６】
ウェイトｗ(φ)は、傾斜部が曲線でもよい。すなわち、360°ずらしたものと加算した時１になるようになっていればよい。
【００７７】
（第２の実施の形態）
本実施の形態によるＣＴスキャナのシステム構成は前記図１と同様であり、体積ＢＰ部16bによる逆投影処理の方法のみが異なっている。
【００７８】
従ってここでは、本実施の形態のＣＴスキャナの逆投影処理（作用）について、図５および図６に示す幾何図、および図７に示すフロー図を用いて説明する。
【００７９】
なお、既出の符号および図より明らかな符号の説明については省略する。
【００８０】
図５は、被検体4に固定した座標ｘ,ｙ,zで見たＸ線焦点Ｆと検出面17の位置である。Ｆの回転角をφとする。
【００８１】
図６も同様で、再構成する体積の各ボクセルをi,j,kで番号づけ、ボクセルサイズを△ｘ,△y,△zとする（△y=△ｘ）。
【００８２】
次に、図７において、ステップS1で360°分の投影データｐは90°分ずつのクオータに分けられ、ステップS2で最初の90°分のφc（計算回転角）ループとなり、ステップS3でデータ収集回転角φを計算する.
φ=φｃ＋ｎｑ・π／２ ・・・（15）
ｎｑ:クオータ番号
ステップS4で、φが収集中のφであるか判定して頭だし、あるいは終了を行なう。
【００８３】
ステップS5で、Ｆのｘ，ｙ座標ｘF,ｙFが計算される。
【００８４】
ｘF＝ＦＣＤ・sinφc ・・・（16）
ｙF＝ＦＣＤ・cosφc ・・・（17）
次に、ステップS6〜ステップS11でセンタリングを行なう。
【００８５】
図５において、センタリングは検出面17上のデータをｘｚ平面上のｐｑマトリックスに逆投影する処理である。
【００８６】
ステップS6でｐループに入り、ステップS7でｎpを求める。
【００８７】
ｘo＝（ｐ−ｐc）・ｃp ・・・（18）
ｘ1=ｘo・cosφc ・・・（19）
ｍａｇ1＝ＦＤＤ／√（（ｘF−ｘo）²＋ｙF²−ｘ1²）） ・・・（20）
ｎp＝ｎc＋ｘ1・ｍａｇ１／ｄ ・・・（21）
ｎp，ｍpq:マトリックスp,qにBPされるチャンネル番号
ｃp，ｃq:センタリングピッチ
次に、ステップS8でｑループに入り、ステップS9でｍpq、ステップS10でセンタリングデータｐc（p,q）を求める。
【００８８】
ｍpq＝ｍｃ＋（ｑ−ｑc）・ｃq・ｍａｇ１／ｄ ・・・（22）
ｐc（p,q）＝ｐφ（ｎp，ｍpq） ・・・（23）
ここで、ｎpとｍpqは一般に整数とならないので、補間計算を行なう。
【００８９】
次に、ステップS11で、次のｑ，ｐを繰り返す。
【００９０】
次に、ステップS12〜ステップS22でＢＰを行なう。
【００９１】
図６において、ステップS12でjループに入り、ステップS13でｐoj、△ｐj、△ｑjを求める。
【００９２】
ｍａｇ２＝ｙF／（ｙF＋（ｊ−ｊc）・△ｙ） ・・・（24）
ｐoj＝ｐｃ＋(ｘF−ｍａｇ２・（ｘF＋ｉc・△ｘ）)／ｃp ・・（25）
△ｐj＝ｍａｇ２・△ｘ／ｃp ・・・（26）
△ｑj＝ｍａｇ２・△ｙ／ｃp ・・・（27）
ｐij,ｑjk:ボクセルi,j,kにＢＰされるマトリックス番号
△ｐj:ｉが１増加したときのｐijの増分
△ｑj:ｋが１増加したときのｑjkの増分
ステップS13で、さらにｐijの初期値を設定する。
【００９３】
ｐij＝ｐoj ・・・（28）
次に、Sステップ14でｉループに入り、ステップS15でＬ²を計算する。
【００９４】
ｘ＝（ｉ−ｉc）・△ｘ ・・・（29）
ｙ＝（ｊc−ｊ）・△ｙ ・・・（30）
Ｌ²＝（（ｘF−ｘ）²＋（ｙF−ｙ）²）／ＦＣＤ² ・・・（31）
ステップS15で、さらにｑjkの初期値を設定する。
【００９５】
ｑjk＝ｑｃ＋(ks−ｋc)・△ｑj ・・・（32）
次に、ステップS16でｋループに入り、ステップS17でボクセルi,j,kへのたし込みを行なう。
【００９６】

ここで、ｐijとｑjkは一般に整数とならないので、補間計算を行なうか、最近傍ｐcを選択する。
【００９７】
次に、ステップS18で、次のｋのためにｑjkを更新する。
【００９８】
ｑjk＝ｑjk＋△ｑj ・・・（34）
ステップS19でｋループを繰り返し、ステップS20で次のｉのためにｐijを更新する。
【００９９】
ｐij＝ｐij＋△Pj （35）
ステップS21,S22でｉ,ｊについて繰り返すことで体積全体についてたし込み、ステップS23でφcを繰り返し一つの体積に逆投影をだし込んで行く。
【０１００】
ステップS24で画像を90度回転し、ステップS25でクオータについて繰り返して逆投影処理が終わり、体積の３次元像ができる。
【０１０１】
以上の逆投影処理の説明では、説明をわかり易くするため最速な計算式になっておらず、式はループ外に出せる部分を多く含んでいる。
【０１０２】
上述したように、本実施の形態によるＣＴスキャナでは、まず、各回転位置φで検出面17上のデータをｘｚ平面に逆投影してセンタリングデータｐcを求めると、ｐcはボクセルの一つの面ｉｋ平面に平行な面上で等間隔で得られるため、ｉｋ面への逆投影計算（具体的には、ｐij,ｑjkの計算）を著しく簡略化することができ、再構成速度を上げることが可能となる。
【０１０３】
また、前述した第１の実施の形態と同様に、回転軸12方向のボクセルのループ、すなわちｋループを最内としているので、対応チャンネルｐij,ｑjk（あるいは△ｑj）の計算をｋループ内で共通にできるため、前述した従来のように、ｋループをi,jループの外側にする場合よりも計算に無駄が無くなり、再構成速度を上げることが可能となる。
【０１０４】
さらに、データを収集した順に他のデータを待つことなく再構成できるため、再構成速度を上げることが可能となる。
【０１０５】
さらにまた、撮影面11からの傾斜に従って逆投影しているため、回転軸12方向の分解能を上げることが可能となる。
【０１０６】
（第２の実施の形態の変形例）
前述した第１の実施の形態の変形例と同様に、ＢＰウエイトは、１／Ｌ²以外にさらに図４で示すように、回転角によるウエイトｗ(φ)を掛けるようにしてもよい。この場合にも、同様の作用効果を得ることができる。
【０１０７】
第２の実施の形態において、体積のｋ方向がｉｊ方向と比較して同等のサイズの場合には、ｉループをｋループの内側にするようにしてもよい。この場合、式（33）は略計算でなく１行目の計算を行なう。
【０１０８】
（第３の実施の形態）
本実施の形態によるＣＴスキャナのシステム構成は前記第２の実施の形態と同様であり、透過像収集時の機構動作と体積ＢＰ部16bによる逆投影処理の方法のみが異なっている。
【０１０９】
従ってここでは、本実施の形態のＣＴスキャナの透過像収集時の作用と、逆投影処理（作用）について、図７に示すフロー図を用いて説明する。
【０１１０】
本実施の形態の場合には、断層撮影を開始すると、回転テーブル5が回転と同時に昇降され(ヘリカルスキャン)、この間に透過像が収集され、360度以上の方向の透過データから体積の３次元像が再構成され、表示部10に表示される。
【０１１１】
また、本実施の形態の体積ＢＰは、前述した第２の実施の形態の場合とほぼ同様であり、図７に示すフロー図、および式（15）〜（35）を用いて説明する。
【０１１２】
すなわち、前述した第２の実施の形態の場合と異なる点は、ｋcが定数でなくφの関数であること、および式（33）が異なることである。
【０１１３】

ここで、図８（ａ）を参照して各式について説明する。
【０１１４】
まず、式（36）でｋc(φ)は、回転角φの時の撮影面11が横切るマトリックスのｋ位置である。
【０１１５】
この式で、ｚpは１回転中に昇降する高さ（ヘリカルビッチ）で、△ｚは高さ方向のボクセルサイズ、φOはｋ＝０を横切る時の回転角である。
【０１１６】
図８（ａ）で、ｗφ（ｚ）は回転角φの時のｚ方向のボクセルのＢＰのウェイトで半値幅ｚpの台形状となる。
【０１１７】
この台形の斜面においては、360°異なるφからのＢＰが合成される(点線)。
【０１１８】
ｗφ(z)をｋ目盛に変更したウエイトがｗφ(k)で、図８（ｂ）に示す中心がｋc(φ)、半値幅ｚp／△ｚの台形である。
【０１１９】
ｋでのｚp／△zは回転角で、360°に相当する。
【０１２０】
ここで、斜面の半長ｋαは角度αと、
ｋα＝Ｚp／△z・α／２π ・・・（37）
の関係があり、補間合成する角度半幅αを設定するとｋαが決まる。
【０１２１】
式（33'）においては、ＢＰウェイトにｗφ（ｋ）を用いるため、ヘリカルスキャンを行なった場合に、φについて連続してＢＰを行なっても、各ボクセルに360°分のＢＰがなされる。
【０１２２】
上述したように、本実施の形態によるＣＴスキャナでは、前述した第２の実施の形態と同様に、再構成速度を上げること、および回転軸12方向の分解能を上げることが可能となる。
【０１２３】
また、ヘリカルスキャンで効率よく、かつ撮影面11からのビームの傾斜を大きくせずに撮影できるため、回転軸12方向に広くかつ高品質の３次元像を得ることができる。
【０１２４】
さらに、ヘリカルスキャンで回転、昇降を一定速度で滑らかに広い範囲をスキャンできるため、被検体4に加速度がかからず、動き易い被検体4でも高品質の画像を得ることができる。
【０１２５】
また、台形のウェイトｗφ(k)を掛けてＢＰしているので、360°の回転の前後が平均されるため、撮影中に被検体4が動いても偽像が生じ難くなる。
【０１２６】
（第３の実施の形態の変形例）
ウエイトｗφ(k)は、図８（ｃ）に示すように、傾斜部が曲線であってもよい。すなわち、ｚp／△zずらしたもの(点線)と加算した時に１になるようになっていればよい。
【０１２７】
（第４の実施の形態）
本実施の形態によるＣＴスキャナのシステム構成は前記第３の実施の形態とほぼ同様であり、ＬＯＧ変換部15と再構成部16との間に窓関数掛け部を備えている点、および回転中心が上から見てＸ線ビーム2の中心からずれて設定されている点が異なっている。
【０１２８】
図９は、回転中心ＣとＸ線ビーム2との位置関係を示す概要図である。
【０１２９】
これは、回転テーブル5をずらして設定して、被検体4をＸ線ビーム2から片側にはみ出させて載置し、大きな被検体4も撮影可能としたＣＴスキャナであり、この撮影法はオフセットスキャンと呼ばれる。
【０１３０】
本実施の形態によるＣＴスキャナにおける作用は、前記第３の実施の形態と同様であり、ＬＯＧ変換後の投影データｐφ(ｎ,ｍ)に対して、図１０に示すような窓関数ｗ(ｎ)を掛ける点のみが異なっている。
【０１３１】
窓関数ｗ(ｎ)は中心ｃｈ，ｎｃで、０．５でここを中心に傾斜しており、傾斜領域の外側の一方は０もう一方は１である。
【０１３２】
再構成部16は、前記第３の実施の形態と同様に再構成を行なう。
【０１３３】
上述したように、本実施の形態によるＣＴスキャナでは、オフセットスキャンの場合でも、データ列を再編成（パラレルデータヘの変換等）することが不要となるため、高速で再構成することができ、さらに中心ｃｈを挟んで滑らかなウエイトが掛かるため、投影データの急激な変化が避けられ、偽像（リング状）の少ない３次元像を得ることができる。
【０１３４】
さらに、ヘリカルスキャンを行ないながらオフセットスキャンを実施することができ、前述した第３の実施の形態と同様の効果を得ることができるのに加えて、大きな被検体4に対応できるＣＴスキャナを実現することが可能となる。
【０１３５】
（第４の実施の形態の変形例）
第４の実施の形態によるオフセットスキャンは、前述した第１または第２の実施の形態に対して適用することもできる。
【０１３６】
また、窓関数ｗ(ｎ)は傾斜部が曲線であってもよい。すなわち、中心ｃｈで０．５、軸に直交する左右側に傾きが対称な０から１まで変化する傾斜部を有し、傾斜部の外は片側が０逆側が１であればよい。
【０１３７】
（第５の実施の形態）
図１１は、本実施の形態によるＣＴスキャナのシステム構成例を示す概要図で、前記第１の実施の形態とほぼ同様であり、空気補正部14とＬＯＧ変換部15との間に散乱線補正部19を備えている点、およびＸ線ビーム2を撮影面11を挟んだ薄厚コーン状に制限する可動のコリメータ18を備えている点が異なっている。
【０１３８】
本実施の形態によるＣＴスキャナにおける作用は、前記第１の実施の形態と同様であり、断層撮影時にＸ線ビーム2を薄厚コーン状に制限する点と、空気補正後のデータｈ（ｎ，ｍ）に対して散乱線補正を行なう点が異なっている。
【０１３９】
図１２は、散乱線補正の内容を説明するための図である。
【０１４０】
図１２は、検出面17上のデータｈ（ｎ，ｍ）を示している。
【０１４１】
Ｘ線ビーム2の照射部20以外はコリメータ18で遮られる部分で、被検体4等からの散乱線のみ検出している。
【０１４２】
照射部20内のデータｈ（ｎ，ｍ）に対して、散乱線補正

を加える。
【０１４３】
ここでは、ｈ（ｎ，ｍ）に近い上下の２チャンネルを１次補間して散乱線を求め、減算している。
【０１４４】
上述したように、本実施の形態によるＣＴスキャナでは、散乱線測定の専用の検出器を用いることなく、２次元検出器のコリメータ18で遮られる部分のデータを用いて散乱線補正をすることができる。
【０１４５】
また、専用の検出器に対して、２次元検出器は全チャンネルが一度に製造されるため、特性が均質である。
【０１４６】
さらに、測定点に近い位置で、同時にしかも均質な条件で散乱線が測定できるため、正確に補正を行なうことが可能となる。
【０１４７】
（第５の実施の形態の変形例）
第５の実施の形態において、補正を行なうに当たっては、２チャンネルのデータでなく、もっと多数のチャンネルデータを用いてもよく、１次補間でなくてもよい。
【０１４８】
この多数のチャンネルデータを用いることにより、統計精度を上げることが可能となる。
【０１４９】
第５の実施の形態における散乱線補正は、前記第１の実施の形態に付加するだけでなく、前記第２乃至第４の実施の形態に対しても付加することができ、さらに単独で図13に示すような従来の２次元検出器を用いるＣＴスキャナに対しても付加することができる.
（第６の実施の形態）
本実施の形態によるＣＴスキャナのシステム構成は前記第１の実施の形態とほぼ同様であり、やわらかな変形し易い被検体4の撮影に適用される点が異なっている。
【０１５０】
本実施の形態の場合、被検体4をテーブル5に設定し、準備回転として、滑らかな一定速度の回転を行なわせ、十分に被検体4を安定させた後に、断層撮影を開始する。
【０１５１】
装置は、そのまま同じ回転速度を保ったままＸ線をONさせ、透過像を収集して３次元像を作成する。
【０１５２】
ヘリカルスキャンの場合は、同様に準備回転を行なわせておき、断層撮影開始時に昇降を徐々に開始させて透過像を収集開始する。
【０１５３】
また、準備回転の時間が短くてよい場合には、準備昇降をいっしょに行なってもよい。
【０１５４】
上述したように、本実施の形態によるＣＴスキャナでは、回転始動時に生じた被検体4の動きを一定速度回転を続けて終息させ、その後に回転速度を変えずに撮影するため、変形し易い被検体4でも動きを終息させて撮影でき、高品質な３次元像を得ることができる。
【０１５５】
（第７の実施の形態）
本実施の形態によるＣＴスキャナのシステム構成は前記第１の実施の形態とほぼ同様であり、フィルタ関数の自動選択を行なう点が異なっている。
【０１５６】
すなわち、データ処理部9は、フィルタ掛け前のデータ（ｐあるいはｈ）の信号ノイズ比(平均)を計算して、フィルタ関数を自動選択する。
【０１５７】
信号ノイズ比は、低周波数成分を信号、高周波数成分をノイズとして、その比である信号／ノイズを求める。
【０１５８】
信号ノイズ比の小さな場合、フィルタ関数は低いカットオフ周波数のものを選択して、空間分解能を少し低下させる代りに画像ノイズを少なくさせる。
【０１５９】
上述したように、本実施の形態によるＣＴスキャナでは、撮影倍率や管電圧、管電流、スキャン時間等の撮影条件を任意に変更しても、透過データの信号ノイズ比でフィルタ関数を選択するため、画像ノイズと空間分解能との関係が最適な断面像を得ることができる。
【０１６０】
【発明の効果】
以上説明したように、本発明のコンピュータ断層撮影方法および装置によれば、撮影の高速化あるいは画像の高品質化に寄与することが可能となる。
【図面の簡単な説明】
【図１】本発明によるＣＴスキャナの第１の実施の形態を示す概要図。
【図２】同第１の実施の形態のＣＴスキャナにおける作用を説明するための幾何図。
【図３】同第１の実施の形態のＣＴスキャナにおける作用を説明するためのフロー図。
【図４】本発明によるＣＴスキャナの第１の実施の形態の変形例を示す図。
【図５】本発明の第２の実施の形態のＣＴスキャナにおける作用を説明するための幾何図。
【図６】本発明の第２の実施の形態のＣＴスキャナにおける作用を説明するための幾何図。
【図７】本発明の第２の実施の形態および第３の実施の形態のＣＴスキャナにおける作用を説明するためのフロー図。
【図８】本発明の第３の実施の形態およびその変形例のＣＴスキャナにおける作用を説明するための図。
【図９】本発明によるＣＴスキャナの第４の実施の形態を示す部分概要図。
【図１０】同第４の実施の形態のＣＴスキャナにおける作用を説明するための図。
【図１１】本発明によるＣＴスキャナの第５の実施の形態を示す部分概要図。
【図１２】同第５の実施の形態のＣＴスキャナにおける作用を説明するための図。
【図１３】従来の高分解能型ＣＴスキャナのシステム構成例を示す概要図。
【符号の説明】
1…Ｘ線管
2…Ｘ線ビーム
3…放射線検出器
4…被検体
5…回転テーブル
6…回転・昇降機構
7…シフト機構
8…機構制御部
9…データ処理部
10…表示部
11…撮影面
12…回転軸
13…Ｘ線制御部
17…検出面
18…コリメータ
19…散乱線補正部
20…照射部。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a computer tomography method and apparatus among non-destructive inspection apparatuses, and more particularly to a computer tomography method and apparatus capable of contributing to high-speed imaging or high image quality.
[0002]
[Prior art]
In recent years, high-resolution industrial computer tomography apparatuses (hereinafter referred to as CT scanners) have been manufactured for the purpose of inspecting small electronic components and the like with high resolution.
[0003]
FIG. 13 is a schematic diagram showing a system configuration example of this type of conventional high-resolution CT scanner, in which both a transmission image and a cross-sectional image are obtained.
[0004]
In FIG. 13, an X-ray tube 101 and a detector 103 that detects a cone-shaped X-ray beam 102 emitted from the X-ray tube 101 with two-dimensional spatial resolution are arranged to face each other. A transmission image of the subject 104 inside is obtained.
[0005]
The transmitted image can be displayed as a moving image in real time, or can be added by the data processing unit 109 and displayed with reduced noise.
[0006]
When the rotary table 105 and the rotary lifting mechanism 106 are moved closer to or away from the X-ray tube 101 by the shift mechanism 107, the shooting distance is changed and the shooting distance is reduced, the shooting magnification can be increased.
[0007]
When taking a cross-sectional image, a large number of transmission images are obtained while rotating the subject 104 on the rotary table 105 by the rotary elevating mechanism 106.
[0008]
The multiple transmission images are processed by the data processing unit 109, and a cross-sectional image on the imaging surface 111 is obtained from the transmission images passing through the imaging surface 111 orthogonal to the rotation axis 112.
[0009]
As the reconstruction method, for example, a filter-corrected back projection method (FBP method) shown in, for example, “CT scanner” (Yoshinori Iwai edition: Corona) is used.
[0010]
The cross-sectional image position of the subject 104 is changed by moving the subject 104 up and down in the direction of the rotation axis 112. However, a helical scan that rotates and raises and lowers the rotary table 105 simultaneously is performed on the imaging surface 111 in one imaging. There is also a method for obtaining a plurality of substantially parallel cross-sectional images (three-dimensional images).
[0011]
Further, there is a method of performing a helical scan and obtaining a plurality of cross-sectional images substantially parallel to the imaging tomographic plane 111 using a transmission image passing outside the imaging plane 111.
[0012]
The reconstruction method at this time is described in the following literature as an application of the FBP method.
[0013]
(Reference 1) “CT System” Toshiba Corp. Hiroshi Arata, Shinjiro Nambu, “Japanese Patent Laid-Open No. 4-224736”
In the first reconstruction method here, since the transmission image plane passing outside the imaging plane 111 is regarded as a parallel plane and tilted with respect to the imaging plane 111, it is backprojected. In contrast, the second reconstruction method disclosed in the same document performs backprojection according to the tilt, so that the resolution in the direction of the rotation axis 112 can be increased.
[0014]
This second reconstruction method can be said to be a method (Feldkamp method) described in the following document applied to a helical scan.
[0015]
(Reference 2) “Practica1 cone-beam algorithm” L. A. Fe1dkamp, L. C. Davis, and J.W. Kress J.0pt.Soc.Am./Vol.1,No.6,pp.612-619/June1984
[0016]
[Problems to be solved by the invention]
By the way, recently, there is a strong demand for examinations using a CT scanner, and the types of examination objects and examination contents are expanding. For this reason, there is a tendency that the demand for higher image quality and the demand for higher imaging speed are increasing.
[0017]
However, when a plurality of cross-sectional images are obtained by a single scan using the cone-shaped X-ray beam 102 with the above-described conventional high-resolution CT scanner, the time required for reconstruction of the cross-sectional images is long. There is a problem of becoming a bottleneck in speeding up.
[0018]
Further, the above-described literature 1, literature 2, etc. do not describe a specific method for speeding up reconstruction when the cone-shaped X-ray beam 102 is used (and simultaneously helically scanned).
[0019]
On the other hand, the above-described conventional high-resolution CT scanner has a feature that a high-resolution image can be obtained by bringing the subject 104 close to the focal point of X-rays. Since the subject 104 is irradiated with the ray beam 102, the scattered X-rays scattered by the subject 104 and incident on the detector 103 increase, and the cross-sectional image becomes unclear depending on the subject 104, so that sufficient inspection can be performed. There is a problem that can not be done.
[0020]
In addition, the subject 104 may be soft and easily deformed, and there is a problem that a movement occurs during imaging and a cross-sectional image becomes unclear.
[0021]
Furthermore, depending on the arbitrarily selected shooting conditions, the reconstruction filter function may be inappropriate, resulting in a noisy image. In such a case, the filter function is selected again and reconstructed.
[0022]
An object of the present invention is to provide a computer tomography method and apparatus capable of contributing to high-speed imaging or high image quality.
[0023]
[Means for Solving the Problems]
  In order to achieve the above object, a computed tomography apparatus according to a first aspect of the present invention provides a radiation source that emits a radiation beam and a radiation detection that detects the radiation beam from the radiation source with two-dimensional spatial resolution. A rotation means for relatively rotating the subject within the radiation beam, and a translation means for relatively moving the subject within the radiation beam in the axial direction of the rotation. The rotation by the rotation means and the translation means In a computed tomography device that collects transmission data while performing the relative movement by almost simultaneously,
  Two-dimensional transmission data from multiple directions of the subject obtained by the radiation detector during rotation by the rotation meansOn the other hand, the rotation axis position is 0.5, and the right and left sides orthogonal to the rotation axis have an inclined portion that changes from 0 to 1 in which the inclination is symmetric. A window function multiplication unit for multiplying the window function, and two-dimensional transmission data multiplied by the window function3D imageA volume backprojection unit that backprojects to a volume element;Reconstructing means for creating a three-dimensional image of the subject.
[0026]
  Therefore, in the computer tomography apparatus of the invention corresponding to claim 1, for the purpose of imaging a large subject, a rotation center is set at the end of the radiation beam detected by the radiation detector, Transmission data is obtained by protruding the subject outside the beam. ( Offset scan ) In some cases, by multiplying the window function, sudden changes in the projection data can be avoided. ( Ring shape ) Can be obtained. Also reorganize data columns ( Conversion to parallel data, etc. ) Can be reconfigured at high speed..
[0028]
  Also, Claims1In the computed tomography apparatus according to the invention, a three-dimensional image over a wide area in the direction of the rotation axis of the subject can be obtained at a time.
[0029]
  Furthermore, in the invention corresponding to claim 2, in the computed tomography apparatus of the invention corresponding to claim 1,The volume back projection unit back projects the two-dimensional transmission data multiplied by the window function onto a centering surface substantially parallel to one surface substantially parallel to the rotation axis of the volume element of the three-dimensional image. Is projected back to each volume element for each set of volume elements approximately parallel to the centering plane.
[0030]
Therefore, the claims2In the computer tomography apparatus of the invention corresponding toSince the second back projection becomes back projection between parallel planes, the calculation time of the back projection coefficient can be shortened, and reconstruction can be performed at high speed..
[0031]
    Meanwhile, claims3The computer tomography apparatus according to the invention corresponds to a collimator that restricts the radiation beam to a fan beam along a plane of rotation, and the transmission data of the passage blocked by the collimator is used to scatter the transmission data that is not blocked. And a scattered radiation correcting means for correcting radiation.
[0032]
  Therefore, the claims3In the computed tomography apparatus according to the invention, the intensity of only the scattered radiation is obtained from the transmission data of the blocked passage, and the transmission data of the blocked passage close to the two-dimensional position of the transmission data is obtained from the transmission data that is not blocked. By subtracting the transmission data, the scattered radiation can be corrected, and a high-quality cross-sectional image can be obtained.
[0033]
  Meanwhile, claims4In the computer tomography apparatus according to the invention, the reconstruction unit selects a filter function based on a signal-to-noise ratio of the transmission data, and performs a filter correction back projection to create a three-dimensional image. Computer tomography equipment.
[0034]
  Therefore, the claims4In the computed tomography apparatus according to the invention, the filter function is selected based on the signal-to-noise ratio of the transmission data even if the imaging conditions such as the imaging magnification, the tube voltage, the tube current, and the scan time are arbitrarily changed. A cross-sectional image in which the relationship between noise and spatial resolution is optimal can be obtained.
[0037]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0038]
(First embodiment)
FIG. 1 is a schematic diagram showing a system configuration example of a CT scanner according to the present embodiment.
[0039]
In FIG. 1, as an X-ray tube 1 as a radiation source, a microfocus X-ray tube having a focal point F of a radiating X-ray beam 2 of several to several tens of μm is used. X-ray planar solid state detector (or X-ray ll (image intensifier tube) and TV camera) is used.
[0040]
The X-ray tube 1 and the radiation detector 3 are arranged to face each other and supported by a support member (not shown) on the floor.
[0041]
The subject 4 is placed on the rotary table 5 and rotated along the imaging surface 11 in the X-ray beam 2 by the rotation / elevating mechanism 6 and is moved up and down at right angles to the imaging surface 11.
[0042]
Further, the subject 4 is moved between the X-ray tube 1 and the radiation detector 3 along the imaging surface 11 by the shift mechanism 7 supported on the floor together with the rotary table 5 and the rotation / lifting mechanism 6. The shooting magnification is changed.
[0043]
Although not shown, the mechanism portion is surrounded by X-ray shielding.
[0044]
On the other hand, as other components, a data processing unit 9 that processes a transmission image from the radiation detector 3, a display unit 10 that displays a processing result, and a mechanism unit are controlled by commands from the data processing unit 9. A mechanism control unit 8 and an X-ray control unit 13 for controlling the tube voltage and tube current of the X-ray tube 1 are provided.
[0045]
The data processing unit 9 and the display unit 10 are ordinary computers, which include a CPU, a memory, a disk, a keyboard, an interface, and the like, and store software and the like for reconstructing a three-dimensional image from tomographic sequences and data.
[0046]
The operator uses the data processing unit 9 and the display unit 10 to perform menu selection, condition setting, manual operation of the mechanism unit, start of tomography, device status reading, 3D image display, 3D image analysis, etc. Do.
[0047]
The data processing unit 9 includes an air correction unit 14 that processes digital data from the radiation detector 3, a LOG conversion unit 15, and a reconstruction unit 16, and the reconstruction unit 16 further includes a filtering unit 16a. And a volume BP portion 16b.
[0048]
Next, the operation of the CT scanner configured as described above according to the present embodiment will be described.
[0049]
In FIG. 1, when obtaining a transmission image, the operator places the subject 4 on the table 5, sets the tube voltage and tube current, turns on the X-ray, and displays the transmission image on the display unit 10.
[0050]
In the case of a three-dimensional image, the subject 4 is moved up and down by the rotation / lifting mechanism 6 so that the center of the examination position is aligned with the imaging surface 11.
[0051]
This can also be done while looking at the transmission image.
[0052]
When tomography starts, the rotary table 5 rotates, and during this time, a transmission image is collected by the data processing unit 9.From transmission data in the vicinity of the imaging surface 11 obtained at intervals of Δφ in the 360 ° direction, A three-dimensional image is reconstructed and displayed on the display unit 10.
[0053]
First, the transmission image is corrected by the air correction unit 14 for each channel by taking a ratio with the data da when there is no subject 4 collected in advance.
[0054]
h = (d-doff) / (da-doff) (1)
Here, doff is data when the X-ray is OFF.
[0055]
Next, each channel is logarithmically converted by the LOG converter 15 and converted to projection data p corresponding to the line integral of the absorption coefficient.
[0056]
p = LOG (1 / h) (2)
The projection data of the n and m channels at the rotation angle φ is described as pφ (n, m), and then a high frequency enhancement filter is applied in the n direction along the imaging surface 11 by the filter application unit 16a.
[0057]
This is done by performing a Fourier transform in the n direction, applying a filter function that is approximately proportional to the frequency in the frequency space, and returning it by an inverse Fourier transform.
[0058]
Next, the volume BP unit 16b performs back projection on each volume element (voxel).
[0059]
This back projection processing will be described with reference to the geometric diagram shown in FIG. 2 and the flowchart shown in FIG.
[0060]
Note that description of the above-described reference signs and signs that are obvious from the drawings is omitted.
[0061]
FIGS. 2A and 2B show the positions of the X-ray focal point F and the detection surface 17 viewed at coordinates x, y, and z fixed to the subject 4. Let the rotation angle of F be φ.
[0062]
Each voxel of the volume to be reconstructed is numbered by i, j, k, and the voxel size is Δx, Δy, Δz.
[0063]
Here, Δy is preferably set equal to Δx, and Δz is also preferably equal to Δx.
[0064]
Next, in FIG. 3, a φ loop for 360 ° is obtained in step S1, and x, y coordinates xF, yF of F are calculated in step S2,
xF = FCD · sinφ (3)
yF = FCD · cosφ (4)
FCD: Distance between focal point and center of rotation
In steps S3 and S4, the loop of voxels j and i is entered. In step S5, nij, Δmij, L²Ask for.
[0065]
nij, mijk: Channel number BP to voxel i, j, k
△ mij: increment of mijk when k increases by 1
1 / L²: BP weight
x = (i−ic) · Δx (5)
y = (jc−j) · Δy (6)
△ x, △ y, △ z: Voxel size
φ ′ = arctan ((xF−x) / (yF−y)) (7)
θ = φ−φ ′ (8)
nij = nc + FDD.tan.theta. / d (9)
L²= ((XF-x)²+ (YF-y)²) / FCD²      ... (10)
Δmij = FDD / (√ (L²) ・ FCD ・ cosθ) ・ △ z / d (11)
d: Channel spacing
FDD: Focus, distance between detection surfaces
In step S5, an initial value of mijk is further set.
[0066]
mijk = mc + (ks−kc) · Δmij (12)
Next, in step S6, the k loop is entered, and in step S7, addition to the voxels i, j, k is performed.
[0067]

Here, since nij and mijk are generally not integers, interpolation calculation is performed.
[0068]
Next, in step S8, mijk is updated for the next k.
[0069]
mijk = mijk + Δmij (14)
Steps S9, S10, and S11 are repeated for k, i, and j, and the entire volume is added. In step S12, φ is repeated and backprojection is added to one volume, and the backprojection process is completed, and the volume is three-dimensional. I can make an image.
[0070]
In the above description of the backprojection process, the formula is not the fastest for easy understanding, and the formula includes many parts that can be put out of the loop.
[0071]
As described above, in the CT scanner according to the present embodiment, the loop of voxels in the direction of the rotation axis 12, that is, the k loop is the innermost, so the calculation of the corresponding channel nij, mijk (or Δmij) is performed within the k loop. Since it can be made common, there is no waste in calculation compared to the case where the k loop is outside the i and j loops as in the conventional case described above.
[0072]
In addition, since the data can be reconfigured without waiting for other data in the order in which the data is collected, the reconfiguration speed can be increased.
[0073]
Furthermore, since the back projection is performed according to the inclination from the imaging surface 11, the resolution in the direction of the rotation axis 12 can be increased.
[0074]
(Modification of the first embodiment)
BP weight is 1 / L²In addition to this, as shown in FIG. 4, a weight w (φ) depending on the rotation angle may be multiplied.
[0075]
In this case, data collection is performed during 360 ° + 2α rotation. When this weight is applied, before and after 360 ° rotation is averaged, even if the subject 4 moves slightly on the rotary table 5 during imaging, a false image is less likely to occur.
[0076]
The weight w (φ) may have a curved slope portion. That is, it is only necessary to be 1 when added with the one shifted by 360 °.
[0077]
(Second Embodiment)
The system configuration of the CT scanner according to the present embodiment is the same as that shown in FIG. 1, and only the method of back projection processing by the volume BP unit 16b is different.
[0078]
Therefore, here, the back projection processing (action) of the CT scanner of this embodiment will be described with reference to the geometric diagrams shown in FIGS. 5 and 6 and the flowchart shown in FIG.
[0079]
Note that description of the above-described reference signs and signs that are obvious from the drawings is omitted.
[0080]
FIG. 5 shows the positions of the X-ray focal point F and the detection surface 17 viewed at coordinates x, y, and z fixed to the subject 4. Let the rotation angle of F be φ.
[0081]
Similarly in FIG. 6, each voxel of the volume to be reconstructed is numbered by i, j, k, and the voxel sizes are set as Δx, Δy, Δz (Δy = Δx).
[0082]
Next, in FIG. 7, the projection data p for 360 ° in step S1 is divided into quarters for 90 °, and in step S2, the first 90 ° φc (calculation rotation angle) loop is formed. Calculate the collection rotation angle φ.
φ = φc + nq · π / 2 (15)
nq: quota number
In step S4, it is determined whether or not φ is the φ being collected, and the head is cued or terminated.
[0083]
In step S5, the x and y coordinates xF and yF of F are calculated.
[0084]
xF = FCD · sinφc (16)
yF = FCD · cosφc (17)
Next, centering is performed in steps S6 to S11.
[0085]
In FIG. 5, centering is a process of back-projecting data on the detection surface 17 onto a pq matrix on the xz plane.
[0086]
In step S6, a p-loop is entered, and np is obtained in step S7.
[0087]
xo = (p−pc) · cp (18)
x1 = xo ・ cosφc (19)
mag1 = FDD / √ ((xF−xo)²+ YF²-X1²)) ... (20)
np = nc + x1 · mag1 / d (21)
np, mpq: Channel numbers BP to matrix p, q
cp, cq: Centering pitch
Next, a q loop is entered at step S8, mpq is obtained at step S9, and centering data pc (p, q) is obtained at step S10.
[0088]
mpq = mc + (q-qc) .cq.mag1 / d (22)
pc (p, q) = pφ (np, mpq) (23)
Here, since np and mpq are generally not integers, interpolation calculation is performed.
[0089]
Next, in step S11, the following q and p are repeated.
[0090]
Next, BP is performed in steps S12 to S22.
[0091]
In FIG. 6, the j loop is entered in step S12, and poj, Δpj, and Δqj are obtained in step S13.
[0092]
mag2 = yF / (yF + (j−jc) · Δy) (24)
poj = pc + (xF−mag2 · (xF + ic · Δx)) / cp (25)
Δpj = mag2 · Δx / cp (26)
Δqj = mag2 · Δy / cp (27)
pij, qjk: matrix number BP to voxel i, j, k
Δpj: increment of pij when i increases by 1
Δqj: increment of qjk when k increases by 1
In step S13, an initial value of pij is further set.
[0093]
pij = poj (28)
Next, in step S14, the i loop is entered, and in step S15, L²Calculate
[0094]
x = (i−ic) · Δx (29)
y = (jc−j) · Δy (30)
L²= ((XF-x)²+ (YF-y)²) / FCD²      ... (31)
In step S15, an initial value of qjk is further set.
[0095]
qjk = qc + (ks−kc) · Δqj (32)
Next, in step S16, the k loop is entered, and in step S17, addition to the voxels i, j, k is performed.
[0096]

Here, since pij and qjk are not generally integers, interpolation calculation is performed or the nearest neighbor pc is selected.
[0097]
Next, in step S18, qjk is updated for the next k.
[0098]
qjk = qjk + Δqj (34)
The k loop is repeated in step S19, and pij is updated for the next i in step S20.
[0099]
pij = pij + △ Pj (35)
In steps S21 and S22, the entire volume is added by repeating i, j, and φc is repeated in step S23 to add back projection to one volume.
[0100]
In step S24, the image is rotated 90 degrees, and in step S25, the quota is repeated to complete the back projection process, and a three-dimensional image of the volume is obtained.
[0101]
In the above description of the backprojection process, the formula is not the fastest for easy understanding, and the formula includes many parts that can be put out of the loop.
[0102]
As described above, in the CT scanner according to the present embodiment, first, when the centering data pc is obtained by back projecting the data on the detection surface 17 to the xz plane at each rotational position φ, pc is one surface ik of the voxel. Since it is obtained at equal intervals on a plane parallel to the plane, the backprojection calculation to the ik plane (specifically, calculation of pij and qjk) can be remarkably simplified, and the reconstruction speed can be increased. It becomes.
[0103]
Further, similarly to the first embodiment described above, since the loop of voxels in the direction of the rotation axis 12, that is, the k loop, is the innermost, the calculation of the corresponding channel pij, qjk (or Δqj) is performed within the k loop. Since it can be made common, there is no waste in calculation compared to the case where the k loop is outside the i and j loops as in the conventional case, and the reconstruction speed can be increased.
[0104]
Furthermore, since the data can be reconfigured without waiting for other data in the order in which the data is collected, the reconfiguration speed can be increased.
[0105]
Furthermore, since the back projection is performed according to the inclination from the imaging surface 11, the resolution in the direction of the rotation axis 12 can be increased.
[0106]
(Modification of the second embodiment)
Similar to the modification of the first embodiment described above, the BP weight is 1 / L.²In addition to this, as shown in FIG. 4, a weight w (φ) depending on the rotation angle may be multiplied. Also in this case, the same effect can be obtained.
[0107]
In the second embodiment, when the volume k direction is the same size as the ij direction, the i loop may be located inside the k loop. In this case, equation (33) is not an approximate calculation, but the first row is calculated.
[0108]
(Third embodiment)
The system configuration of the CT scanner according to this embodiment is the same as that of the second embodiment, and only the mechanism operation at the time of transmission image collection and the back projection processing method by the volume BP unit 16b are different.
[0109]
Therefore, here, the operation at the time of transmission image collection and the back projection processing (operation) of the CT scanner of the present embodiment will be described with reference to the flowchart shown in FIG.
[0110]
In the case of this embodiment, when tomography is started, the rotary table 5 is moved up and down at the same time as the rotation (helical scan), and transmission images are collected during this time, and the three-dimensional volume is obtained from transmission data in directions of 360 degrees or more. The image is reconstructed and displayed on the display unit 10.
[0111]
The volume BP of the present embodiment is substantially the same as that of the second embodiment described above, and will be described using the flowchart shown in FIG. 7 and equations (15) to (35).
[0112]
That is, the difference from the case of the second embodiment described above is that kc is not a constant but a function of φ, and equation (33) is different.
[0113]

Here, each equation will be described with reference to FIG.
[0114]
First, in equation (36), kc (φ) is the k position of the matrix that the imaging surface 11 crosses at the rotation angle φ.
[0115]
In this equation, zp is the height (helical bitch) that moves up and down during one rotation, Δz is the voxel size in the height direction, and φO is the rotation angle when crossing k = 0.
[0116]
In FIG. 8A, wφ (z) is a weight of the BP of the voxel in the z direction at the rotation angle φ and has a trapezoidal shape with a half width zp.
[0117]
On this trapezoidal slope, BP from φ different 360 ° is synthesized (dotted line).
[0118]
The weight obtained by changing wφ (z) to k scale is wφ (k), and the center shown in FIG. 8B is a trapezoid with kc (φ) and half width zp / Δz.
[0119]
zp / Δz at k is a rotation angle and corresponds to 360 °.
[0120]
Here, the half length kα of the slope is an angle α,
kα = Zp / Δz · α / 2π (37)
Therefore, kα is determined by setting the angle half width α to be interpolated.
[0121]
In Expression (33 ′), wφ (k) is used as the BP weight. Therefore, when helical scanning is performed, BP corresponding to 360 ° is applied to each voxel even if BP is continuously performed for φ.
[0122]
As described above, in the CT scanner according to the present embodiment, it is possible to increase the reconstruction speed and the resolution in the direction of the rotation axis 12 as in the second embodiment described above.
[0123]
In addition, since the image can be taken efficiently by the helical scan without increasing the inclination of the beam from the imaging surface 11, a wide and high-quality three-dimensional image in the direction of the rotation axis 12 can be obtained.
[0124]
Furthermore, since a wide range can be scanned smoothly at a constant speed of rotation and elevation by helical scanning, the subject 4 is not accelerated and a high-quality image can be obtained even with the subject 4 that is easy to move.
[0125]
In addition, since the BP is multiplied by the trapezoidal weight wφ (k), before and after 360 ° rotation is averaged, even if the subject 4 moves during imaging, a false image is hardly generated.
[0126]
(Modification of the third embodiment)
As shown in FIG. 8C, the weight wφ (k) may have a curved slope. That is, it is only necessary to be 1 when added with the one shifted by zp / Δz (dotted line).
[0127]
(Fourth embodiment)
The system configuration of the CT scanner according to this embodiment is almost the same as that of the third embodiment, and is provided with a window function multiplying section between the LOG conversion section 15 and the reconstruction section 16, and the rotation center. Is different from the center of the X-ray beam 2 when viewed from above.
[0128]
FIG. 9 is a schematic diagram showing the positional relationship between the rotation center C and the X-ray beam 2.
[0129]
This is a CT scanner in which the rotating table 5 is set to be shifted and the subject 4 is placed on one side so as to protrude from the X-ray beam 2 and the large subject 4 can be imaged. Called a scan.
[0130]
The operation of the CT scanner according to the present embodiment is the same as that of the third embodiment, and a window function w (n as shown in FIG. 10 is applied to the projection data pφ (n, m) after LOG conversion. ) Is only different.
[0131]
The window function w (n) is center ch, nc, and is inclined about 0.5 at the center. One outside the inclined area is 0, and the other is 1.
[0132]
The reconstruction unit 16 performs reconstruction in the same manner as in the third embodiment.
[0133]
As described above, in the CT scanner according to the present embodiment, it is not necessary to reorganize the data string (conversion to parallel data, etc.) even in the case of offset scanning, and can be reconstructed at high speed. Since a smooth weight is applied across the center ch, a sudden change in projection data can be avoided, and a three-dimensional image with few false images (ring-shaped) can be obtained.
[0134]
Furthermore, an offset scan can be performed while performing a helical scan, and in addition to obtaining the same effects as those of the third embodiment described above, a CT scanner capable of handling a large subject 4 is realized. It becomes possible.
[0135]
(Modification of the fourth embodiment)
The offset scan according to the fourth embodiment can also be applied to the first or second embodiment described above.
[0136]
The window function w (n) may have a curved slope. That is, it is only necessary that the center ch has an inclined portion that changes from 0 to 1 on the right and left sides orthogonal to the axis, and the inclination changes from 0 to 1, and the outside of the inclined portion is 0 on one side and 1 on the opposite side.
[0137]
(Fifth embodiment)
FIG. 11 is a schematic diagram showing a system configuration example of the CT scanner according to the present embodiment, which is substantially the same as that of the first embodiment, and the scattered radiation correction is performed between the air correction unit 14 and the LOG conversion unit 15. A difference is that the unit 19 is provided, and a movable collimator 18 that restricts the X-ray beam 2 to a thin cone shape sandwiching the imaging surface 11 is provided.
[0138]
The operation of the CT scanner according to this embodiment is the same as that of the first embodiment. The X-ray beam 2 is limited to a thin cone at the time of tomography, and the data h (n, m after air correction). ) Is different from that in FIG.
[0139]
FIG. 12 is a diagram for explaining the content of the scattered radiation correction.
[0140]
FIG. 12 shows data h (n, m) on the detection surface 17.
[0141]
Other than the irradiation unit 20 of the X-ray beam 2, only the scattered radiation from the subject 4 or the like is detected at the part blocked by the collimator 18.
[0142]
Scattered ray correction for data h (n, m) in the irradiation unit 20

Add
[0143]
Here, the scattered light is obtained by subtracting the upper and lower channels close to h (n, m) by linear interpolation, and subtracted.
[0144]
As described above, in the CT scanner according to the present embodiment, the scattered radiation correction can be performed using the data of the portion blocked by the collimator 18 of the two-dimensional detector without using a dedicated detector for scattered radiation measurement. it can.
[0145]
Further, since the two-dimensional detector is manufactured at a time for the dedicated detector, the characteristics are uniform.
[0146]
Furthermore, since the scattered radiation can be measured at a position close to the measurement point at the same time and under a uniform condition, it is possible to correct accurately.
[0147]
(Modification of the fifth embodiment)
In the fifth embodiment, when performing the correction, not a 2-channel data but a larger number of channel data may be used, and the primary interpolation may not be performed.
[0148]
By using this large number of channel data, the statistical accuracy can be improved.
[0149]
The scattered radiation correction in the fifth embodiment can be added not only to the first embodiment but also to the second to fourth embodiments. It can also be added to a CT scanner using a conventional two-dimensional detector as shown in FIG.
(Sixth embodiment)
The system configuration of the CT scanner according to the present embodiment is almost the same as that of the first embodiment, except that the CT scanner is applied to the imaging of the subject 4 that is easily deformed.
[0150]
In the case of the present embodiment, the subject 4 is set on the table 5, and as the preparation rotation, the rotation at a smooth constant speed is performed, and the tomography is started after the subject 4 is sufficiently stabilized.
[0151]
The apparatus turns on X-rays while maintaining the same rotational speed, collects transmission images, and creates a three-dimensional image.
[0152]
In the case of the helical scan, the preparatory rotation is performed in the same manner, and when the tomography is started, the elevation is gradually started to start collecting transmission images.
[0153]
If the preparation rotation time may be short, preparation lifting may be performed together.
[0154]
As described above, in the CT scanner according to the present embodiment, the movement of the subject 4 generated at the start of rotation is stopped at a constant speed and then imaged without changing the rotation speed. The specimen 4 can be photographed with the movement stopped, and a high-quality three-dimensional image can be obtained.
[0155]
(Seventh embodiment)
The system configuration of the CT scanner according to this embodiment is almost the same as that of the first embodiment, except that the filter function is automatically selected.
[0156]
That is, the data processing unit 9 calculates the signal noise ratio (average) of the data (p or h) before filtering and automatically selects the filter function.
[0157]
The signal-to-noise ratio is determined by taking the low frequency component as a signal and the high frequency component as noise, and determining the ratio of signal / noise.
[0158]
If the signal-to-noise ratio is small, the filter function is chosen to have a low cut-off frequency to reduce image noise instead of slightly reducing spatial resolution.
[0159]
As described above, in the CT scanner according to the present embodiment, the filter function is selected based on the signal-to-noise ratio of the transmission data even if the imaging conditions such as imaging magnification, tube voltage, tube current, and scan time are arbitrarily changed. A cross-sectional image in which the relationship between image noise and spatial resolution is optimal can be obtained.
[0160]
【The invention's effect】
As described above, according to the computed tomography method and apparatus of the present invention, it is possible to contribute to the speeding up of imaging or the improvement of image quality.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing a first embodiment of a CT scanner according to the present invention.
FIG. 2 is a geometric diagram for explaining the operation of the CT scanner according to the first embodiment.
FIG. 3 is a flowchart for explaining the operation of the CT scanner according to the first embodiment;
FIG. 4 is a diagram showing a modification of the first embodiment of the CT scanner according to the present invention.
FIG. 5 is a geometric diagram for explaining the operation of the CT scanner according to the second embodiment of the present invention.
FIG. 6 is a geometric view for explaining the operation of the CT scanner according to the second embodiment of the present invention.
FIG. 7 is a flowchart for explaining the operation of the CT scanner according to the second embodiment and the third embodiment of the present invention.
FIG. 8 is a diagram for explaining the operation of the CT scanner according to the third embodiment of the present invention and its modification.
FIG. 9 is a partial schematic diagram showing a fourth embodiment of a CT scanner according to the present invention.
FIG. 10 is a view for explaining the operation of the CT scanner according to the fourth embodiment.
FIG. 11 is a partial schematic diagram showing a fifth embodiment of a CT scanner according to the present invention.
FIG. 12 is a diagram for explaining the operation of the CT scanner according to the fifth embodiment.
FIG. 13 is a schematic diagram showing a system configuration example of a conventional high-resolution CT scanner.
[Explanation of symbols]
1 ... X-ray tube
2 ... X-ray beam
3. Radiation detector
4 ... Subject
5 ... Rotating table
6 ... Rotation / lifting mechanism
7 ... Shift mechanism
8 ... Mechanism controller
9 ... Data processing section
10 ... Display section
11… Shooting surface
12 ... Rotation axis
13 ... X-ray controller
17 Detection surface
18 ... Collimator
19: Scattered ray correction unit
20: Irradiation part.

Claims (4)

A radiation source that emits a radiation beam; a radiation detector that detects the radiation beam from the radiation source with two-dimensional spatial resolution; a rotating means that relatively rotates a subject within the radiation beam; In a computed tomography apparatus having translation means for relatively moving the subject in the axial direction of rotation, and collecting transmission data while performing rotation by the rotation means and relative movement by the translation means substantially simultaneously,
For the two-dimensional transmission data of the subject obtained from the radiation detector during the rotation by the rotation means , the tilt is 0.5 on the rotation axis position and on the left and right sides orthogonal to the rotation axis. A window function multiplying section for multiplying a window function that has a symmetric slope portion from 0 to 1 and one side is 0 on the other side and 1 outside the slope portion, and two-dimensional transmission data multiplied by the window function Reconstructing means for creating a three-dimensional image of the subject , having a volume backprojection unit that backprojects a volume element of a three-dimensional image;
A computer tomography apparatus comprising: The computed tomography apparatus according to claim 1,
The volume back projection unit back projects the two-dimensional transmission data multiplied by the window function onto a centering surface substantially parallel to one surface substantially parallel to the rotation axis of the volume element of the three-dimensional image. The computed tomography apparatus is characterized in that the value of is projected back to each volume element for each set of volume elements substantially parallel to the centering plane . The computed tomography apparatus according to claim 1 or 2,
A collimator that restricts the radiation beam to a fan beam along the plane of rotation; and a scattered ray correction means that corrects the scattered radiation of the transmission data that is not obstructed, using transmission data of the passage obstructed by the collimator; A computer tomography apparatus comprising: The computed tomography apparatus according to any one of claims 1 to 3,
The computer tomography apparatus according to claim 1, wherein the reconstruction unit selects a filter function based on a signal-to-noise ratio of the transmission data, and performs a filter correction back projection to create a three-dimensional image.

Images